Transition metal sulfides

Progress in Solid State Chemistry, Vol. 10, Part 4, pp. 207-270.

Pergamon Press.

Printed in Great Britain.

T R A N S I T I O N METAL S U L F I D E S C. N. R. RAOt and K. P. R. PISHARODY Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India

formation of itinerant electrons. A lower transfer energy would mean an increase in the critical distance, Re, below which the cation-cation interacDuring the last decade, considerable effort has tions are sufficiently strong to form itinerant been directed towards understanding the chemistry and physics of transition metal chalcogenides. As a cation-sublattice d-bands. Such cation-sublattice result of these investigations, a variety of new bands would be expected in sulfides like TiS and materials with interesting properties have been VS. Due to the higher covalency, the d"+' mandiscovered. Unlike the transition metal fluorides ifolds of the heavier cations fall closer to the top of which contain localized 3d electrons, sulfides and the valence band. The t~* bands in CoS (or NiS) oxides of transition metals contain either localized would be broader than in the corresponding oxide or itinerant 3d electrons or both simultaneously. and the energy separation between high- and lowStudies of transition metal oxides have been in- spin CoS is reduced. The small value of U in CuS structive in understanding the behavior of d elec- could bring the d 1° manifold at a copper atom trons in solids; ~'-3~however, properties of transition sufficiently close to the top of the valence band that metal chalcogenides are quite different from those the valence state of the copper ions become amof oxides since d electrons in'chalcogenides partici- biguous. pate in covalent bonding. Such covalent bonding in Structures of transition metal sulfides are quite chalcogenides reduces the formal charge on transi- different from those of the corresponding oxides. tion metals and favors formation of metal-metal The factors that contribute to these differences bonds. Metal-metal interactions indeed play a sig- are{ 7~(a) hybridization of anion 3s and 3p orbitals nificant role in determining the properties of many with the anion 3d orbitals could stabilize six of the transition metal chalcogenides. The influence nearest-neighbor cations in a trigonal-pyramid conof such interactions on the electronic structure of figuration; (b) sulfur-sulfur bonding may give rise these solids can be studied by means of their to molecular anions; (c) larger covalency reduces transport properties and these interactions become the effective charges on the ions, and (d) the large particularly important in transition elements sulfi- polarizability of the anions favor formation of layer des of the second and third series. structures with van der Waals forces between Electrical and magnetic properties of transition layers. metal sulfides have been studied e x t e n s i v e l y Y ~ Transition metal monosulfides generally crystalWhile the electrical properties vary from semicon- lize in the NiAs (B8) structure (or its variations) ducting to metallic (superconducting in some containing hexagonally close-packed anions rather cases), magnetic properties vary from weak than in the rock salt structure of the monoxides diamagnetism (through localized magnetism) to (with octahedral coordination of metal ions). Hextemperature-independent Pauli paramagnetism. Jel- agonal packing involves octahedral sites, the strings linek ~6) has discussed these variations in general of octahedral sites sharing common faces parallel terms on the basis of a qualitative band model. to the c-axis. Further, each anion has six nearestGoodenough ~7~ has recently made a comparative neighbor cations (in a trigonal pyramid) and facestudy of the properties of fluorides, oxides and shared tetrahedral sites form a trigonal bipyramid sulfides of divalent transition elements in terms of interstice (the interstitial positions are partly octhe conceptual phase diagrams containing temperacupied in cation-rich compositions). Most ture, orbital occupancy number and near-neighbor monosulfides are cation-deficient and cationtransfer energy as variables. The general features vacancy ordering gives rise to distinct phases. of transition metal monosulfides and disulfides visTransition metal disulfides (as well as did-vis the oxides can be examined following the selenides) form layered compounds containing arguments of GoodenoughJ 7) Just as in transition monoatomic anions as well as compounds with metal monoxides, the monosulfides have the 3d" diatomic anions. In the layered compounds, the manifolds which become more stable with respect six-coordinated cations occupy octahedral interto the 4s bands with increasing atomic number until stices or trigonal pyramidal interstices. The cations the shell is half-filled. The larger covalency in the always form close-packed layers where the intermonosulfides implies greater screening of the ca- stices share common edges and van tier Waals tion electrons from their nuclear charge. This re- forces operate between the layers. The disulfides sults in a small electrostatic energy, U; a smaller U crystallize in a variety of polytypes. Only the first results in a smaller transfer energy required for the few transition metals in a period form such layered compounds since quadrivalency is necessary for ?To whom all correspondence should be addressed: the purpose. When the energy of the trivalent d "÷' Jawaharlal Nehru fellow, Indian Institute of Technology; manifold falls below the valence band, three holes Commonwealth Visiting Professor, University of Oxford are introduced into the anion 3p bands. In such a (1974-75). case, S~- ions are formed; disulfides of heavier I. Introduction

207

208

C . N . R . RAO and K. P. R. PISHARODY

cations like Mn or Fe belong to this category. Generally these compounds crystallize in pyrite, marcasite or arsenopyrite structure, depending on the magnitude of cation-anion interactionsJ 8~When the d" manifold has n = 0, 2 or 4, marcasite structure is favored; when n = 5, a high-spin configuration in the pyrite structure or a low-spin configuration in the arsenopyrite structure is formedfl ~When n ~>6, the pyrite structure is generally formedfl ~ The sulfides MS2 (M = Mn to Zn) all crystallize with pyrite structure, although FeS: also forms a marcasite phase. The cubic pyrite structure is similar to rock salt, but the diatomic anions have axial symmetry and these axes are ordered equally along the four (l 1 l) directions of the cube. The cations occupy octahedral sites and anions are four-fold coordinated to one anion and three cation nearest neighbors. Besides the mono- and di-sulfides, transition metals form sulfides of the general formulae, MS3, M2S, Mt+~S, MS~+~ and so on beside a variety of other well-defined compounds with unique properties. Metal atom vacancies influence the crystal structures of the transition metal sulfides. For example, in the NiAs (B8) structure, very often metal atom vacancies exist; when the vacancies are negligible, the NiAs structure is not very much favored and monoclinic distortions or superstructures are formed. The vacancies in the NiAs structure are usually limited to the alternating planes of the metal atoms and they are ordered. When the composition reaches M0.~X, CdI2 or MoSs type structure is formed. In some transition metal sulfide systems there is a continuous transition from NiAs to CdI2 structure as in the TiS-TiS: system. When strong metal-metal interactions are present, zig-zag chains of the transition metal atoms run along one of the crystallographic directions. Many Nb, Mo and Re sulfides where such zig-zag chains exist have been reported. It is interesting that in most of the superconducting transition metal compounds such metal-metal chains are present or the metal-metal distance is very small. The overlap between the d-orbitals of the metal ions give rise to narrow d-bands with high density of states. Even though exact energy band calculations have been attempted only for a few sulfides, certain general observations can readily be made. The valence band is usually built up of the 3s and 3p orbitals of sulfur and (n + 1)s and (n + 1)p orbitals of the transition metals. The valence and the conduction bands are separated by few eV (1-10 eV). T h e nd orbitals of the transition metal ions can remain localized on the ion or form a band, depending on the crystal structure and internuclear distances. In some cases, the d-band is very narrow and in some others very broad. The d-levels or bands are situated between the valence band and the conduction bands. In some instances, overlapping with either the valence or the conduction band also occurs. The d-bands are usually split up into several sub-bands due to ligand fields, Jahn-Teller ,distortion and interatomic exchange interactions. Besides these factors metal-metal interactions can also split a broad d-band into narrow sub-bands. In this article, we have attempted to survey the results of investigations (reported in the literature

up to June 1974) on binary transition metal sulfides and also on some interesting ternary sulfides. We have presented important data on sulfides ~9~in the form of tables at the end of the article to serve as ready reference material on these fascinating systems. II. Sulfides of group IIIB transition metals Transition elements of group IIIB, Sc, Y and La, react with sulfur to form a number of binary sulfides. In spite of their being in the same group, the sulfides of these elements exhibit varying physical properties. The common oxidation state of these elements is + 3, but binary sulfides of other oxidation states are also stable. As in the case of sulfides of other transition elements, deviation from stoichiometry and polymorphism are common features amongst these sulfides. All the sesquisulfides are semiconductors, but the sulfides of lower oxidation states are usually metals. The cationic radii range from 0.7 ~, for Sc(III) to 1.06 for La(III) and the radius ratio is an important factor in deciding the bonding and structure of these chalcogenides. 2.1. S c a n d i u m - s u l f u r system The scandium-sulfur system is important because of the potential applications of the sulfides as wide-band semiconductors. Like other transition metal sulfides, scandium forms non-stoichiometric sulfides. SczS3 Sc2S3 was first prepared by Klemm. "°~ A more detailed investigation of the crystal structure and properties was carried out by Dismukes and WhiteJ "'~2) Sc:S3 is prepared ~1~ by heating Sc203 in H2S or CS2 atmosphere. Single crystals are grown by chemical transport reactions. (9'"~ The unit cell of Sc:S3 can be described as a cation-deficient NaCI structure. "~'n~ The orthorhombic unit cell with sixteen formula units contains twelve rock salt units with several ordered cation vacancies. The brown-violet-colored phase reported earlier by Menkov et al. "3~ is probably a non-stoichiometric phase. Sc2S3 is a semiconductor with an optical band gap of 2.78 e V f 4~Doping Sc2S3 by Mg and Zn, or P, As and Sb have been attempted to produce p - t y p e material, but without s u c c e s s f 4~These dopants fill the voids present in the structure instead of occupying substitutional sites. Lashkarev et a l l 5~ have reported Sc2S3 to be n-type between 300 K and 1500 K and have discussed their results in terms of a band model where the filled valence band is formed by 3p levels of S -2 ions and the conduction band is made of 3d levels of scandium. ScS The metallic gold-colored ScS was prepared by Dismukes and White ~11~by reacting the elements at 1150°C for 70hr. ScS has a NaC1 type structure. The metallic nature is due to the presence of excess d-electron in the conduction band. N o band energy calculations are available and optical studies of this compound may be interesting.

Transition metal sulfides Sc~ xS2(Sc:+x $3) Dismukes and White ~"~ have tried to prepare a series of compounds of compositions between Sc,.S3 and ScS. They report several nonstoichiometric phases between Sc2S3 and ScS with a discontinuity at Sc,.37S2. The formula for the metal +3 rich phases may be written as Sc2+x(e )3xS:-2 in which the excess electron is delocalized and contributes to the metallic conductivity. These nonstoichiometric phases are black metallic compounds with charge carrier concentrations of 102~cm -3. Sc,3~$2 is reported to have a rhombohedral unit cell,¢'L'2)where the excess metal atom may be filling the voids present in Sc:S3. NaScS:, MgSc2S4, MnSc2S4 and FeSc:S4 are also known. NaScS2 has a NaCrS:-type structure while others are normal spinels. ¢~6''7)CuScS2 is a semiconductor with NiAs type structure. ¢~8~ 2.2 Y t t r i u m - s u l f u r system Like scandium, yttrium also forms a series of binary sulfides ~19-241 but unlike scandium sulfides, these sulfides are similar in properties and structure to the rare earth sulfides. Even though several sulfides such a s Y S 2 , t19) Y5S7, (2°'22) Y S tzl) and Y 2 5 3 (23'24) have been reported, properties of only a few are known. The preparation of these sulfides in pure and crystalline form is one of the hurdles for such investigations. Y2S3 Y:S3 has been prepared along with other rare earth sesquisulfides by Flahaut et a l Y ) and recently by Sleight and PrevittJ TM Flahaut et al. classified the rare earth sesquisulfides as a,/3, y, 6 and e depending on the crystal structure. Sleight and Previtt t2s) have changed this nomenclature to A, B, C, D and E respectively. A-type is orthorhombic, C is cubic (ThsP4), D is monoclinic and E is rhombohedral (corundum); the structure of B is not confirmed; some of the rare earth oxysulfides have been reported to have this structure. (:~ Y2S~(D) has the same crystal structure as HozS3 in which half the metal atoms are coordinated to six and the other half to seven sulfur atoms while two-thirds of the sulfur atoms are coordinated to four metal atoms and one-third of the sulfur atoms to five metal atoms. Thus, the average coordination is 6½ for the metal and 4~ for sulfur. Henderson et al. ~'~ have grown single crystals of Y:S~ and measured the electrical resistivity and the thermoelectric power in the 300--1300K range; the results show Y:S3 to be a semiconductor. Magnetic susceptibility data (:7'~ indicate that the paramagnetism of YzS~ is not due to the localized spin-only moment. A detailed temperature variation study of the properties of YzS3 would be worth while. Solid solutions of Y2S3 and La,.S3 and several other rare earth sesquisulfides with CaS, SrS, BaS, MnS, MgS, etc., have been prepared and such compounds, ML2S4, exist in different crystal structures depending upon the radii of the cationsJ :9~Recently, solid solutions, (L,L'):S~, like E r s S c S 6 , Y S c S 3 , NdYbS3, have also been preparedJ ~°) YS Yttrium monosulfide has been reported ~:1~to have JPSSC Vol. lO, No. 4~B

209

the NaC1 type structure. It is metallic and paramagnetic. A complete study of the properties of this compound will be of much interest especially since it has the same structure as ScS. and Y5S7 Y4S3 and Y5S7 were discovered from X-ray studies, ~:'~ but no physical properties are reported, except that they are semimetals. In the monoclinic Y5S7, both seven-fold and distorted octahedral coordinations of the metal atom occurJ TM Part of the Y metal in Y5S7 can be replaced by divalent cations like Cd +2, Mg +2, Fe +2, etc., to give MY4S7J z2~ Y453

YS_, A tetragonal crystal of the composition YzS3.80 was prepared by Flauhaut et al. °9~ by reacting sulfur and yttrium at 500-600°C with excess sulfur. This is a paramagnetic semiconductor. No physical measurements have been reported on this compound. 2.3. L a n t h a n u m - s u l f u r system In contrast to scandium and yttrium sulfides, sulfides of lanthanum and other rare earths exist in different structural modifications and their properties are highly influenced by the presence of the low-lying 4f orbitals (which are localized). The conduction band is presumably made of d-orbitals and probably 4f levels lie in the band gap region. It is of interest to see how the conduction electrons are affected by these localized electrons. Application of pressure on these compounds may cause the 4f electrons to be delocalized to some extent and thus the properties may be changed dramatically. The known sulfides of lanthanum are LaS, ~3t'32~ L a 2 S 3 c25) and L a S 0 . 7 ~ _ t s 3 f 1~ La2Sa Lanthanum sesquisulfide exists in two different structures, a and y, like many other rare earth sesquisulfidesJ TM Sleight and Previtt (25) reinvestigated these structures along with other rare earth sesquisulfides and found La2S3 to belong to the a-form. Flauhaut et alJ 33~ have reinvestigated the t~-form and have, however, observed that both the /3- and y-forms can be converted to the a - f o r m by prolonged heating at 1000°C and that this conversion is facilitated by the presence of excess sulfur. The a - f o r m is a brick red compound and belongs to the orthorhombic system. Structural studies on the /3-form are yet to be carried out. /3-La2Ss powder has been reported to be superconducting at low temperature under high pressures. ~34)Five superconducting phases with the following transition temperatures have been reported: 5.9-6.6 K, 7.2-7.6 K, 8.3-8.6 K, 10.4--10.7 K, 14.1-14.5 K. The chemical purity of these samples is not clearly stated, a-La2Ss is reported to be semiconducting at room temperature. Detailed studies on both the a - and /3-phases would be desirable. LaS Lanthanum monosulfide has the NaC1 structure and is metallic. Electrical resistivity and thermoelectric power of LaS have been measured between 300 and 1300KJ 3~ It is likely that in LaS,

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the valence band is formed mainly from the sulfur 3p orbitals and metal 6s orbitals. The conduction band is formed from 5d orbitals of lanthanum. It is interesting to see the effect of pressure on rare earth sulfides, since it may delocalize t h e / - o r b i t a l s and merge with the d-band, t36) Low-temperature measurements on LaS would also be of some interest. Thermal conductivity data t37) on LaS between 80 and 480 K show that the lattice component is considerable. Magnetic susceptibility measurements (3s~ show weak paramagnetism which is temperature-independent. Thermodynamic properties of LaS have been discussed by Westrum. t3s~ A nonstoichiometric phase LaS075(La~S3) has been reported, <:" but the properties have not been studied. Sulfides of the other lanthanides have not been reviewed here since they would be outside the scope of this article. Interested readers may refer to the reviews by Flahaut and Laurelle ~'~9) on this subject. IlL Sulfides of group IVB transition metals The binary sulfides of group IVB elements, Ti(3d), Zr(4d) and Hf(5d) form an interesting class of compounds which have been studied rather extensively during the last few years. Interest in these sulfides is mainly due to their widely different electrical, magnetic and optical properties, despite their close resemblance in crystal structure. These sulfides along with other chalcogenides have been reviewed by a few authors. ~ ) Properties of several of the IVB compounds are not yet fully studied and would be most worthwhile to pursue.

cL o~-

1/3c

TiS2

Ti2S3

Ti5SB

Ti4S 5

Ti8S 9

Ti3S ~

TiS

FIG. 1. Sections through the hexagonal (1120) planes of titanium sulfides in the range TiS-TiS2. Open circles, sulfur positions; closed circles, fully occupied metal positions; hatched circles, partly occupied metal positions. (After Wieger and J ellinek.C's~)

linear Ti atoms share common octahedral-site faces along the c-axis; the Fermi energy would fall below the 4s band here as well. F o r TiS,-x (with approx. 45at.%S), Bertram ~42~ observed an ordered NiAs structure which he called TiS~-x I (x = 0.2). For preparations with still lower sulfur content (approx. 40 at.%S), Bertram observed two phases which he called TiS~-xlI and TiSI_~III. According to Jellinek t45~both TiSl-x II and TiSa ~III are NiAs phases 3.1. Titanium-sulfur system with various degrees of sulfur disordering. It is not The sulfides of titanium have been studied by clearly understood how the vacancies are ordered many workers ~-") and Jellinek (4~)has reviewed the in these phases. Many other workers ~°-5~) have various titanium-sulfur compounds reported until reported sulfides of lower sulfur content than in 1963. Titanium forms a series of compounds with TiS, but no definite homogeneity ranges and tempsulfur, c~:> several of which are non-stoichiometric. erature regions of stability of these phases have Some of the reported sulfides are: ~48)TiS, Tfi_~S, been reported. Franzen and Gilles ~54)have reported TiSI-~, Ti3S4,Ti2S3, Ti3Ss, TiS2, TiS3, Ti4Ss, Ti4S3 and a thermodynamic study of the vaporization of TiS and they have estimated the melting point of TiS to Ti~Ss. Possibility of many more phases has been indicated by a recent electron microscopic study. <'9~ be 2200 -+ 20°C. TiS is reported to be a brown metallic compound In addition to the variation in the oxidation states of titanium in these compounds, stacking faults in the with a resistivity of 10-" ohm cm, the actual value depending on stoichiometry: '''55'56~ crystal structure give rise to superlattice structures. Some of these superlattices are shown in Fig. 1. TiS2 TiS2 crystallizes in the CdI2 (C6) structure. This TiS Hahn and Harder ~4°)reported two modifications of can be prepared by reacting the elements in TiS. If TiS is prepared at 700°C, NiAs type is stoichiometric quantities at 900°C. Single crystals obtained. Heating this form at 1000°C results in an can be grown by chemical transport reactionsJ 9'5~ irreversible transformation to the rhombohedral Jeannin and Benard (~'Ss)had indicated that the range phase. Bartram ~42)has suggested that the high temp- of non-stoichiometry is in the range Tfi.04S2-TH05S2. Preparations at temperatures higher than 1000°C erature form is metal deficient Ti,_~S(x~0.1), while the low-temperature form has the ordered always give non-stoichiometric compounds with NiAs structure. TiS has an anomalously large c/a the lattice parameters of the hexagonal unit cell ratio (1.9) whereas most B8 compounds have a ratio increasing with the non-stoichiometry. The crystal structure of TiS2 consists of planar considerably smaller than the ideal value 1.63 for hexagonal close packing of the anions. This large sheets of (TiS2), units with TiS6 octahedra joined at ratio indicates stabilization of the e* band relative the edges and the different sheets held together by to the a* band and the Fermi energy is below the 4s van der Waals forces. It is easy to intercalate metal b a n d : ~ Cation-deficient Ti0.9S has two to one hex- atoms between the sulfur layers where there are agonal to cubic stacking and strings of three col- vacant octahedral holes.

Transition metal sulfides Electrical resistivity of TiS: has been measured by various workers. ~'47'~s'~7) The temperature dependence of the resistivity of TiS: shows it to be metallic. Conroy and Park ~7~ attribute this to the deviation from stoichiometry and regard TiS~ to be a degenerate semiconductor. Titanium atom being in an octahedral symmetry, the d-band would be split into a six-fold t~g and a four-fold e~ band. Conductivity should thus be due to the presence of charge carriers in the tzg band. This was the band model given by Wilson and YoffeJ ~ F r o m optical studies, Wilson and Yoffe suggested that the d band is separated from the valence band by about 1 eV. Recent studies ~59~on the electrical and optical properties of (Ti/Ta)S2 show that TiS: must be metallic due to the overlap of the d-band with the valence band (Figs. 2 and 3). Takeuchi and Katsuta ~°) have also investigated the electrical and magnetic properties of crystals of TiS~.0 to TiSL95 and have suggested that the metallic conduction comes from the slight overlapping of the bottom of the conduction band with the top of the valence

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-

~/~q

o,2 0.4 o.~ o~ ~.o X

$2 ko-2

)

i , l l l

,

i ,t

8O 150 240 Temperature[* K]

FIG. 2. Electron transport properties of (Ti/Ta)S~ as a function of composition. Resistivity of TiS2 increases with temperature whereas 1T-TaS2 is semiconducting. Substituting Ti in 1T-TaS2 quenches the phase transitions. (After Thompson et alp ~ ) ----

TiS 2

- -

Tio9TOoI52

-----

TIO 2100852 ToS2(IT Phase)

.....

/

,\

/

00

0'~

band. These workers have suggested that the Fermi level of TiS2.0 is in the vicinity of a minimum of the density of states curves resulting from the overlap of bands. The Fermi level seems to be deviating to higher energies with excess titanium atoms. These conclusions are also supported by magnetic susceptibility measurements, ~57'6°)which show that TiS2 is Pauli paramagnetic. Optical measurements of TiS2(61'62)show a gap of 0.9eV; free carrier absorption for TiS2 is below 0.5 eV. Infrared reflectivity measurements ~63-6s) of layered dischalcogenides suggest that in TiS:, bonding is metallic while in ZrS2 and HfS2 it is ionic. This reasoning has also been supported by the radius ratio in these chalcogenides. In titanium dichalcogenides, the d-band is overlapping with the valence band while in HfS2 and ZrS2 the d-band is at much higher energies and no overlap with the valence band is possible. A tight-binding calculation on group IV dichalcogenides by Bromley et al. ~66"67~ shows that the valence band is made up of the sulfur p-orbitals predominantly and that the metal d-levels form the conduction band. Another interesting aspect of TiS2 and similar layered materials is the formation of chemical intercalation complexes in which the vacant interlayer spacings are filled with alkali metals (68) or Lewis bases. (69'7°) On such intercalation, the c-axis expands accompanied by changes in physical properties. Lewis bases donate electrons to the conduction band in these compounds.

[in TixTO 1_xS2]

1oo~k

211

~!~'

,'~

~-0'

~'~ ~ ! ; - ~ 0

FIG. 3. Reflectance spectra of various (Ti/Ta)S2 as a function of composition. (After Benda et al/62~)

TisSs(Ti0 6S) Between TiS2 and TiS a number of titanium sulfides have been discovered. In these compounds, the metal vacancies are ordered to form different structures (Fig. 1). TisS8 is based on a close packing of sulfur with partial filling of the octahedral holes. The structure has been described a s (cchh)3. (48'71-73) The vacancies are located at alternate layers as [C(1.0)C(0.2)h(1.0)h(0.2)]3 where the cation occupancy number is given in parenthesis. Since preparation of pure phases is difficult and often contaminated with other phases, properties of this compound as well as of similar phases are not known. TiES3(Ti0.629S) This compound was first studied by Hahn and Harder. (4°)Benard and Jeannin ~Ss~as well as McTaggart and Wadsley"" have reported the structure. According to Wadsley, the structure of Ti2S3 consists of alternately filled and partly occupied sheets of metal atoms. The vacancies are limited to every second metal layer. The sulfur atoms are arranged as hexagonal and cubic close-packed alternately, but each second layer of octahedral holes is only one-third occupied. This can be described as [C(1)h(1/3)]2. The lattice parameters of the hexagonal Ti2S3 depend on the stoichiometry/5s~ Benard and Jeannin ~Ss)observed a superlattice structure for Ti~S3 when the reaction was carried out at 800°C for S/Ti ratio of 1.5. This new phase was identified as TisS~2 with a = 3.43 ~ and c = 34.3 A.(7,) A recent electron microscopic study (49~ has revealed more complex phases near Ti2S3.

212

C . N . R . RAO and K. P. R. PISHARODY

Ti354(Tio.755) The crystal structure of Ti3S4 is similar to Ti2S3 with vacant octahedral holes in every second layer half-filled, tT~ Jellinek et alfl s'7~) assigned a rhombohedral structure to this phase. This structure is built up of cubic and hexagonal close packing of sulfur atoms with partial occupancy of the octahedral holes in the layer as {c(0.95) h(0.7) c(0.7) h(0.95) c(0.5) h(1.0) h(0.5)}~, viz. (chchchh)3 (see Fig. 1). This irregular occupation of holes is interesting indeed. ThSs(Ti0.8oS) Like other sulfides, this structure is also based on close packing of sulfur atoms. This can be described as (chchh)2 with repeating units of ten close-packed sulfur layers. The occupancies of the holes are t4s'7" [(c(0.90) h(0.9) c(0.6) h(1.0) h(0.6)h. TisSg(Ti0.sgS) This phase was reported by Bartramfl 2) Jellinek and coworkers ~76~ have described this structure based on partial occupancy of the close-packed sulfur layers. The stacking can be represented as [c(0.83 h(1.0) h(0.83)]3 (Fig. 1). Ti~S~ In the range Ti~.~05S2 to Ti~.255S2, Jeannin ~5s'77)observed weak additional lines in the X-ray pattern indicating the presence of a new phase which they ascribed to Ti3Ss. Picon t78)had also reported a new phase at this composition. However, according to Jellinek, ~48)the phase at this composition is TisSa and not Ti3S5. TiS3 Monoclinic TiS3 has been characterized by several workers ~4°'77'79~and the structure is the same as that of ZrSe3 ~') (Fig. 4). The cations are in trigonal

FIG. 4. Projection of the structure of TiS3, ZrS3 or ZrSe3 in the a, c plane. Atoms with light contours lie at y = ~and with heavy contours at y = ~-. The cross-section of the prismatic coordination of metal atom is a strongly compressed triangle with the result S-S bonding is formed. prismatic coordination and form additional bonds in the middle plane of the prism connecting infinite anion prisms by shifting the adjacent prisms in the cell by half the unit cell. TiS~ can be regarded as Ti+4S-2($2)-2. TiS3 is a diamagnetic semiconductor.

Ti2S The crystal structure of Ti2S was first reported by Owens et alfl °) This compound crystallizes with an orthorhombic structure similar to Ta2P and the unit cell contains twelve formula units. In Ti2S, metal atoms are surrounded by ten or eleven metal and non-metal atoms with eight of these atoms at the comers of more or less distorted c u b e s : " Ti2S is metallic and brittle. Bonding in this compound has been interpreted in terms of electron-deficient directional bonds formed by sulfurfl °) Titanium is presumably in the quadrivalent state with 3d24s 2 electrons involved in bonding. A metal-rich sulfide, TirS, with a superstructure of the titanium metal has also been reportedfl °~ Ternary sullides o f titanium. Ternary sulfides of titanium like BaTiS3 and SrTiS3 have hexagonal structures of the BaNiO3 typefl 2-~) In these structures, Ti atoms form a chain parallel to the c-axis. These are semiconductors at low temperatures. The ternary sulfide, PbTiS3, is metallicfl 5~ Although a large number of compounds are formed in the titanium-sulfur system, the properties of several of these phases are not known due to experimental difficulties in preparing them in pure form. Partial filling of the octahedral holes in the alternate layers of sulfur packing gives rise to several phases which are described as "occupation waves". ~48)The recently reported superconducting compound LixTiHS2(x =0.4) would presumably belong to one such systemfl ~'sT)It would be interesting to know whether the superconductivity is influenced by the concentration of lithium or by the structure of the particular phase. The disappearance of superconductivity when this phase was cooled slowly, instead of quenching, indicates that the structure of the particular phase plays a predominant role in the superconductivity. Intercalation of the other metal excess TipS phases with alkali metals may throw more light regarding this aspect. A survey of the intercalation compounds has been given by Leblanc-Soreau et al. tuJ Single crystals of TiSxSe2-x and TiSxTe2_, have been prepared by Rimmington and Balchin. <.9~ 3.2. Z i r c o n i u m - s u l f u r system The first systematic study of the Zr-S system was conducted by Strotzer and coworkers c~ who reported a trisulfide ZrS3.2-2.s, a disulfide ZrS2.0-~.s, a sesquisulfide ZrS~.5_~.2 and two subsulfides ZrS0.75 and ZrS0.33. Hahn et al. ~9~)and McTaggart and Wadsley t4" undertook a detailed investigation of this system and found the number of phases in the Zr-S system to be less than in the Ti-S system. This is probably due to the fact that higher oxidation states become more stable for 4d and 5d transition elements compared to 3d elements. Due to the instability of the lower oxidation states, we do not have the compounds in the range ZrS-ZrS2 (akin to the Ti-S system) formed by filling octahedral holes. The following are some of the well-established sulfides of zirconium ZrS3, ZrS2, Zr2S, Zr3S2, Zr,-x S and ZrSm-~.Jellinek ~45~had reviewed the chemistry of the Zr-S system up to 1963. ZrS2 This has a structure similar to TiS: with the metal

Transition metal sulfides atom in the octahedral hole between the sulfur layers. Solid solutions between isostructural SnS~ and ZrS~ like Sn~Zr~_~S: have been reported, but the properties have not been reportedJ n~ ZrS~ is a diamagnetic semiconductor. ~:'6t) Recently a band energy calculation for ZrS: and HfS: has been carried out by Murray et al. ~ employing the tight binding method (Fig. 5).

0.4

0-6

A c~ u.I I

0.8

_E2_

r~-

_

r;

. . . .

r'~

Ka

m F FIG. 5. Band structure of ZrS~ using tight binding method. (After Murray et al. ~ ) Optical properties of ZrS2 show that the basic energy gap is an indirect o n e : " Near the top of the absorption edge, two main peaks are observed. Such peaks which are also seen in other dichalcogenides, probably arise from spin-orbit splitting of the valence band. ~ Unlike in TiS:, there is no overlap of the d-band with the valence band in ZrS2 (as well as HfS2) due to the increased ionic nature of the latter. Widely separated d-band and valence band would give rise to semiconductivity. Infrared reflectivity measurements ~6~do indeed indicate the ionic nature of ZrS~. Photoelectric properties of ZrS2 have been studied. ~9~ Alkali metals can be intercalated between alternate sulfur layers, when ZrS2 becomes metallic and superconducting at low temperatures) 9'~ The structures of these intercalated disulfides and the ordering of the vacancies in these compounds after intercalation are not known. Probably the vacancies are ordered to form a superlattice and may belong to one of the "occupation waves" described by Jellinek: ~ ZrS2 intercalation compounds have been surveyed by Leblanc-Soreau et al. ~ It will be interesting to study whether any particular superlattice favors the superconductivity, tg~) ZrS and Zro.77S Zirconium monosulfide was first reported by McTaggart and Wadsley t4~ and the preparation has been more recently reported by Steeger and Carter) 95> This is a metallic phase and has the NaC1 structure ~97~ (unlike TiS which has a hexagonal

213

structure). Zro.77Shas a superstructure of NaC1 type with the ordering of the Zr vacancies: 97~ A cubic Zr3S4 has also been reported by Stocks et al. <98~ ZrgS2, Zr:S, Zr2~S8 Unlike the Ti-S system where between TiS2 and TiS a number of sulfides with close packed layers are known, ~48)the Z r - S system does not have such compounds. However, a number of metal excess compounds have been discovered and they are all metallic with different structures. Conrad and Franzen (99~ have studied the metal-rich phases, Zr2S, Zr2,S8 and Zr9S2. Zr2S is isostructural with Ti2S with Ta2P-type structure. "°°~ In this structure, the sulfur atoms have capped trigonal prismatic coordination while the metal atoms have capped cubic coordination. Zr2~S8 is isostructural with Nb21S8 and the coordination is similar to that in Zr2S. "°" ZrgS2 is a body-centered tetragonal phase "°~ where the coordination of Zr is very different from that of Zr2S or Zr2~S8. There is a variety of coordination for the Zr atoms and the coordination of sulfur is square antiprismatic. Zr3S2 and Zr3S3 These phases were first reported by Hahn and Ness. (~°3~ Zr3S2 has a WC-type structure, but no properties have been studied. Tetragonal Zr4S3 has been later identified as ZrSiS. ~4~:°4) ZrS3

This is a semiconducting diamagnetic solid with monoclinic structure c4t'55'79~and is prepared at low temperatures. It is isostructural with TiS3 and ZrSe3 (Fig. 4). Jellinek et al. "°5~ have discussed the structure and bonding in ZrS3 and ZrSe3; sulfur in ZrS3 is present as S~2 and S -2. T e r n a r y sulfides. CaZrS3, BaZrS3 and SrZrS3 have been prepared) s2-~ BaZrS3 has the perovskite structure below 800°C. PbZrS3 has been prepared at high pressures and has an orthorhombic structurefl 5'~°6~ ZrSiS (and HfSiS) have been reported. "°<~°7~ Properties of many of these ternary systems have not been studied extensively. 3.3. H a f n i u m - s u l f u r s y s t e m Hafnium sulfides are quite similar to zirconium sulfides, the influence of the ionic nature of bonds being more pronounced in hafnium sulfides. Hf2S, HfS, Hf2S3, HfS2, HfS~ and Hf3S4 are the known members of this system. McTaggart and Wadsley ¢41~ investigated the H f - S system as early as 1958. HfS: Hafnium disulfide is isostructural with ZrS2. It is an insulating and diamagnetic c o m p o u n d : 7'1°8~ A band structure calculation using the tight-binding method has been carried o u t : 6~ HfS2 forms red crystals and its optical properties have been studied by Wilson and Y o f f e : ~ Infrared reflectivity measurements ~63~indicate the ionic nature of HfS2 just as in the ZrS2. HfS McTaggart and Wadsley ~4~ reported HfS with an orthorhombic structure. More recently, Franzen

214

C. N. R. RAO and K. P. R. PISHARODY

and Graham "°9) have prepared HfS and proposed either a WC or a disordered WC structure (which is quite different from a NaCI type ZrS structure). Physical properties of this compound are not known.

ABCD 1T. 2H(o)

2H(b) 3R

Hf~S, This has been reported t~) to have a cubic structure like Zr3S4. Properties are not known. Hf2S3 According to McTaggart and Wadsley, (41)the cell dimensions of the hexagonal Hf2S3 are very close to those of HfS2. Hf2S3 is a semiconducting diamagnetic compound. It is probably formed by partly filling the vacant octahedral holes between the alternate sulfur layers in HfS2.

,.Jf'x

o~ o.-.7

4H(G)

4H(b)

HfS3 HfS3 has the monoclinic structure of ZrSe3. 0L55'79) This is a semiconducting diamagnetic compound. Hf2S Hf2S was first reported by Franzen and Graham. t'°) In Hf2S, the metal atoms are found in a distorted octahedron formed by three metal and three sulfur atoms. This coordination is quite different from that in Ti:S or Zr2S. Smeggil~tt~ suggests that the coordination in Hf2S may due to the different electronic states of the metal involved in bonding. IV. Sulfides of group VB transition metals

The sulfides of V, Nb and Ta exhibit quite dissimilar physical properties. For example, niobium and tantalum dichalcogenides are superconductors at low temperature while vanadium dichalcogenides are normal metals with paramagnetic behavior. Non-stoichiometry and polymorphism are common in all the group VB disulfides and a number of sulfides with varying metal to sulfur ratios have been reported. The dichalcogenides have layered structures with different stacking of S - M - S layers (Fig. 6). Both octahedral and trigonal prismatic coordinations of the metal atoms are found in the different polytypes. Metallic behavior is more pronounced in structures with trigonal prismatic coordination of the metal atoms (e.g. 2H-TaS2) while compounds with octahedral coordination (e.g. 1T-TaS2) are usually non-metallic. Vanadium disulfide seems to exist in only one polytype with the vanadium located at the center of a distorted octahedron. In niobium and tantalum chalcogenides, metal-metal interaction plays a significant role in determining the physical properties.

4.1. Vanadium-sulfur system The various compounds of vanadium with sulfur were first investigated by Biltz and Kocher °~2) and by Klemm and Hoschek. "~3) Many compounds in the range V3S to VS5 have been reported, "5) but some of these have not been fully characterized. S - S bonding is usually present in sulfur-rich van-

Z,H(c)

~ e~.

6R

T c

FIG. 6. Sections through (1120) planes of the hexagonal unit cells for the different kinds of polymorphic transition metal disulfides, MS2, of groups IV, V and VI. Metal atoms are in black circles and S atoms are in open circles. (After Huisman and JellinekY8") adium sulfides and the V - S bond shows some ionic character. VS Stoichiometric VS is unstable at atmospheric pressure and seems to disproportionate to cationdeficient V7S8 and cation-rich V9S8. This structural instability indicates the presence of narrow, cation sublattice d-bandsY J In the neighborhood of the composition corresponding to V S , on the metaldeficient side, two phases have been reported"12'lt3): a phase Vo.98S-V0.~S, of NiAs type structure, and another Vo.87SV0.6~S, of lower symmetry. Pederson and Gronvold ~;14)have reported that in the range of V0.9~S-V0.sgS, three different hexagonal phases with superstructure of NiAs type exist while in the range Vo.s:S-V0.74S three different monoclinic phases with NiAs type superstructure exist. The non-stoichiometric VS;-x phase ( 0 . 8 5 a S/V ~< 1.05) has the MnP structure derived through an orthorhombic distortion of the NiAs structure. Franzen and Westman " ' ) did not observe a twophase region between NiAs type VS and MnP type VSI-x. The transition between NiAs and MnP structures is of second order, m6~ The non-stoichiometry is primarily due to randomly distributed sulfur vacancies. The stability of the MnP structure in V S , has been interpreted to be associated with a particular range of electron concentration tn6) (0.3electron/~3). Recent Knight shift measurements °~7~ on VS~ with NiAs and MnP structures show a change in the shift at V/S = 0.94 indicating a change associated with the conduction band. Band structure calculations show that the N i A s - M n P transition of VS is due to the charge density wave in the conduction band. "Is)

215

Transition metal sulfides The non-stoichiometric V,-~S system has been studied recently by Delamaire et al. ~119)For x = 0.16 to 0.365, VI-~S exists as a single homogeneous non-stoichiometric phase with gradual change in lattice parameters. No discontinuity is observed in the isothermal studies of phases. Electrical conductivity and Seebeck coefficient have been measured as functions of non-stoichiometry.{'9~ Gr0nvold et al. ~2°~ have investigated the V-S system with various S/V ratios. According to them, VS~.x phases have the NiAs-orthorhombic structure with (a = 5.825 ,~, b = 3.303 ,~ and c = 5.860 for VS) the a - and b-axes doubled. The compounds VS~, where x is 1.025, 1.05, 1.075, 1.10 and 1.125, all have orthorhombic or hexagonal structures with a = 2a0X/3 and b = 2a0 and c = 6c, 5c, 4c and 3c respectively for the c-axis. Analogous structures are known in pyrrhotites as well. V7S8 and VTSes have superlattice structures closely related to NiAs type with ordering of the vacanciesJ n~ Besides, the hexagonal compound with a = 2a' and c = 4c', a new monoclinic compound with a = 2a'X/3 and, b = 2a' and c = 4c' has been reported (a' and c' refer to the NiAs cell). Magnetic susceptibilities of various V ~ S phases have been recently studied ~=~ (see Fig. 7) and the results show that vanadium atoms in these phases have both localized and itinerant d-electrons.

f

*

/o

- ,

M55,

.oOp

/.',o'.'/f / ...¢-.- f X.'P'_ • -." / ~-

-

o e

Qcoo

j

A

i

'P

M3S4

l.

FIG. 8. Ordering of vacancies in cation deficient NiAs type transition metal sulfides like M3S,, M~Ss, M~S~ to produce superstructures• Filled circles, metal atoms; squares, vacancies. (After Chevreton et alJ n3"n4~)

z,O00r106%Qt (cm3/gal V)

t\

spiral along the c-axis with the cation lying between the pair of rectangles, VS4 is a diamagnetic semiconductor: ns) The occurrence of alternate long and short V-V bonds along the c-direction in the crystal is responsible for the diamagnetism. These V-V bond lengths vary between 2.83 A and 3.22 A. Pyrolysis of VS4 at 400°C yields vanadium-rich sulfides.'~9)

.

20

L,0

60

80

tOO

120

"K

FIG. 7. Magnetic susceptibility of vanadium sulfides as a function of temperature and composition. (After De Vries and Haas. "22~)

VsS8

V5S8 has a monoclinic structure derived from NiAs with a superstructure because of the ordering of vacancies ~113'n4~ (Fig. 8). VsSe8 and VsTes also have the same structures. Magnetic susceptibility studies "=~ of VsS8 show contributions from both localized and itinerant electrons; the Neel temperature of V5S8 is 30 K (Fig. 9). VS4 (Patronite) VS4 can be prepared from the elements at 400°C or by heating VS15 and S at 400°C for several weeks. (nSt It has a monoclinic cell(126~ with 8VS4 groups per unit cell. Investigations of the structure of VS4 by Allmann et alJ nT~ have shown that VS4 consists of two types of S-S d.umbbells with $1-$2 and $3-$4 at distances of 2.03 or 2.04A respectively. Every dumbbell model forms a rectangle at a distance of 3.14 A. Between each pair of rectangles, a vanadium atom is situated. The coordination of vanadium is intermediate between a square prism and a square antiprism. The rectangular S-S units

V3S4

V3S4 can be prepared from the elements at 800-1000°C. It has a monoclinic unit cell(~3°:3"which can be described as a defect NiAs type structure with vanadium vacancies confined to every second metal layer and ordered within the layers. Its structure is similar to Fe3Se4 (Fig. 8). V~S~ is metallic(13°)with a Seebeck coefficient of + 13.1/xV/deg. The metallic conduction is due to electrons in the d-band. V3S4 exhibits weak antiferromagnetic coupling,~n2'm~ probably between the layers in which vanadium exists as V +2 and V .3 alternatively as in Cr3S~.

1500

t/",cal (gat V / c m 3 ) . experiment . . . . theory ~

ys~'~--

50(3

~.', , . L . . . .

b ....

i .........

,

0 100 2o0 300 400 5oo*K FIG. 9. Reciprocal magnetic susceptibility of V5S8as a functionof temperature. (After De Vries and Haas. "=>)

216

C. N. R. RAO and K. P. R. PISHARODY

V2S3 V2S~ has a monoclinic structure derived from NiAs. According to Loginov, <~n) V2S3 is an antiferromagnet with Curie-Weiss dependence between 100 ° and 500°C. The magnetic coupling between the localized d-electrons on the V +3ions gives rise to the ordering. Other properties of this sulfide have not been studied. V3S Two modifications of V3S have been reported: ~4) one, a high-temperature form, a-V3S, and the other, a low-temperature form, /3-V3S with a transition between 825 ° and 950°C. The structure of a-V3S is related to Ni3P structure while fl-V3S may be an intermediate between the a - f o r m and the /3tungsten structure. The a - f o r m has a bodycentered tetragonal unit cell, while /3-V3S has a tetragonal structure. The unit cell contains 8V3S units. These phases are metallic, but not superconducting. V5S4

V5S4, VsZe4, NbsSe4, NbsTe4, NbsSb4 and Ta~Sb4 are all reported to have the Ti~Te4 structure. <~33~V~Sa has a tetragonal unit cell with I4[m space group with 2V in (a) and 8V and 8S in (h) positions in the unit cell.
system was first investigated by Jellinek et al. <~37) Kadijk and Jellinek but not confirmed. Nb3S4 Niobium reacts with S, Se and Te to give the respective chalcogenides ~39~ Nb3S4, Nb3Se4 and Nb~Te4. In Nb3S4, each Nb atom is surrounded by six sulfur atoms at the corners of a distorted octahedron. <138'~4°>These NbS6 octahedra are joined to other octahedra to form three-dimensional networks. Each octahedron is linked to two other octahedra by common faces and to four other octahedra by common edges. The Nb atoms are not at the centers of the octahedra, but are shifted by 0.306 ,~ in the direction of a face, sharing edges with other octahedra. Thus, Nb-Nb--Nb chains are formed running in the c-direction. The short N b - N b distances in the chain are comparable to those in the Nb metal. The short metal-metal bonds bestow many interesting properties. The metallic behavior and superconductivity (below 2K) are perhaps influenced by the d-orbital overlap of these close niobium neighbors. The presence of large hexagonal holes which cover about one-third of the structure are interesting. Intercalation of alkali metals or alkaline earth metals in these holes, which run as channels in the c-direction, could be attempted. NbS3 Niobium trisulfide is isostructural with ZrSe3 with a monoclinic unit cell (Fig. 4) and is unstable above 600°C. The b-axis in NbS3 is twice that expected for ZrSe3 type structure. Since ZrSe3 is regarded as Zr+4(Se~)-2Se-2, it is possible that Nb exists in quadrivalent state with S - S bonding. Doubling of the b-axis has also been suspected to be due to Nb--Nb pairs. "38> Infrared and Raman studies <~4"indicate that the S-S vibration frequency is above 550 cm -~. NbS3 is a diamagnetic semiconductor.(l~8:42: 43) Nb~-xS (0 ~
217

Transition metal sulfides

gated by Wilson and Yoffe~5)who have proposed an approximate band model where the non-bonding d-orbitals split into a doubly degenerate (dz~) band at lower energy and into a four-fold degenerate (d:-y:, dxy) band (with dip mixing) at a higher energy (Fig. 10a). In NbS:, the d : band is half full resulting in metallic properties. Another band NbS2 model using the tight binding method has been Polytypism is common in group V and VI dichal- reported by Jellinek and coworkers~44~ (Fig. 10 b cogenides and the different types of polytypes and c). Even though ordering of the d bands is generally encountered are: ~'~ 1T-MX2, 2H-MX2, qualitatively the same as in the Wilson-Yoffe 3R-MX2, 4H-MX~ and 6R-MX2 where M is the model, the separation between the bands are differtransition metal and X is the chalcogen (Fig. 6). For ent. In the Wilson-Yoffe model, the d-states lie in NbSz, only the hexagonal (2H) and the rhom- the region between the sp valence band and the bohedral (3R) types have been reported so far ~t38~ empty antibonding conduction band. According to and the existence of some other polytypes cannot Jellinek and coworkers, "44j the d : band overlaps be ruled out. The thermodynamics of these phase with the valence band and the higher empty d-states transitions have not been completely studied. It is overlap with the conduction band. Recent photointeresting that such polytypism has not been ob- emission studies "45~have shown that in 2H-NbSe2, served for group IV dichalcogenides which also which is isotypic with 2H-NbS2, the maximum crystallize in layered structures. density of states, N(~), in the d : band is about 0.4 eV 2H-NbS2 is stable above 850°C and 3R-NbS2 is below the Ev and the slope of the N(E) is negative at made below 800°C. Using the common description EF. This is in agreement with the results of the for close-packed structures, 2H-NbS2 can be de- intercalation experiments with alkali metal which scribed as AcA, BcB, etc., where the upper-case indicate that on intercalation, the density of states letters represent the sulfur position in the closed- decreases and the superconducting transition temppacked structure and the lower-case letters repres- erature decreases. Van Maaren and coworkers <'46''47~have measured ent the metal positions. Similarly, 3R-NbS2 can be written as AcA, CbC, BaB. The repeat unit consists the resistivity, Hall coefficients, thermoelectric of two and three layers for 2H-NbS2 and 3R-NbS2, power and heat capacity of Nb and Ta dichalrespectively. Metal atoms are filled in alternate cogenides. These results have been interpreted in vacant sites available between the sulfur atom terms of a band overlapping with the sp valence layers. Tendency to fill the vacant octahedral and band. The negative thermoelectric power is due to tetrahedral holes available in these vacant inter- the large number of low mobility electrons in the layer spacings is quite common amongst these narrow d-band while the positive Hall coefficient dichalcogenides and thus non-stoichiometric arises out of the higher mobility of the holes in the Nb,.~S2 is rendered possible. In both 2H- and valence band. 3R-NbS2, Nb atoms are in a trigonal prismatic Quite recently, the band structures of 2H-NbSe2 coordination of the sulfur atoms, while sulfur is and 2H-MoS2 have been calculated by Mattheiss "~8~ employing the APW method. (~49~These calculations coordinated to three Nb atoms. Optical properties of NbS2 have been investi- show a 1 eV hybridization gap within the d: and d~,

The structure of low-temperature Nb~-~S is a superstructure of NiAs type with the a-axis twice that of the NiAs cell and c-axis equal to that of the NiAs sub-cell. In this phase, triangular clusters of Nb are present instead of Nb-Nb chains; this phase is also metallic and Pauli paramagnetic.

J

e'* e*

e~ -F A B

e~

--~-~" N(~ ) (a) MoS 2

-~-~" N(e) (b) WTe2(C3v)

~N(~) (c) 2H-ToS2(D3h)

FIG. 10. Schematic one electron energy level diagrams for group V and group VI dichalcogenides. (a) Wilson and Yoffe model for trigonal prismatic MoS2 with d : band full; (b) Jellinek model for trigonally distorted octahedral WTe2; and (c) for 2H-TaS2 (trigonal prismatic). A and B in the absorption spectra of group V and group VI dichalcogenides arise due to spin orbit coupling of the valence band and the conduction band, according to Wilson and Yoffe model but correspond to a '~* and e'* transitions in Jellinek model, (From refs. 5 and 144.)

218

C . N . R . RAO and K. P. R. PISHARODY

d ~-~ manifolds of the Nb or Mo 4d bands. This gap is larger than and not related to the ligand field splitting between d,: and d~Ly~, d.,y. In 2H-NbS2 or NbSe2, the Fermi level falls near a slightly broadened two-dimensional logarithmic singularity in the density of states. The A P W calculations also show that the density of states, N(Ev), is 3.0 states per eV, agreeing well with the value of 2.8 derived from superconductivity "5°~ and heat capacity data. (14~)According to Mattheiss (~4s)the existence of such a fine structure in the density of states near the Fermi level can lead to distortions at low temperatures resulting in the splitting of the density of states peak. Such structural distortions have indeed been observed for 2H-NbSe2. ('~L~2) NbS2 undergoes a superconducting transition temperature at 6 K depending upon the polytype. ~47~ 3R-NbS2 is superconducting below 5.5 K while the 2H-NbSz has a T, between 5,8 K and 6.2 K. The superconducting transition temperature decreases in metal-rich Nbl+~ $2. The lowering of Tc in Nb~÷~$2 may be due to the change in the density of states at the Fermi level. Substitution of Nb by Ta decreases T~; Mo has even a stronger effect. (~) Tellurium substitution also acts in the same direction. (~) Magnetic properties of NbS~ have been studied by various workers. (~s''~) Both 3R- and 2H-NbS2 show temperature-independent paramagnetism. 2H-NbSe2 and 2H-TaSe2 have been reported to be antiferromagnetic, (~-t~v but recent investigations "~') using (9~) Nb resonance have shown the absence of any antiferromagnetism. Electrical measurements on single crystals of 21-1- and 3R-NbS2 have not been reported, but such measurements on 2H-NbSe2 have been carried out. (1~7)NbSe2 shows an anomalous sign change of the Hall coefficient at low temperature (40 K). Such changes have been reported for TaSe2 and TaSz at different temperatures (Fig. I1) and have been considered to be due to structural distortions "~s) where changes in lattice parameters are accompanied by changes in the position of the d-band. Magnetic measurements (~2't~v show a peak in the susceptibility curve at this temperature. Although similar measurements on NbS2 have not been made, the results are expected to be quite similar.

Preparation of single crystals by the vapor transport method often results in the formation of Nb,+x $2; these cation excess sulfides are not superconducting and their properties differ from those of pure stoichiometric NbS2. Intercalation of alkali metals and transition metals (~59) in the vacant holes available in NbSz or NbSe2 were discussed earlier under group IV dichalcogenides. Such intercalation is also possible with group V and VI dichalcogenides and it affects their superconducting properties. In NbS2 and TaS2, up to two-thirds of the octahedral holes can be occupied by Mn, Fe, Co, Ni. "6°'~6~)At the composition MNb3S6 (or MTa3S6) ordering of the octahedral cations give super cells with the a-axis V 3 times that of the original unit cell. Ordered MnB3S6 also exists with Ti, V and Rh. (4'~6~) CrNb3S6, CrNb~Se6 and MnNb3S6 become ferrimagnetic at low temperature while the magnetic moments of Fe, Co, Ni order antiferromagnetically."~°'~6' Copper and silver have been reported to be inserted into the tetrahedral holes rather than octahedral holes between the trigonal prismatic layers. (~62'I63)In these compounds, only one-third of the available tetrahedral holes are filled up. Cu~NbS2, Ag~NbS2, Cu, TaS2 and Cu~NbSe2 have 2H-MoS2 structure, ('6') but such compounds have been reported to be non-superconducting, t') The compounds TNbzS4, where T = Mn, Fe or Cu, have been reported to possess the orthorhombic structure. ~6s) Organic molecules like pyridine, aniline, amides, etc. (which are good Lewis bases) as well as NH3 can be intercalated in NbS2 and TaS2 "66) (see Fig. 12). Intercalation of alkali metals (~9) from liquid NH3 is also very easy, but such intercalations decrease T~ of NbS2 and increase T~ of TaS2. Nb21S8 Franzen et al. ~'~69'm) have investigated the lower sulfides of some of the transition metals with a desire to understand the bonding of the sulfur atoms in such compounds. They have reported (~7°)a new niobium sulfide, Nb2,Ss, with a tetragonal structure in which each sulfur is surrounded by a slightly distorted trigonal prism of Nb atoms with an additional Nb atom on a line perpendicular to the

0"9

ToS2a - a x f s v 0.8 * 0 0.7 0 o

..... ___ -- - --

TaSs(py)l/2 a-axis / ~ TaS2C-axis ~ ' " TaSs(py)I/2 C - a x i s / ~ J

/

~

~

/ ' / / ~

0-6 /

E 0"4

_

2~

z

/ f

02

"~ 0-1

g~

0

20

410

dO iO lO0 120 '40 [;0 180 2;0 220 240 T e m p e r a t u r e [o K ]

FI~. 11. Electrical resistivity of 2H-TaS2 and 2I-I-TaS=(Py),/=(along a and ¢ direction of the crystal) plotted against temperature. (After Thompson et al. (1~"))

219

Transition metal sulfides Layer

earamide

~7~

FIG. 12. Crystal lattice of 2H-TaS: after intercalation with stearamide. (After Gamble et al."~) face of the prism. The bonding in this compound seems to involve delocalized electron-deficient metallic type bonds. Nb2,Ss is metallic. Investigation of physical properties at low temperatures have not been reported, but it is likely that this will be superconducting at low temperatures. Nb~,S~ This high-temperature phase has a structure with capped trigonal prismatic coordination of the Satoms and a high coordination number for Nb; ~'~ this is a metallic phase. Nb:S is another sub-sulfide of Nb reported to have been prepared by heating the elements at high temperature. "7:~ This phase was later identified to be Nb~S8. Thus, the existence of NbzS has not been confirmed. Nb~S3 The preparation and physical properties of the compound with this composition have been investigated by Oganesyan and others. °73-~76) This is a metallic phase with about 10:~ charge carriers per cm ~. The Hall coefficient, resistivity, Seebeck coefficient, etc., have been determined on the powdered sample. According to Jellinek, ~13s)Nb:S3 is a metal excess 2H-Nb~+~S2 with a hexagonal unit cell. This is quite different from Nb:Se3 which has the Mo~S3 structure. (t77) 4.3. Tantalum-sulfur system Early investigations on tantalum sulfides were carried out by Biltz and K6cher ~Ts~ and also by H/igg and Sch6nbergJ ~79)Biltz and K6cher reported four tantalum sulfides: TaSo.3_~.0,TaSL0-Lg, TaS: and

TaS3. Later investigations on the T a - S system were carried out by JellinekJ ~s°~Besides TaS3, 1T-TaS2, 2H-TaSk, 3R-TaS2 and 6R-TaS2 (Fig. 6), several metal-rich phases like 2H-Ta,+2S2, 3R-Ta~+~ $2 and 6R-Ta~+xS2 have been reported. Jellinek "8°~ has shown TaS0.3_,.0to be a mixture of 2H-Ta~+x $2 and Ta and TaSLo-L9 to be a mixture of 3R-Ta~+~S2 and 2H-Ta,+x $2. Sulfides of Ta are quite similar in their structure and properties to the sulfides of Nb which were discussed earlier. These sulfides ~4'~)have been reviewed earlier, but recent studies on these compounds have produced exciting results and we shall examine these results here. TaS2 Jellinek ~tS°~ has described four polytypes of TaS2; 1T-TaS2, 2H-TaS2, 3R-TaS, and 6R-TaSk_. 4H-TaS2 is also known "8L~82)and it behaves as if it is a mixture of 1T-TaS: and 2H-TaS: (Fig. 6). 1T-TaS: has the same structure as that of TiS2 where each Ta atom is surrounded by six sulfur atoms in an octahedral c o o r d i n a t i o n 9 3 ~ The atomic arrangement in lT-TaS2 can be described as IAbCIAbCI where the upper-case letters denote the sutfur sites in a hexagonal close packing while the lower-case letters represent the Ta sites. Recent X-ray studies "sS~ show evidence of superstructure in 1T-TaS2 with a = a ' V ' ~ and c = 3c'; similar superstructure is also found in IT-TaSe.. ~'~6~ 1T-TaS2 is prepared by reacting the elements at 1000°C followed by quenching to room temperature. 1T-TaS2 ~'s3'~''sT)is a diamagnetic semiconductor (Fig. 13). Temperature variation of the electrical

"~ t~ ,o-2'_'~.L_ I

IZ

IO-~

IO-40

I

80

I

160

I

I

240

Temperature

520

400

(°K)

FIG. 13. Electrical resistivity of IT-TaS2 as a function of temperature. (After Thompson et a l 9 ~) resistivity of 1T-TaSz shows two phase transitions at 190 and 348 K. ~s3) The high-temperature transition is accompanied by a change from semiconducting to metallic behavior while at the lowtemperature transition resistivity increases by an order of magnitude. Normally one would expect 1T-TaS2 to be metallic due to the presence of the excess d-electrons in the conduction band. It is possible that there is strong metal-metal interaction which causes the t2g band to split into a filled and empty band. Recently Jellinek and coworkers ~8~

220

C . N . R . RAO and K. P. R. PISHARODY

have put forward a spin orbit coupling mechanism which splits the t2, band into a four-fold band at lower energy and a two-fold band at higher energy. This spin orbit coupling stabilizes the ground state as d l + d ~ compared to the d : + d ° state. Thus, conduction which involves the change from d ~+ d ~ to d 2 + d ° (hopping) requires higher activation energy than the band width and makes it nonmetallic. Substitution of Ti for Ta in 1T-TaS2 is readily possible; physical properties like heat capacity, ~62) resistivity, ~ ) Seebeck coefficient, etc., have been measured in such systems (Figs. 2 and 3) and these substitutions quench the phase transitions in 1T-TaS> "83~ The phase transitions are also very sensitive to pressure. "9°~ IT-TaS2 is not superconducting down to 0.3 K. 2H-TaS2 is a black metallic phase which can be obtained by annealing 1T-TaS: at 500°C for several days. It is isostructural with 2H-NbS2. Electrical and magnetic properties of 2H-TaS2 (Figs. 11 and 14) show evidence of a phase transition at 70 K due to a small crystallographic distortion. "~s~ At 70 K, the Hall coefficient changes sign from a positive to a negative value accompanied by a change in the slope in the resistivity c u r v e . 091) Such structural distortions have also been observed for 2H-TaSe2 and 2H-NbSe2. ~'~ 2H-TaS: is superconducting below 0.8 K. A band model proposed by Jellinek "4~ explains many of the physical properties of 2H-TaS: and 2H-NbS~. According to this model (Fig. 10), the d-band is split in a trigonal prismatic field into a non-degenerate a, and two doubly degenerate e'and e"-bands. The metallic property of TaS~ and NbSz is due to the half-filled bands. 2H-TaS: and 2H-NbS: can be intercalated with various organic Lewis bases like pyridine, stearamide, etc. Intercalation increases the separation between the layers (Fig. 12) and electrical and magnetic properties are also altered considerably (Figs. 11 and 14). On intercalation with organic molecules and inorganic ions, the superconducting transition temperature is raised to about 4K. "~ Magnetic susceptibility measurements "9~ on intercalated 2H-TaS: indicate that the superconducting

fluctuations start around 35 K. Electrical resistivity measurements on such intercalated TaS2 do not show any phase transitions. "~s~ These results have been explained on the basis of a band model in which the valence band and the d-band overlap. This overlap has been correlated to the lattice parameters and thus a change in the lattice parameters at the transition temperature affects the overlap which, in turn, determine the density and sign of the charge carriers. As in the case of NbS2, transition metals can be intercalated in the vacant octahedral and tetrahedral holes to give the compounds MxTaS:. ~1~°~ At low temperatures, these compounds show some magnetic ordering. Recently, an NMR study of NH3 intercalated TaS2 has been reported "93~ and the study provides evidence for the two dimensional motion of NH3 molecules. The ordering of the atoms in 3R-TaS2 is described as [AbA[BcBICaC[ while in 6R-TaS2 it is IAbAIBcAIBcBICaBICaCIAbCI. In 6R-TaS2, Ta atoms have both octahedral and trigonal prismatic coordinations. These polytypes are stable between 500° and 800°C; phase transformations between them have not been studied in detail. It has been observed that except 1T-TaS2, all the polytypes of TaS2 form non-stoichiometric Tal+x $2. In the case of 2H-TaS2, the superconducting transition temperature seems to increase to about 3 K in Ta~.03S:.t194~The system TaS2-x Sex has been studied in detail recently. "9~) Optical spectra and energy loss spectra of tantalum dichalcogenides have been reported in the literature. ~Sa~ TaS3 The existence of the trisulfide of tantalum was confirmed by JellinekY 8°) Later, Bjerkelund and Kjekshus " ~ established that TaS3 belongs to the orthorhombic system, unlike TaSe3 which belongs to the monoclinic system. "9~'~ Single crystals of TaSj are prepared by chemical transport. (2°°) TaS3 is diamagnetic, but the susceptibility data show an anomaly near room temperature below which the susceptibility falls more rapidly as the temperature is decreased. "97~ Such behavior is not seen in TaSe3. TaS3 is not metallic (unlike TaSe3)

0.8 E

0"7

~ .a0'6

2H - "los 2

,+~0-5 0"4 To S2(Pyridine) I/2 u 0"2 :V-

g O.I 0

210 410'' 60

130 Ic)o I~I0 140 160 180 2()0 2:~0 2,40 Ternperoture E°K ']

FIG. 14. Magnetic susceptibility of 2H-TaS2 and 2H-TaS2(Py)I~2 vs. temperature. (After T h o m p s o n et al. .58~)

221

Transition metal sulfides and the resistivity at low temperatures does not follow a simple exponential relationship. "9~':°~ Whether this is due to the presence of extrinsic carriers or due to some crystal structure deformations is not clear. Ta~S and Ta~S Ta~S is prepared by the reaction of the elements at high temperature followed by annealing. It has an orthorhombic structure/2°:~ The structure may be viewed as chains of body-centered pentagonal antiprisms of Ta atoms sharing faces in one direction and interconnected via sulfur atoms in the other directions. Ta~S is also prepared by the high-temperature reaction (1600°C) of the elements/~°3~ This has a monoclinic cell with eight units per unit cell. The Ta~S structure is closely related to Ta:S. Properties of these metal excess sulfides have not been studied.

4.4. Ternary sulfides of V, Nb and Ta

5.1. Chromium-sulfur system The sulfides of chromium were first investigated by Haraldsen <2~5'2~6~ who observed broad homogeneity ranges in their compositions and also interesting magnetic properties. A reinvestigation of this system by Jellinek (2~7~indicated that in the range CrS-CrS~5, six phases exist with narrow homogeneity ranges: CrS, Cr758, Cr5S6, Cr354, Cr:S3 (trigonal) and Cr2S3 (rhombohedral). A new phase, Cr:Ss, has been reported, <2ts~ but its structure and properties are not known. The structures of the chromium sulfides are related to the hexagonal NiAs structure, with metal vacancies in every alternate layer. The vacancies are ordered below a certain temperature so as to exhibit cooperative magnetic properties at low temperatures. Electrical properties show a rather gradual change from semiconducting CrS and CrzS3 to metallic Cr556 and Cr758. (219'z20) All the sulfides of chromium are paramagnetic with a Curie-Weiss law dependence at high temperatures. At low temperatures, most of them are antiferromagnetic except Cr5S6 which is ferrimagnetic. The d-electrons in most of the chromium sulfides appear to be localized. Thus, in CrS there is a cooperative Jahn-Teller distortion of the octahedral sites which are occupied by highspin Cr 2+ ions. This demonstrates that the e~ orbitals of these ions are localized; the distortion is completely suppressed by deviations in stoichiometry.~7~ Single crystals of some of the chromium sulfides have been grown by the chemical transport method Cz2~-~23~or by the flux methodJ 22°~

Besides the intercalation compounds discussed earlier, several other ternary sulfides of V, Nb and Ta are known. BaVS3, BaNbS3 and BaTaS3 have been reported/2~2°6~ In these compounds, the M +~ ions have octahedral coordination and form chains along the c-axis direction. BaVS3 is a semiconductor below 130K and becomes metallic above this temperatureJ 2°~ BaNbS3 and BaTaS3 are both semiconductorsfi °'~ The Seebeck coefficient of these compounds is about - 60/x V/deg. It is rather surprising that despite the relatively short M - M distances, these materials are semiconductors. Re- Cr~-x S and CrS cently, ~2°~it has been shown that these compounds Crj ~S (0 ~ x ~<0.12) has been studied by several have formulae like BaTaosS3 with pentavalent Ta. authors. Haraldsen ~2~5~noticed a NiAs superstrucCuTaS3, PbNbS3, PbTaS3, PbVS3 and SrTaS~ ture in these compounds. Phase relations between have also been reported/:°7'~°s~ PbNbS3 and PbTaS~ several of these compounds were determined by are both superconductors below 3K/~°9~ Com- Popma and Van Bruggen ~2:°~(Fig. 15). Both CrS and pounds of V, Nb and Ta having the formula K3MS,, TI3MS4 and Cu~MS, in which MS43 anions exist are 125C also known/~°-2~ In these compounds, the M atom m.p. of a-Cr is surrounded by 4S atoms tetrahedrally and by six }T°K Cu atoms octahedrally in a cubic cell. Mixed anion NiAs / IOOC ~ '~c~{om2 / ternary compounds like NbS:C12 and N b P S are known) 2~3':14)N b P S is superconducting below 12 K. Several carbides sulfides of Nb and Ta like 75C Nb~SC~_~ and TazS~C which have interesting struc(6C tural relationships with the disulfides are also (550) known and these have been intercalated with orrH= 450 50C i ganic molecules/n~ NIAs type

\

F Jperstr. iAsfyp~

superstr

J(2o, 3c)

mor~ocl

V, Sulfides of group VIB transition metals

The sulfides of Cr, Mo and W have some similarities with those of the V, Nb and Ta. Chromium sulfides are more ionic compared to the sulfides of Mo and W. Many non-stoichiometric sulfides are known to be stable, but chromium disulfides and trisulfides are not stable. In the case of molybdenum and tungsten, there are not many non-stoichiometric sulfides, but the di- and trisulfides are stable compounds. All these sulfides tend to crystallize in the NiAs-type structure.

,~ype /re,o)

•TH= 125 CrsS

CrzS 8

CrS

Q-Cr

FIG. 15. Phase diagram of Cr-S system. (After Popma and Van BruggenJ 22°) Cr,-x S have different structures at different temperatures/22°) For example, slowly cooled Cr0~S is monoclinic, contaminated with 12.5% of CrvS8 while a quenched sample contained CrS with 6.5% Cr7Ss. Similarly, slowly cooled Cr0~sS consists

222

C . N . R . RAo and K. P. R. PISHARODY

mainly of Cr7S8 (contaminated with small amounts of CrS); quenched Cr0.ssS exhibits the NiAs-type structure. When heated, this compound separated into Cr7Ss and a small amount of monoclinic CrS at 350K. Above 590K, ordering of the vacancies disappears and NiAs structure is obtained. Monoclinic CrS (Cr0.~S) can be prepared by the flux method ~°> by heating with CrI2 at llO0°C. Exact stoichiometric CrS has not been prepared and all preparations of CrS are likely to be contaminated with Cr7S8. The unit cell dimensions of CrS depend on the cooling rate of the sample during the preparation. The Cr +2is surrounded by four sulfide ions at 2.43 ]k and two more sulfide ions at 2.88 A,. Magnetic properties of Cr0,9~S have been studied as a function of temperature. <22°~Around 450 K, this shows an antiferromagnetic-paramagnetic transition. A neutron diffraction study ~z:"~of monoclinic CrS also confirms the presence of such a magnetic transition at 450 K. Electrical properties of CrS~ with 1.00 ~< x ~< 1.20 have been studied by Kamigaichi et alJ 22~) The compounds with x ~< 1.12 are p - t y p e semiconductors and those with x ~< 1.13 are metallic conductors. Electrical conductivity of the semiconducting compounds increase anomalously between 300 ° and 500°C due to the structural transition from the monoclinic to the NiAs-type structure. Phase relations of non-stoichiometric CrS~ in the range 1.20 ~< x ~< 1.480 as functions of the sulfur pressure and temperature have been reportedJ ~26) The equilibrium sulfur pressure over CrS~ increases monotonically with increasing sulfur content in the ranges 1.20 ~

agonally and the metal atoms in the octahedral holes, with vacancies in alternate layers of metal atoms. Ordering of these vacancies gives rise to NiAs-type supercells. In trigonal Cr2+~$3, the a-axis is V'3 times the a ' - a x i s of the NiAs subcell and the c-axis is doubled. In rhombohedral Cr2S3, the caxis is tripled (Fig. 16). The atomic positions in both the forms of Cr2S3 have been determined by Jellinek. <217) Magnetic susceptibility of rhombohedral Cr2S3 has been studied by several workers, te~9,221':22azT-23°) Cr2S3(r) shows ferrimagnetic behavior below 120 K with a maximum at about 85 K. The results of the susceptibility measurements by van Bruggen ~23"and Bertaut et alJ 229~differ considerably in the value of the magnetization at 0 K. According to the former, Cr2S3 has a spontaneous magnetization of 0.91 × 10-5 p,B per Cr atom whereas the latter workers claim complete disappearance of the magnetization of Cr2S3 at 0 K. It is possible that the actual value depends on the stoichiometry. Neutron diffraction experiments ~229) on Cr2S3 indicate collinear spin structure with magnetic sublattices. Experiments of P o p m a et al. <23°a32) indicate that the spins lie in a plane perpendicular to the trigonal axis. Electrical resistivity of n-type Cr2S3 increases as the temperature is decreased down to 125 K (To) at which temperature it has the maximum value and then decreases showing a minimum at 85 K. ~2~9a~8> F r o m the measurement of Hall coefficients (which were independent of the magnetic field) and resistivities, the number and mobility of charge carriers have been calculated, t219~ The mobility shows a minimum at Tc and a maximum at 85K. The number of charge carriers is maximum at To. Similarly, the negative magnetoresistance also shows a pronounced maximum around To. There seems to be some coupling between the charge carriers and the magnetization in Cr2S3(r). t219) Properties of Cr2S3 at high temperatures depend on the partial pressure of sulfur and hence on the deviations from stoichiometry. ~2") Cr2S3 is ordinarily an n-type semiconductor and becomes p -type at Cr2S3 Both rhombohedral and trigonal CrzS3 have been high sulfur partial pressure. ~2"~Electrical properties reported <2~) (Fig. 16). Cr2S3 is prepared by heating of non-stoichiometric Cr2S3 suggest a hopping mechanism for conduction. <234> Magnetic as well as electrical properties depend on the ordering of vacancies in the metal layers. Thus, magnetization of both CrsS6 and Cr2S3 decrease if the ordering of the vacancies is decreased by high-temperature quenchingJ 23~>Order-disorder transitions occur at 340°C and 560°C in CrsS6 and Cr2S3(r) respectively. The crystal structure of trigonal Cr2S3 is similar to that of CrsS6 except that both the 3(d) and the 2(a) sites are vacant (Fig. 16). <2~7)This gives rise to Trigonol Cr~ S~ an arrangement wherein two of each three possible metal sites are vacant. Trigonal Cr~S3 is paramagnetic above about 125 K. ~236'237~As the temperature is I~homboheclrol Cr2S 3 lowered, magnetization increases with a maximum FIG. 16. Unit cells of trigonal and rhombohedral Cr2S3. at 95 K and further on it decreases. Between 15 K Filled circles represent the Cr atoms. (After Flahaut°6~ and 4.20 K, CrzS3(tr) behaves as an antiferromagnet. and Jellinek.12~7)) At 4.2 K, the magnetization at H = 0 has zero value, but at 8 0 K it has ~ 1 0 2 I~B/Cr atom. Neutron the elements at 1000°C for several days, a~9) the diffraction measurements on Cr2S3(tr) at 4.2, 78 and material being n-type or p - t y p e depending on the 300 K show that the maximum in the magnetization conditions. <227'228) Cr2S3 belongs to NiAs-Cd(OH)2 cannot be attributed to any change in the spin type structure. The sulfur atoms are packed hex- arrangement. Qualitatively, the magnetic structures

223

Transition metal sulfides of Cr2S~(tr) at 4.2 K and 78 K are essentially the same. The spin arrangement can be described as a screw type spiral structure with a periodicity of exactly twice the crystallographic c-axis. Electrical properties of Cr:S3(tr) have been studied by Oganesyan.( ~73,238,2~9~ Cr586 Cr5S6 has a trigonal superstructure of NiAs-Cd(OH)2 type with ordered vacancies in every alternate layer. (2~7)At room temperature, the cell volume is 6 times that of the NiAs subcell. The three-dimensional order of the vacancies disappear at about 600K, but up to 900K the vacancies remain confined to every other metal layerJ 237) CrsS~ is antiferromagnetic below 158K, ferrimagnetic between 158K and 305K and paramagnetic above 305 K (23°235"237'24°-245) (see Fig. 17).

,

(a)

I

7 k --8500oe 6 ~000oe 5- CrSx IA:x=lI19/, 4 "B:x=1-178

(b) FIG. 18. (a) Antiferromagnetic spin arrangement for Cr~S6; (b) ferrimagnetic spin arrangement for Cr5S6. (After Van Laar/24°))

b 2 1 ~ . o

.......

0

~ 100

B

200 T(*K)

300

400

FIG. 17. Magnetization of CrsS, at various temperatures for compositions CrS,,~, and CrSHT~. (After Dwight et al. ~24z~)

Various mechanisms have been proposed to explain these magnetic transitions. Yuzuri et al. ~243) proposed a triangular configuration of spins with a discontinuous change of the angle between these spins at the magnetic transitions. Van Laar, (24°) based on a neutron diffraction study, has argued that the spin configuration in Cr556 is spiral-type below the transition temperature (160K); above this temperature, the spiral unwinds to give rise to collinear Neel-type ferrimagnetism (Fig. 18). According to this mechanism, the transition should be of second order, whereas the thermal hysteresis observed in the transition indicates a first-order transition. Dwight et al. ~246) have investigated this transition as a function of pressure, temperature and applied field. The values of (OT~/OP) and (OT,/OP) are 1.83deg/kbar and 0.04deg/kbar respectively. Their studies show that for the observed ground state spiral configuration, not only that all the nearest neighbor interactions should be antiferromagnetic, but also that the antiferromagnetic next-nearest-neighbor interactions should be present. The secondary magnetostrictive forces are responsible for the thermal hysteresis. The pressure dependence of ~r0 indicates that some of the magnetization arises from band electrons.

Popma and Haas c230~have carried out ferrimagnetic resonance experiments on Cr5S6 to examine the magnetic anisotropy. The resonance field was dependent on temperature; at To, the resonance field was 12.5 kOe which corresponds to g = 2. CrsS6 is a metallic conductor with resistivity of the order of 10-3 ohm cm and with a very small Seebeck coefficient. Cr384 Cr3S4 has the defect NiAs-Cd(OH)2 monoclinic structure with ordered vacancies in every other metal layer (217~ (see Fig. 8). Cr3S4 is metallic (~3°'247~ whereas Cr3Se4 is a semiconductor. (:'8) Cr3S4 is antiferromagnetic with a Neel temperature of 280 K. (248'249)The magnetic unit cell does not coincide with the crystallographic unit cell. The unit cell is doubled in the a and c directions with spins in (101) planes in "phase". Cr7S8 At low temperatures, this has a NiAs-Cd(OH)2 type unit cell similar to other chromium sulfides with a superlattice of a = 2a' and c = 3 c ' . (220,239) The structure resembles that of Fe7Se8 (Fig. 8). ~2~°'251) Above 590 K, the structure is intermediate between NiAs and Cd(OH)2 types with disordered vacancies. Above 800 K, NiAs type is obtainedJ =°) Cr788 is metallic (m) and antiferromagnetic with TN = 125 K.

Cr, S8 CrsSs has been prepared from the elements under high pressure (90 kbar) and at high temperature

22,1

C. N. R. RAo and K. P. R. PISHARODY

(1200°C).~s) This has a defect NiAs-Cd(OH)2 structure with ordering of vacancies (Fig. 8). CrsSs is a metallic phase with a Curie-Weiss dependence of magnetic susceptibility. 5.2. Ternarysulfides of chromium The disulfide of chromium is not stable, but NaCrS2, KCrS2 are stable compounds with rhombohedral structure (253-255~(these are not intercalation compounds). Sulfur atoms in these compounds form a cubic close-packed structure and the alkali metal atoms and chromium atoms occupy the octahedral holes in alternate layers. Analogous compounds with copper and silver are also known; t256)in these compounds, Cr .3 ions are in octahedral holes while Cu or Ag ions are in the tetrahedral holes. Magnetic and optical properties of these compounds have been studied and the presence of antiferromagnetism suggests localized d-electron behavior in these compounds. LiCrS2 exhibits interesting magnetic properties at low temperature s. (257.258) Several compounds of the formula, MCr2S,, where M is Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd and Hg are known. (6) The structures of these compounds are of the normal spinel type or of Cr3S4 type (defect NiAs) depending upon the M atom. In the NiAs type (Cr3S4), all the atoms are in octahedral holes of the hexagonally close-packed sulfur lattice. Transformation from the spinel structure to NiAs type structure is possible under high pressureY 9>Thus, MnCr2S4, FeCrES4 and CoCr2S4 transform to the monoclinic structure at 65 kbar and 1000°C. Such spinel-NiAs transformations have been discussed by Albers and Rooymans. t26°~ Generally, oxides (hard acids) form normal spinels whereas the more polarizable anions of Se and Te (soft acids) form the defect NiAs type structure. Sulfides fall in between and form compounds in both these structures. Magnetic properties of the NiAs type ternary chalcogenides have been studied by Tressler and Stubican, ~26' who have observed ferrimagneticS-anti ferromagnetic transitions accompanying the spinel ~,~ NiAs transformations. In MnCr2S4, FeCr2S4, CoCr2S4 and NiCr2S4, the antiferromagnetic Neel temperatures are 70 K, 200 K and 200K respectively. Most of the spinel type semiconducting ternary sulfides become metallic in the NiAs phasef162) This is probably due to the closer metal-metal distances causing the greater overlap of the orbitals in the latter structures. Electrical and optical properties of several of these spinels have been studied in detail. ~263-267~ Metallic TiCr2S4 and CrV2S4 have the Cr3S4 type structure while VCr,.S4 is a spinel with semiconducting properties. (13°)The spinel ZnCr2S4 is an antiferromagnetic semiconductor and CdCr2S4 is a ferromagnetic semiconductor, t268'~9~ HgCrES4 is metamagnetic below 25 K and ferromagnetic between 25 K and T~.~7°) CUCrES~ is a metallic ferromagnet. (27L2~2) Ternary sulfides of rare earth elements like LaMS~ where M is Cr, Mn, Fe and Co have a/so been reported. ~7~ LaCrS~ is monoclinic while LaFeS3, LaMnS~ and LaCoS~ are hexagonal. Opti-

cal properties of these compounds have been studied recently. ~27~Similar compounds with Y, Gd, Dy, Ho and Er have also been reported. ~:74'~75~ LaCrS~ is a paramagnetic insulator while others are semiconductors. 5.3. Molybdenum-sulfur system Properties and structures of the sulfides of molybdenum are quite different from those of chromium. Unlike chromium sulfides where Cr-S bonding is fairly ionic and localized d-electron behavior is common, Mo--S bonds have appreciable covalency giving rise to the trigonal prismatic coordination of molybdenum in most of these compounds. M02S3, MoS3 and MoS2 have been characterized, while MozS5 and MoS4 and other polysultides are not been fully examined. MoS3 MoS3 is prepared by precipitation from solution or by decomposition of tetrathiomolybdates like (NH~)2MoS4. It is a dark brown to black solid and amorphous to X-rays. t276) When heated to about 1000°C, MoS3 decomposes to give MoSs and to M02S3 at still higher temperatures. Equilibria between these phases have been studied by Stubbles and Richardson. ~27r~ Mo2S3 Mo2S3 can be prepared from the elements by heating at 1300°C for several hours "77)or by thermal decomposition of MoS2 in vacuum. ~27s~This sulfide belongs to the monoclinic system~279~and generally has the metal excess composition corresponding to Mo2.0~$3.~28°'~8" The crystal structure can be described as obtained by filling the octahedral holes with Mo atoms in a "Chh" close packing of sulfur atoms, in such a way that the Mo atoms are shifted from the center of the octahedron to form zig-zag chains. The Mo-Mo distance in this chain (2.86/~,) is close to that found in Mo metal. Two structural transitions have been observed in Mo2S3."77~ At 37°C, a superstructure with all the crystal axes doubled is formed and below -80°C, the lattice undergoes a triclinic distortion. Electrical resistivity, Seebeck coefficient and magnetic susceptibility of Mo2S3 have been measured at various temperatures. "77'27s~The magnitude and sign of the Seebeck coefficient change as the temperature is increased. Resistivity and susceptibility data also show some anomalies, some of which cannot be attributed to the structural phase transitions alone. The phase transitions are significantly affected when Mo is partly substituted by Cr or Cu. ~2~2) MoS2 MoS2 exists in two polytypes, 2H-MoS2 and 3R-MoS2, depending upon the arrangement of the S-Mo-S layer(45) just as in NbS2 or TaS2. In 2H-MoS2 (natural molybdite) the arrangement can be described as AbA, BaB while in 3R-MoS2 the sequence is [AbAIBaBICaC I. In both forms, the Mo atom is surrounded by S-atoms forming a trigonal prism. Preparation and structures of both types of MoS2 have been discussed by several work-

225

Transition metal sulfides ers. <137':76'28~)Single crystals of MoSz are grown by chemical transport reactions using Br: or 12.t2u~Very often, the synthetic crystals possess stacking faults. Under high pressure, 2 H - M o S : is easily transformed to 3R-MoS~J ~ss~ MoS: is a diamagnetic semiconductor; because of the layered nature of the crystal, electrical properties are highly anisotropicJ 2s~-2~°) MoS~ exhibits photoconducting propertiesJ ~9~-29~)MoSz has not excellent lubricating properties ~:97) and catalyzes many reactionsJ zg~ Studies of optical properties and photoemission of MoS: have been carried out in detail to understand the band structure of MoS2.15'291'29s311)One of the characteristics of the absorption spectrum is the presence of an excitonic doublet below the fundamental absorption edgeJ ~gs) The pair of excitons probably result from spin-orbit splitting of the valence band ~'~°s~ (Fig. 10) which is about 0.2 eV; other interpretations for this excitonic fine structure have also been put forward) ~4s~ Electrical and optical properties of MoS~ have been qualitatively explained by an energy band model, according to which the tzg band of Mo is split into a filled d~2 band at lower energy and an empty e', band. Such a splitting is expected in a trigonal prismatic surrounding of the sulfur atoms (Fig. 10). Theoretical calculations of the band structure have recently been carried out by several authors ~'66'~'~5'14s'~'313)(Fig. 19).

÷

4

5-,6-

1÷ 6¸

3"

J M T' K T r-AA FIG. 19. Band structure of 2H-MoS2 using APW method. (After Matheiss."4"))

pounds has the general formula, MMo, S,+1 where M is Sn, Pb, Cu, Mg, Cd, etcJ 32°'32" The transition temperature, Tc ranges from 2.5 K for CdM05S6 to 13 K for PbMo6Ss. These ternary compounds are related to the Mo3Se4 structure. In this arrangement there are void channels running across the lattice and extra metal atoms can be placed to give the ternary compounds of the formula M~Mo3Se4. M03S4, however, is not a stable compound. Ternary Mo sulfides with the spinel structure have been reported; ~32z) for example, Gao.sM02S4 is a ferromagnetic semiconductor with T, of 16K. 5.4. Tungsten-sulfur system Sulfides of W are quite similar to those of Mo in chemical and physical properties; WSz and WS3 are the only stable compounds known. W2S3 or W3S4 is not reported. WS~ The trisulfide of W is prepared by precipitation or by decomposition of the tetrathiotungstate with acid. ~2~6~WS3 is amorphous to X-rays like MoS3. The chocolate-colored WS3 is easily oxidized in air and decomposes to give WSz on heating at high temperatures. This is diamagnetic semiconductor. WS2 Both hexagonal, 2H-WS2, and rhombohedral, 3R-WS2, have been reported ~276) with structures similar to the corresponding molybdenum disulfides. Under high pressure and temperature, 3R-WS2 can be prepared in pure form. t2s3)Both have layered structures with the vacant holes in alternate layers. Intercalation compounds with alkali metals have been prepared with WS2 as well, ~3~5~but their properties at low temperatures have not been reported. Single crystals prepared by transport reactions ~237'2s') usually yield the 3R-form while the powder prepared under normal pressure contain mostly the hexagonal type. Optical properties of WS2 have been studied (5~ and the spectra show two excitonic peaks near the absorption edge as in the case of MoS2, although the splitting is different in magnitude. A gradual change in the splitting has been noted in the mixed system (Mo/W)Se2J ~ Electrical properties (287) and energy bands of WS2 are expected to be similar to those of MoS2 even though detailed studies have not hitherto been carried out.

1"

VI. Sulfides of group VIIB transition metals

Because of the d 5 configuration of Mn +2 ion, properties of Mn +2 sulfides are expected to be MoS2, having a layered structure with vacant similar to ions like Zn +2 and Cd +2 which have the octahedral holes in every alternate layer, can take closed shell configuration. MnS and MnSz are familup additional metal atoms like alkali metals °~4-3~6~ iar compounds, and MnS3 is yet to be estaband transition metals, ~3~7)similar to group IV and V lished. °23) M n - S bonds are highly ionic and this is dichalcogenides. Alkaline earth metals and Yb have reflected in the properties of the sulfides. The also been intercalatedJ 3~s) These intercalated com- d-electrons in these sulfides are highly localized pounds are superconducting at low tempera- and show characteristic paramagnetism and ligand turesfl ~'~Another interesting class of superconduct- field spectra. ing compounds of molybdenum sulfides has been Sulfides of Tc and Re are quite different from reported by Matthias et a/J 3!9) This class of corn- those of Mn. Thus, ReS2 has some similarity with J P S S C Vol. 10, No. 4---C

226

C. N. R. RAO and K. P. R. PISHARODY

the layered dichalcogenides of Mo and W. The R e - S bond is partly covalent and R e - R e bonds are present in some of the chalcogenides.

Specific heat measurements on a - M n S at various temperatures have been carried out to determine the Neel temperature; °38'345'354) TN is between 152K and 155K. At temperatures below 4 K , a large contribution from nuclear hyperfine interaction to 6.1. Manganese-sulfur system the heat capacity is observed. °55) Specific heat measurements also show that antiferromagnetism MnS in /3-MnS occurs at lower temperatures, t~4s> Electrical conductivity of a - M n S increases with MnS occurs in three modifications: the green form, a - M n S , with the NaCI structure and the pink temperature as in semiconductors ~356-361) and the form, 13-MnS, obtained by precipitation, with the zinc magnitude of conductivity depends on the blende or wurtzite structure, t324~Each Mn ÷2 ion has stoichiometry: 3~9'36°)There is a knee in the conductwelve nearest neighbors in all the forms, but tivity curve near the Neel temperature. Seebeck different numbers of S -2 neighbors (six S -2 in NaCI coefficient of MnS is nearly constant below the structure; 4S -2 in zinc blende and wurtzite struc- Neel temperature. ~3~ The conduction mechanism in tures). In a-MnS, each Mn +2ion is bonded through a MnS probably involves hopping. sulfur atom to its twelve nearest neighbors by 90°Mn EPR studies of a - M n S have shown that the S-Mn linkages and to its six second nearest neighbors resonance line shape is Lorentzian with a g-value by 180° M n - S - M n linkages. In the B-forms, each (2.005) quite close to the value of dilute Mn ÷2 in Mn ÷2is bonded tetrahedrally through sulfur atoms to diamagnetic crystals. ~362) The intensity of the line its nearest Mn neighbors. Crystal lattice parameters decreases as the temperature is decreased and there of MnS have been determined at different tempera- is no residual absorption below the Neel temperatures and p r e s s u r e s : 32~-32s) ture. ~362-3~)The close correspondence of the g value /3-MnS is meta-stable at room temperature and is with that of the Mn ÷2 ion suggests that there are no readily converted to a - M n S around 200°C. ~328'329) free conduction electrons and the conduction is /3-MnS is also converted to a - M n S at high pres- through a hopping mechanism. (365'366~ sures: 33°) o~-MnS undergoes a small rhombohedral Optical properties of MnS show that the elecdeformation below the Neel temperature. <331)Single tronic transitions in MnS are due to the d-electrons crystals of a - M n S are prepared by the vapor in the weak ligand field of sulfide i o n s . ~365-369) Three transport method. <332'333) Single crystals of /3-MnS main absorption peaks (A, B and C) at 16,420, 19,398 and 22,016 cm -~ corresponding to 6Alg ~4Tlg, have been grown from silica gels under proper conditions of temperature and concentration, t334) 6Alg -">'u'r2g, and 6A~g~ 4A~, 4E transitions are seen in Magnetic properties of a - M n S have been investi- the spectrum. Besides these peaks, a strong absorpgated by several workers, t335-345) It is antifer- tion edge near 2.8 eV is seen due to the chargeromagnetic with a Neel temperature of 152 K. t3~s)At transfer band. A band model for a - M n S to explain higher temperatures, the susceptibility obeys the these features has been suggested by Huffman and Curie-Weiss law with the magnetic moment corres- Wild. (3~) The absorption peak C is split into small ponding to five unpaired electrons. Both forms of peaks probably due to crystal distortion. ~36s) The 13-MnS are also antiferromagnetic. The Neel temp- Racah parameters, B and C for a - M n S are 808 cm-' and 3751 cm -~ and the Dq value is 1025 cm -~. The erature of the zinc-blende-type MnS is around 100 K, while in the wurtzite type, antiferromagnet- peaks corresponding to the 6Alg-->4Tlg and 6Alg 4Tzg transitions have anomalously high intensity ism occurs at still lower temperatures probably due to stacking faults: 3~34s) with oscillator strengths of 10-3-10 -4. The intensity increases with increasing temperature below the Magnetic structures of all the three forms of MnS have been deduced by neutron diffractionfl4°) In Neel temperature and saturates above the Neel point. This has been interpreted as due to side band a - M n S , the moments within the ( I l l ) plane are ferromagnetically aligned and successive (111) absorption associated with magnons: 37°) Optical properties and conductivity data of a planes are antiferromagnetically coupled. In the MnS can be explained by band models in which the zinc-blende-type /3-MnS, the spins lie in planes which are perpendicular to one of the crystallog- partially localized electrons are affected both by the crystal field and by spin polarized exchange raphic axes. ~349~In the wurtzite type /3-MnS, the energy. (37~'~72) The band gap in a - M n S is 3.2 eV hexagonal close-packed planes of Mn are arranged which explains its insulating behavior. as ABAB . . . . etc., with the sulfur atoms in the tetrahedral holes. The internal arrangement of the spins in each of the close-packed planes is such that MnS2 MnS2 crystallize in pyrite structure ~325)where Mn two-thirds of the nearest neighbors is parallel and one-third is antiparallel. In adjacent planes, the and Si ~ are arranged in such a way that the S - S spins are reversed. Antiferromagnetism in all these bond is parallel to the body diagonal. Crystals of compounds can be explained by the superexchange MnS2 have been grown by the hydrothermal mechanism involving intermediate anions. ~3~°)In t~- method ~373'374)besides by other methods. ~375~MnSz is essentially ionic just as MnS and exhibits antiferMnS, interaction may involve the d : and d : - : orbitals of Mn and the 3p orbitals of sulfur, romagnetic properties. Magnetic susceptibility of whereas in/3-MnS, the superexchange may involve MnS2 has been measured at various temperatures d~, d~, d~ orbitals of Mn and some sp hybrid of and the magnetic unit-cell is twice the chemical unit sulfur orbitals. <3~s'3~°)Theoretical calculations of the cell.°76) MnS: undergoes an antiferromagneticexchange interactions between ions have been car- paramagnetic transition at 48.20 K as determined by ried out by many workers. ~339'~49'3~-3~3) susceptibility measurements, <377)in agreement with

227

Transition metal sulfides the transition temperature of 47.93 K found by heat-capacity measurementsJ 378'379)In MnSz, Mn has twelve nearest neighbors out of which eight are antiparallel and four are parallel. Also, it has four parallel and two antiparallel next nearest neighbors. The various exchange interaction constants in MnS~ have been calculated by Lin and H a c k e r : ~77> MnSz is a semiconductorJ ~s°~Electrical properties of MnS2, as well as of other pyrites, have been qualitatively explained by a semiempirical molecular orbital schemeJ ~s~)Of all the pyrites, only MnS: appears to exhibit localized electron behavior; <7)in the other pyrites, covalent mixing with the two ~r-bonding orbitals of e symmetry is strong enough to create a narrow cr*-band of itinerant electron states as well as a low-spin state of the cation.

RezS7 RezS7 is prepared by precipitation from acid solutions of perrhenates by H2SJ 389'396'397)Re~S~ has been reported to have a tetragonal structure, ~388'389) but other workers ~8~'397) have noted that it is amorphous to X-rays. Re~S7 decomposes on heating to give ReS2J ~8~'397) Technitium sulfides. Amongst the sulfides of technetium, only Tc:S7 and TcS~ have been confirmed) 4'38~'~98) Tc:S7 can be prepared in the same way as Re2S7 by precipitation. Its structure and chemical properties are not known. TcS: is a layered compound with triclinic symmetryJ ~8~) It has been described to have a distorted Cd(OH): type unit cell with hexagonal close-packed anions. TcS~ is a semiconductor ~4~with an optical absorption edge around 1.0 eV. 1385)

6.2. Ternary sulfides of manganese Ternary manganese sulfides having spinel structures like MnCr:S4, ~382>MnSc2S4,"6~ MnV:S~ (Cr3S4type), MnC02S4, MnFe2S4, MnNi2S4, MnTi2S4 ~4~are known. These are magnetic semiconductors. Compounds like KzMn3S~, Rb2Mn3S4, Cs~Mn3S4 form a new class of ternary sulfides where metal-metal bonds significantly influence the magnetic and electrical propertiesJ ~ In these compounds, the manganese and alkali layers are separated by sulfur layers. The Mn *: ions are surrounded by tetrahedra of sulfur atoms which are joined at the edges. The alkali metal has a 8 + 2 coordination of sulfur. These are antiferromagnetic compounds with localized spin on the Mn. ~

6.3. Sulfides of technetium and rhenium Rhenium sulfides have been investigated by several workersJ 3~39" Re2S7 and ReS2 are well characterized, but Re2S5,~388'391)ReS3, (388'389'391)Re,.S3~3~) and ReS <392)have not been studied fully.

VII. Sulfides of group VIII transition metals

Sulfides of the elements of group VIII have been extensively studied, but not fully understood due to the complexity of the phases. A large number of phases with broad as well as narrow homogeneity regions have been observed in these sulfides. The sulfides of Fe, Co and Ni can be precipitated from aqueous solutions of their salts, but the precipitates generally contain large amounts of water. The precipitate obtained from ferric solutions is not Fe2S3 but the non-stoichiometric monosulfide. From hot solutions, a spinel-type phase of variable composition is also obtained. From Co +2 solutions, an amorphous product having the composition Co(SH, OH): is obtained. This product on drying gives non-stoichiometric Co~_~S. From Ni +2 solutions, the precipitate Ni(SH, OH)2 is obtained and this on drying, yields NiS or Ni3S4 (in presence of oxygen); at low pH, rhombohedral NiS and at higher pH, NiAs-type NiS are obtained.

7.1. Iron-sulfur system ReSz The crystal structure of ReS: (or ReSe2) is related to the CdCI2 type structure with anions forming a distorted cubic close-packed array. ~385'393~The stacking repeats itself after three layers as AcB, CbA and BaC. It does not have the same structure as MoS2 as reported earlierJ 386'387~ Single crystals of ReS: can be prepared by chemical transport reactionsJ 3s~ Thermal decomposition of Re2S7 followed by annealing at l l00°C also gives crystalline ReS3fl ss~ R e - R e bonding in ReS2 is so predominant that Red clusters are formed by displacement of Re atoms from the centers of the anionic octahedraJ 5,393~ ReS2 is a diamagnetic semiconductor with an activation energy of 0.2 eV below 300°C and 0.5 eV at higher temperaturesJ 385'387'394~Optical properties of ReS~ have been investigated by various authorsJ 5'385'39s~A sharp absorption edge (1.33 eV) at lower energies than that of MoS2 or WS2 has been observed. <5) ReS2 forms intercalation compounds, MxReS2, with alkali metals just like MoS2 and WS2. ~3~5~The mixed anion dichalcogenide, ReSSe with the same structure as ReS2 and ReSe2 has been reportedJ 385~

In nature, iron sulfide exists in a variety of minerals like pyrrhotite (Fe,-x S), troilite (FeS) and mackinawite (Fel+x S). A recent review by Ward °99~ on some of these compounds summarizes various interesting aspects of investigations on these compounds. Most of these sulfides adopt a NiAs-Cd(OH)2 type structure or the pyrite structure. Formation of different structures are influenced by the d-orbital overlap of the metal atoms and by the presence of vacancies. Iron sulfides with a wide range of compositions ranging from FeS~ to Fel÷x S (x = 0.05) have been reported and many of these (FeS2, FezS3, Fe3S4, Fe7S8, Fe~ xS, FeS and Fe~+xS) have been studied in detail. The phase diagram for these sulfides has not been completely understood j399.400~ FeS2 FeS2 occurs in two forms, pyrite (C2) and marcasite (C18). ~4°1-'°3~The pyrite structure is cubic with the rock salt structure involving Fe ÷2and S f ions at the lattice points. Each Fe ÷2 is in an octahedral surrounding of sulfur atoms and each sulfur is surrounded by three Fe ÷2ions and one sulfur. In the marcasite structure, which can be considered as a

228

C . N . R . RAO and K. P. R. PISHARODY

deformed rutile or CaCI2 structure, the sulfur atoms are packed hexagonally and half of the octahedral holes are filled with Fe atoms. The crystal structures and the lattice parameters of both pyrite ~4°''*°4) and marcasite c4°3)have been determined at different temperatures and pressures/~'4°~) The relative stabilities of pyrite and marcasite structures in different transition metal sulfides ~8~was discussed earlier in the article. Magnetic ~'~'~2> as well as electrical properties (4°~4'3) of pyrite have been extensively investigated. FeS2 is a non-magnetic semiconductor with a Seebeck coefficient(~9-4~ of 500/zV/deg at room temperature; the Seebeck coefficient decreases with increasing temperature. "°9) Depending upon the chemistry of the material, both p-type and n-type semiconductivity (with wide variations in the magnitudes of the electrical parameters) have been reported. The non-magnetic nature of FeS2 is attributed to the pairing of d-electrons and Mossbauer spectroscopy confirms the t~ configuration of Fe ÷2in FeS2.t 4 ~ ) It is reported that marcasite is more conducting than pyrite. "~) A band model to explain the physical properties of pyrite has been proposed by Bouchard and coworkers. "~°'4~2) Goodenoughcn has proposed an electron energy diagram for pyrite. FeS~ has an optical band gap of about 0.9 eV, "~°'41'while the activation energy from electrical measurements is about 0.2eV around room temperature and around 0.5 eV at higher temperatures. Pyrite can be synthesized by heating the elements together while marcasite can only be prepared hydrothermally. Marcasite undergoes an irreversible transformation to pyrite at about 400oc. (418)

complete understanding of the properties has not yet been achieved. Alsen "19) reported that pyrrhotite has the NiAstype structure with hexagonal close packing of sulfur with Fe atoms in octahedral holes. This type of crystal structure was later confirmed by Hagg and Sucksd6rff. (42°) By means of X-ray powder diffraction and magnetic susceptibility measurements, Haraldsen (4z" studied several nonstoichiometric synthetic pyrrhotites and prepared a magnetic phase diagram. Between the compositions FeS and FeSl.0s, two magnetic phase transformations, a and/3, were observed for all the pyrrhorites. The a-transformation temperature, T~, was lowered with increasing sulfur content while the /3-transformation temperature, T~, was almost constant at about 325°C. A third transformation, called the y-transformation, was observed for Fel_~ S with 0.04 ~
Fe7SrFet-x S-FeS Non-stoichiometric phases of FeS have been known to exist as minerals, commonly called pyrrhotite. Even though their chemical and physical properties have been studied by various workers, because of the complexity of these phases, a

I° Ic

2C

•

/

"

I

I

I

0.98

0-96

0.94

2C

COMP (xin FexS) V NA

Nc

"

"1

I

I IIq 5C 0.92

~

0.90

~

T

•

x

N A + 4 C + MC

o

IC

0-88

~

6C IlC 5C

A 2C÷6C NC

!

4C

MC

FIG. 20. Phase diagram of the pyrrhotites (FeS-FeTS.). (After Nakazawa and Morimoto.(~°))

Transition metal sulfides A, c = 1C); 2C(a = ~ , c = 2 C ) ; 3C(a = 9 0 A , c = 3C); 5C(a = 2A, c = 5C); 6C(a = 2A, c = 6C); l l C ( a = 2A, c = l l C ) ; and 7C(a = 2A, c =7C). A monoclinic type pyrrhotite with 4C superstructure is also known. ~ ) Besides these regular superstructures, several non-integral-type long periodic superstructures like NC, MC and N A have also been reported.~4°°'4zs'429~ Studies of Nakazawa and Morimoto <4°°)have defined the boundaries of these phases with respect to temperature and composition (Fig. 20). 1C type is the stable phase at high temperatures and is non-quenchable. It has a fundamental NiAs type structure. Between Fe0.94S and Fe0.97S, the 1C type, when quenched, changes into a long periodic metastable non-integral type (NC type) structure. When slowly cooled, both 2C and 6C are formed. The boundary between 2C and 1C represents the a-transformation temperature. Between Fe0.94S and Fe0.90S, the products were of NC type at relatively high temperatures. Natural pyrrhotite in this range of composition had 6C, 11C and 5C type structures. These could perhaps be considered as special cases of NC type with stoichiometric composition and integral values of N. The transformation between these phases at room temperature is very sluggish and has not been studied. The NC phase changes to the 1C type at higher temperatures depending upon the composition. At Fe09oS the phase is NC type which changes to N A type at 218°C. Between Fe0.sgS and Fe0.ssS, samples contain NA, MC and 4C phases, but above 305°C, only MC type exists. All the above phases gradually change to l C type at high temperatures. According to Nakazawa and Morimoto, ~4°°~ the /3-transformation corresponds to the change from antiferro- or ferrimagnetism to paramagnetism, and the y-transformation (at 150°C for Fe0.94S and at 210°C for Fe090S) corresponds to the phase boundary between NC and 1C or between NC and 4C. Na phases are ferrimagnetic, NC and MC are antiferromagnetic, while 6C, I1C and 5C are also ferrimagnetic. Magnetic properties of Fe~ ~S have been under investigation for several years by several workers. ~42~4::'"~-~7~ A phase diagram incorporating all the transitions has been given by van den BergJ 438:39) FeS(troilite) is an antiferromagnet with a Neel temperature of 325°C which is constant for all other pyrrhotites: 'z~ According to Goodenough, ~7~ the high Neel temperature implies that the a-spin electrons in the ~r-bonding orbitals are localized; cation clustering at room temperature established itinerant character (below T~) of the sixth d-electron per iron atom which has its spin antiparallel to the net spin on a cation subarray. Around 140°C, FeS undergoes the a-transition. Recently it has been shown that the a-transition corresponds to a ferroelectric to paraelectric transition and the structure changes from 1C to 2C s u p e r s t r u c t u r e . ~.3~:38'439) The polar axis in the ferroelectric 2C phase is parallel to the c-axis. Electrical conductivity of troilite is strongly anisotropic: 44°-4~) The c-axis conductivity changes sharply at the a-transformation. The a-transition is sensitive to pressure which lowers the transition temperature by 2.2 K/kbarJ 4~5-~)

229

Above the a-transition, FeS assumes the IC structure and below it both IC and 2C structures are found. It has been observed that Fe +2ions move together in groups of three in the (001) plane and the sulfur ion suffers a small displacement along the c-axisJ ~9"457)Accompanying the a-transition at x = 0.005, but distinct from it at other compositions, is a spin rotation from parallel to c-axis below To to perpendicular to c-axis above it. (4~1-453~ T~ and Ts (spin flip) occur at different temperatures depending on the stoichiometry: '54'4~ The spin-flip transition is a two-step process and van den Berg has denoted these temperatures as Tu~ and T~k; the spin-flip transition is referred to as the Morin transitionff TM FeTSs is the end member of the nonstoichiometric pyrrhotites. This has a monoclinic 4C supercell (Fig. 8). The Fe vacancies are in every alternate layer and are ordered. (399'456'457)This is a ferrimagnetic compound with the spins in each plane aligned parallel to another, but antiparallel to the neighboring layers. Below TN, the vacancies are present in only alternate planes and thus a net moment exists. Above 325°C, the vacancies are disordered and exist in every plane. Magnetic properties of Fe7S8 have been studied by Benoit ~'12~and other workers. C'2~:36'452'45s-46°)These measurements indicate that the slope of fiX,, ~ersus T plot above 560°C assumes the same value as in FeS. Low temperature magnetic measurements show a broad extremum near 50 K. (~'462~ At 80 K, spin directions in FeTS8 lie at an angle of 70 ° to the c-axis and at room temperature the magnetization is parallel to the c-axis. (~2~ At low magnetic fields, the magnetization of the two sub-lattices are in opposite directions and are slightly different in magnitude. At very high fields, according to Besnus et al. ~3~ the directions should gradually rotate so as to produce absolute magnetic saturation either along the c-axis or in the ab plane. Electrical conductivity of FeTS8 has been reported by several workers ~433'436'46*-u~) both for natural and synthetic samples. The conductivity does not show any anisotropy; Hall effect measurements (467~ indicate that the majority carriers are holes in support of the Mossbauer dataJ ~s'469) In pyrrhotites, both Fe +2 and Fe ÷3 are present along with the vacancies in an ordered arrangement. The saturation magnetic moment obeys a T 312 law which is explained by spin wave theory. (45s'57°-572~ At 0 K all the vacant sites have an ordered arrangement in the layer containing all the Fe +3 ions which will give a theoretical value for M, of 46.0 emu/cc. At higher temperatures, some of the Fe ÷3 ions move to other planes as well. Adaichi and S a t o (459'470'472) have explained the magnetic anisotropy of Fe7Ss by considering the influence of the crystal field on the Fe +2 and Fe +3 ions. Recent Mossbauer studies, ~46~'469~ however, do not indicate the presence of Fe ÷3, but only three different sublattice sites for Fe +2 ions: ~99~ Thus, a complete understanding of the magnetic properties has not yet been achieved. The phase FeI-~S is more complicated than the end members. Besides, the a-transition, a second transition which shifts to lower temperatures with increasing value x has been observed in these compounds. This transition, called the k-transition

230

C . N . R . RAO and K. P. R. PISHARODY

by Thiel and van der Berg, ~'~4~is situated below the a-transition. Magnetic susceptibility measurements by Hirahara ~'~6~also show the presence of such a transition. The /3-transition at 325°C discussed earlier is common to all the pyrrhotites. In the composition range 0.04 < x < 0.09, the magnetic susceptibility shows a peak which is known as ",/-peak}~'~37-~3~) The temperature range of this peak depends upon the stoichiometry. At x = 0.09, spontaneous magnetization is noticed. Besides these three transitions, a Morin transition (473) also occurs .(432-434,452) According to Adaichi, (~7~) this is associated with changes in the c / a ratio. The spin change proceeds in two separate stages which are independent of the a transition} 432'~33)The two temperatures at which the Morin transitions take place depend upon the stoichiometry. Between these temperatures, probably a region with spin perpendicular to the c-axis and a phase with spins parallel to the c-axis exist. (432)According to van den Berg, ~3~) the k- and a-transitions correspond to crystallographic transitions from 2C to 1C while the Morin transitions correspond to two different spin configurations. An exact picture of the phenomena at these transitions is still lacking. Fe,+,S (x ~ 0.05)

temperature of 606K. Mossbauer spectrum of Fe3S4 indicates the presence of Fe in octahedral and tetrahedral sites with antiparallel alignments of the magnetic moments with vacancies ordered at low temperature. (486.489.490.492) Electron diffraction experiments ~493)suggest pyro and piezo electricity along (111) axis of Fe3S4. In the presence of an electric field, and at about 313 K, an anisotropic displacement of the atoms along the (111) axis has been observed. ~4~'495)Use of greigite for computer memories has been suggested} 4~'497) Fe2S3 Fe2S3 exists in both amorphous and crystalline modifications}498'~99~At about 150°C, Fe2S3 converts into Fe3S4. On aging, amorphous Fe2S3 gives crystalline Fe2S3 with a tetragonal cell}498)There is need to investigate Fe2S3 in detail. 7.2. Cobalt-sulfur system Cobalt sulfides closely resemble the sulfides of iron and nickel. Properties like susceptibility and electrical conductivity of cobalt sulfides suggest the predominant role of the partly itinerant character of the d-electrons. The phase diagram of the cobalt-sulfur system was investigated by Rosenqvist tS°°) and later on by Kuznetsov and coworkers (5°':°2) as well as by Hansen (~°3). Based on the measurement of magnetic susceptibilities, Heidelberg et al/~) have compiled a phase diagram for the C o - S system (Fig. 21).

This has been found in nature as mineral mackinawite, ~47~" but can also be synthesized hydrothermally} ~°~ On heating Fe~+~S at 140°C, FeS and Fe are obtained. Takeno ~478) recognizes three different types of mackinawite. The crystal is recos c~s~ '~$3 ported to have a tetragonal structure of layeredt y p e L i O H . (476''179:8°) Coordination around F e is tetrahedral and each sulfur has four adjacent iron atoms forming a square on one side of it. The *~:ooo structure can be described as a distorted cubic 2 close packing of sulfur atoms, in which half the 800 tetrahedral holes are filled by Fe atoms with ocE tahedrai holes vacant. The F e - F e distance in the plane perpendicular to (001) axis is only 2.6A. 600 Additional Fe atoms are placed in the octahedral holes. 400 Mossbauer and neutron diffraction studies of mackinawite show no sign of magnetic ordering. (476) 35 40 45 3O Electrical conductivity on pressed pellets show Sulfur,wt. '/, mackinawite to be a semiconductor (47~ in spite of FIG. 21. Phase diagram of Co-S system based on magnethe short F e - F e distance in the compound. Studies tic susceptibility data. (After Heidelberg et al}~ ) on single crystals would be desirable.

C~8

Fe3S4 Fe3S4 exists in two mineral forms: hexagonal smythite and spinel greigite}~2'4s3)The latter can be synthesized by hydrothermal methods at about 190°C,t~'4sS) and its crystal structure has been described by a few workers/4.6~8) On heating above 280°C, Fe3S4 converts to FeS and FeS2. During preparation of Fe3S4, contamination of FeS and FeS2 has been observed} ~7) Magnetic properties of Fe3S4 are interesting. (4sS'4sg-491)Uda (4s5"4~has found an ordering temperature of 580K and a saturation magnetization of 24emu/g. Spender et al. ~49°~ find Fe~S4 to be semimetallic with a magnetic moment of 2.2_+ 0.3/~B per formula unit at 4 . 2 K and an ordering

Co4S3 This subsulfide has a broad homogeneity range around the composition Co4S3. It is a hightemperature phase stable between 790 ° and 9300C} 5°2'5c~'5°5)This phase has an incongruent melting point at about 1200 K and 30.6%S. The susceptibility is approximately 10-Scgs/g and increases slightly with t e m p e r a t u r e : °4~ The crystal structure and other properties are not known. Co9S8

This phase is peritectically formed at 835°C and has a narrow homogeneity range} ~°°) The crystal structure has been determined by Geller}~ In a cubic close-packed sulfur array, eight Co atoms are

231

Transition metal sulfides in tetrahedral holes and the ninth one is in an octahedral hole. Magnetic susceptibility of CogSs is field dependent and increases from 300K to 1106 K. ~437'5°4)According to Lotgering, (4"~ this is an antiferromagnet with 0 = - 5 0 K . The magnetic properties probably depend upon sample preparation and stoichiometry. Several solid solutions of C09Ss with Ni and Fe have been reported by Knop.~°~'~°"~ CoS Stoichiometric CoS has not been reported but Co~-, S exists with a homogenous region of composition. Kuznetsov et al. ~°~ have reported that Co~-, S exist in two different forms, one with the NiAs-type structure and another with the NiAs superlattice. Another rhombohedral modification has also been reported. ~°~) CoS disproportionates into C09Ss and C03S4 below 475°C; such instability of the B8 phase are characteristic of narrow d - b a n d s ? ~ The phase C01-xS shows a nearly temperature independent p a r a m a g n e t i s m : °4) CoS is expected to have itinerant cr*-electrons; (7~ however, detailed studies on transport properties have not been carried out. Co3S~ This has a spinel type structure whose lattice parameters have been determined using X-ray and neutron diffraction? '°~ It decomposes to CoS~ and Co~-~ S around 914 K ? °~) Co3S. is Pauli paramagnetic and metallic with a Seebeck coefficient of + 4.8 t~ V/deg at 25°C ~"6°'~1"indicating the delocalization of ~-bonding d-electrons. It is interesting that Co~+(Cr~+)S4 is ferrimagnetic and can be described by a localized electron m o d e l ? ~ NMR studies of Co3S. have shown large quadrupole effects from the cobalt ion at octahedral and tetrahedral sites with the possibility of Co +2 in tetrahedral sites: m) Co~$3 This compound was first made by Buerger and Robinson, ~ by heating CoCO~, S and K~CO3 together. The structure is reported to be of spinel type in which some Co atoms are missing from the spinel unit cell. Most of the crystals are twinned and looked hexagonal in shape. Physical properties are not known. CoS2 CoS2 has the pyrite structure like FeS2. Magnetic properties have been studied by various worke r s J 4°7''110'51'1-517)It is a ferromagnetic metallic conductor with a Curie temperature of 120 K. The magnetic properties correspond to one unpaired electron in the e~ orbitals with a partially quenched orbital contribution. Magnetic resonance experiments ~5~7) have confirmed the low spin state of Co <. The Curie temperature is sensitive to pressure with A T , / A P = - 0 . 8 6 ° / k b a r ? 18~The metallic conductivity and reduced moment of CoS2 indicate a small overlap of the a - s p i n and /8-spin bands. ~7) Hightemperature susceptibilities give a molar Curie constant that is greater than the theoretical value (~) for a localized spin S = 1. This observation is typical of a transitional paramagnetism. <7~ This is confirmed by Fel-~CoxS2 which is ferromagnetic with a moment decreasing with increasing x in the range 0.95 < x < 1.0. ~4"~

7.3. Nickel-sulfur system The phase diagram of the N i - S system ~5'~-521~is more complex than that of F e and Co sulfides (Fig. 22). In this system, Ni3+~S2, Ni3S2, Ni4S3+x, Ni6Ss, NbS6, NigS8, NiS, Ni3S4, NiS2 and NiS3 have been reported. Some of these compounds like NiS and NiS2 have been studied extensively, while only superficial studies have been carried out on others. I I0O

30

4O l

,

50 I

I

~

60 I

7

l

/

1007/

@/

100C 900 800 700 4-

P

60C

E

50C 40C

356

30C 20C

20

24

28

32

36

40

44

48

52

56

Weight per ceni S FIG. 2:2, Phase diagram for Ni-S system above 200°C and

from 18 to 56% of sulfur. (After Kullerud and Yund: "'~) Ni3S2 Ni3S2 (mineral heazlewoodite) has a rhombohedral structure at room temperatureJ 519~ This has a slightly distorted body-centered cubic arrangement of sulfur with the metal atom in some of the pseudo-tetrahedral h o l e s ? ='523~At temperatures above 556°C, Ni3S2 converts into a phase, Ni3+x$2, of fcc structure. ¢524~In this phase, the metal atoms are randomly distributed in a cubic close-packed structure of sulfur atoms with several vacancies. This high-temperature phase cannot be quenched and its physical properties are not known. The low-temperature phase is yellow in color and exhibits metallic properties. Ni3S4 Natural (mineral polydymite) as well as synthetic Ni3S4 prepared at 200°C has the spinel structure? 2°~ At high temperatures (400°C), Ni3S4 decomposes into NiS and N i S 9 25~At about 350°C and 400 bars, Ni3S4 is converted into a hexagonal NiAs type structure.~525~ Ni9Ss--NivS6-Ni6S5 Ni7S6 has been reported to exist in two polymorphic formsJ 519) The high temperature phase is orthorhombic ~s2°>and the low temperature one has got hexagonal s y m m e t r y ? °'~ The phase transformation between these two polymorphs is sluggish. Ni6S5 and NigSs have also been reported, C5°1'526~but all these compounds probably belong to the same homogeneous range of composition existing at high temperatures. The crystal structure probably changes from hexagonal through monoclinic to orthorhombic as the stoichiometry changes, from NigSs (through Ni7S6) to Ni6Ss. Properties of these phases are not known.

232

C . N . R . RAO and K. P. R. PISHARODY

NiS2 NiS2 has the pyrite structure. ~'°~)It is a semiconductor with an activation energy of 320 meV at 400K; (41°'411'527'~2s)the activation energy changes to 68 meV between 140 K and 380 K and to 0.45 meV below 140 K. Optical properties of NiS2 have been studied, ~ ) and the data suggest a band gap of 265 meV at 295 K. The band gap is temperaturedependent with a coefficient of - 0 . 4 meV/°C. A model to explain these properties has been proposed by Kautz et al. ~52s) According to this model, the conduction process below 140K involves a hopping mechanism between the acceptor sites; between 140K and 380K, conduction is by holes and above 380 K, by electrons. Thus, a mixed carrier conduction mechanism gives rise to different activation energies. NiS2 has been suggested to be a Mott insulator, (4~°'4")where the e~ band is split into a spin-up and a spin-down band by intraatomic exchange energy. Recent photoemission studies ~29) show an eg band (3.5 eV wide) compared to FeS2 which has a narrow t28 upper band (1 eV wide). NiS2 exhibits anomalous magnetic behavior. ~4~°'4"'53°~The susceptibility changes from 7 × 10-6emu/g to 6 × 10-6emu/g as the temperature is increased from 77 K to 300K. The change in susceptibility obeys a Curie-Weiss law with 0 ~- 1 8 0 0 K . Neutron diffraction studies (~3°) indicate transitions at 40 K and 30 K to magnetically ordered states. Studies by Adaichi et a l : TM show a weak ferromagnetism at 4 K with T,. = 31 K, accompanying the antiferromagnetism with a Neel temperature of 52 K. Recent studies ~32~show that stoichiometric NiS2 exhibits no ferromagnetism below 31 K. The g*-band of NiS2 is half-filled which should give rise to antiferromagnetism although the tr*-band should be broader than in CoS2 (such a broad band would not be compatible with spontaneous magnetism). It is possible that NiS2 barely fulfils the criterion for spontaneous m a g n e t i s m : ) The phase transitions are modified in NiS2-~ Sex which becomes metallic above x =0.5; at x <0.5, there are no significant changes.(Sa~) NiS NiS has two polymorphs with uniquely different properties. ~34-~3~) Below 620 K, NiS has the rhombohedral symmetry (millerite) and at high temperatures, it has the NiAs type structure. ~53s~ In the low-temperature form the Ni atom is surrounded by five sulfur atoms in a tetragonal pyramid coordination, while in NiAs type, each Ni is octahedrally coordinated. The NiAs phase quenched from high temperatures (above 620 K) is stable. The low-temperature form of NiS is semimetallic t~'53~) with temperature independent paramagnetism. ~4'~3~1Electrical and magnetic properties of hexagonal NiS have been investigated in detail, ts37-~ Hexagonal NiS undergoes a first-order phase transition from a semiconducting antiferromagnetic phase to a metallic phase, the T~ depending on the stoichiometry ~*~) (Fig. 23). For stoichiometric NiS, TN is around 264 K. t~3s)The transition temperature is predominantly influenced by the cation-anion ratio rather than the impurities in the sample, t~3) The transition disappears if the nickel content is less than Nio.%S.~3s'543)The transition is accompanied by

Nix5 o x " 1-00

• x" 0'99 x x= 0"98

250'I0-E

A x~, 0.97 V x= 0 . 9 6

200"10-~ --

~x~l~^'~'-~'~~/"

I tO0

I 200

I 300

T *K

FIG. 23. Magnetic susceptibilities (emu/mole) of Nix S as a function of temperature and stoichiometry. (After Barthelemy et al.~5")) a contraction of the lattice (a net decrease in c / a ratio). The transition temperature has negative pressure dependence; ~539) by application of pressure, the antiferromagnetic phase may be completely suppressed: 7'~2) There appears to be no significant change in crystal symmetry accompanying the transition, t542~ Recent studies ~54s) establish that NiS is an itinerant electron antiferromagnet and that the transition may be due to the Debye-Waller modification of the potential rather than due to crystal distortion. <5~) Below the 264 K transition, magnetic moment of Ni is about 1.7/~B. The spins are (antiferromagnetically) parallel to the c-axis and ferromagnetic basal planes are coupled antiparallel along the c-axis as expected of half-filled tr*bands.<7) The susceptibility, Xi, perpendicular to the basal plane in the semiconducting phase is increased by about 20%, while the Xll is very small. ~537~ Similarly, the conductivity is very anisotropic, trl/trll being of the order of 102. At low temperatures the Hall coefficient is positive and the value decreases with increasing temperature. <537'54°~n a r w o o d et a l : TM suggest that the increase in conductivity at the transition is due to the change in mobility rather than carrier concentration. Adler ~5'7)suggested that this is a metal-nonmetal transition entirely due to magnetic ordering. Tyler and F r y t548)have proposed a band model according to which the nickel 3d band (3 eV wide) lies in the middle of a broad 4s band and the Fermi level lies in a minimum in the density of states just below the top of the d band. Mott <549'55°) and Koehler et al. ~3) suggest that the transition is produced due to an exchange splitting of the eg band and consequently NiS behaves as a semimetal below the transition (Fig. 24). 7.4. Some ternary sulfides of Fe, Co and Ni Ternary compounds with alkali metals like MFeS2 (M = Na, K, Rb, CS) have been studied. °Sa) In these compounds, the iron atoms are surrounded by S, tetrahedra and show antiferromagnetic interactions between the iron atoms. Sulfides of the formula LaMS3 (M = Co or Fe) are known. (273)Sev-

233

Transition metal sulfides

Rh3S4 This is a diamagnetic compouno, (565) Lou t the structure has not been established. CF

~r

gC

%

T>T,~

T < Tt

FIG. 24. Energy bands (density of states vs. E) for NiS at T > T, and T > T,. (After White and Mott.~°)) eral spinel type sulfides of the formula FeX2S4, where X is a 3d transition element, are also known. (') FePS, FeAsS, CoAsS and NiAsS are also reported;(~-~6) these are paramagnetic semiconductors. The mineral cubanite, CuFe2S3 has an orthorhombic structure in which Fe and Cu atoms are in tetrahedral holes of the sulfur packing. This is a ferromagnetic material,(~7'559~ where there is rapid electron exchange between Fe ÷: and Fe ÷~. CuFeS: is an antiferromagnetic semiconductor. (56°) The compound CoCr2S4 was referred to earlier. 7.5. Sulfides of Ru,Rh and Pd The sulfides of these elements have not been investigated in detail, despite the fact that a large number of sulfides have been prepared. RuS2 RuS2 is the only established sulfide of ruthenium. Many unstable polysulfides have been reported, but not characterized. (561) RuS~ has the pyrite structure. ~562)It is a diamagnetic semiconductor with an energy gap of about 1.8 eV as found from optical measurements.(5~a) RhS3 This compound has been reported to have a distorted pyrite structure with a formula, Rh2,(S2) 2in which the Rh atom is in tervalent state. (56'~This is a diamagnetic semiconductor like many other transition metal trisulfides. RhS3-~ is a defect pyrite structure and it would be interesting to investigate ordering of the vacancies, if any, at low temperatures. The effect of non-stoichiometry and ordering of vacancies on the properties would also be of interest to study• Rh~S~ has been reported to have a pyrite structure (565~in which the metal vacancies are ordered; (~'566) it is probably the same as RhS3-x (x = 0.3). Rh2S3 In Rh2S3, each Rh atom is surrounded by an octahedron of s u l f u r : 66) Each RhS6 octahedron shares a common face with another octahedron to form octahedron pairs. It is a diamagnetic semiconductor with an optical band gap of about 0.8 eV. (564~ Rh2S3 is isotypic with Rh2Se3 and Ir2S3 and a complete range of solid solubility exists between Rh2S3 and Rh2Se3,(566) but the properties of these compounds are not known in detail.

Rh~7S~ Rh~7S~5,which is isostructural to Pd~7Se~5,(567)has a complex cubic structure. (5~) This is a superconductor below 5.8 K. (~9) The unit cell contains 34 Rh and 30S atoms and the Rh-Rh distance is 2.59A. Octahedral, square and distorted square coordinations of Rh atoms have been found in this structure. The short Rh-Rh distance may develop considerable d-orbital overlap and the metallic properties of this compound should be due to electrons in the narrow d-band. There are larger holes in this structure at 4(C) positions. It is likely that these holes can accommodate additional atoms like those of alkali or alkaline earth metals. Palladium sulfides. The Pd-S system has been extensively investigated by Gr~nvold and Rest; ~57°'57~) PdS, PdS2 as well as several nonstoichiometric phases are known. PdS This has a tetragonal structure. (4:°'572) The metal coordination is a distorted square and that of sulfur a distorted tetrahedron. It is a diamagnetic semiconductor .(536.573) PdS2 PdS2 has a distorted pyrite structure. The unit cell is elongated in one direction giving rise to a structure with the metal in square planar coordination. The metalloid atoms are bonded into pairs. ~76) It is a diamagnetic semiconductor. Pd3S Orthorhombic Pd3S is stable between 554°C and 635°C and can be prepared at room temperature by quenching.(57°:4:5~ This is a metallic phase, but not superconducting.(577~ Pd4S This is a metallic~7~:'~ phase and nonsuperconducting.
7.6. Sulfides of Os, Ir and Pt Studies of •the sulfides of these metals have not f been exhaustive compared to those of the 4d transition metals. Wold (523) has reviewed the solid state chemistry of platinum metal chalcogenides. OsS2 The only sulfide of osmium which has been confirmed is the disulfide with a pyrite structure. (~2'563)It is a diamagnetic semiconductor(57s)with an optical band gap of 2 eV. (~63~ IrS2 IrS~ is reported (s~ to have the same structure as

234

C. N. R. RAO and K. P. R. PISHARODY

IrSe~) ~79)Half the sulfur atoms form S: pairs while the other half do not. The structure could be ~-2 with the iridium atom described as ir+a~-st~ s o2 ~os~ surrounded octahedrally by sulfur. IrS2, with a pyrite structure, has been prepared at 60 kbar pressure around 1500°C. (~°) The difference in the properties IrSs in the two structures are not reported. IrzS3

This has the same orthorhombic structure as RhsS3 <~66)and has been reported to be a diamagnetic semiconductor9 64) Ir3S8 Ir3Ss has a pyrite type structure with ordered metal vacanciesJ ~'5~'5s' It is isotypic with Rh~Ses and Rh3Ss and is a diamagnetic semiconductor.

ing of sulfur is formed. In djurleite and in hexagonal Cu~ S, hexagonal close packing is observed. Chalcocite (CusS) Chalcocite undergoes several phase transitions. Above 104°C it has a hexagonal symmetryf188'5~) Below 104°C it has a superstructure with monoclinic symmetry with metal atoms in triangular coordinationJ 5~'~> The low-temperature phase had previously been reported to have an orthorhombic symmetry due to the twinning of crystals. Hexagonal chalcocite exhibits a range of solid solutions from Cu2S to CuLgsS at 105°C and the range shrinks with rising temperature. Hexagonal chalcocite is stable up to 435°C where it inverts to hightemperature digenite of cubic symmetry. Under high pressure, monoclinic or hexagonal chalcocite can be converted to a tetragonal modification also. (595)

IrS3

Electrical and magnetic properties of Cu2S have been studied by several authors: 5~°-6°2)It is essentially a diamagnetic semiconductor ( p - t y p e ) : ~°~'~) From room temperature to 125°C, the conductivity increases slightly and at 125°C, it sharply dePtS creases; from 125°C to about 380°C, a metallic The crystal structure of PtS is related to CrS and NiAs. Pt is in a square planar coordination while behavior is seen. From 380°C to 480°C, Cu2S behaves as an intrinsic semiconductor, with an activathe anion is surrounded by a deformed tetrahedron of four cations as in PdSJ 5s2`Ss3)PtS is a diamagnetic tion energy of about 1.8 eV. ~5~5~'6°~) The phase transitions at 125°C and 480°C have also been semiconductor (~6) with a small temperatureindependent magnetic susceptibility.(~ss) Specific observed in the measurements of Seebeck coefficients. heat measurements on PtS have been carried out at Optical properties of Cu2S have been investivarious temperatures and no phase transition has gated by several authorsJ 6°°'6°t'6°s-~ Cu2S layer has a been observed. (s~) high reflection coefficient in the 2-25/x region, and its magnitude increases with increasing conductivPtS2 ity up to about 80%. Cu2S gives an optical band gap PtS2 has a CdL (C6) layered type oStructureJ s~'s~s~ of about 1.2 eV at room temperatureJ 6°°'6°' Thin Thus S - S distance in PtS2 is 3.06 A compared to layers of CusS have practical applications in fabri3.19 ,~ in MoSs. PtS~ is a diamagnetic semiconduc- cation of heat reflecting windows, shatter-proof torJ ~6'~sz~s6)Compounds like PtYS where Y is P, As, electroluminescent condensers, shields from high Sb have been examined by Hulliger; ~sT~they are all frequency radio waves and solar cells. metallic with small paramagnetism. IrS5 is isotypic with RhS3 with rhombohedrally distorted pyrite structure; ~6'~ it is a diamagnetic semiconductor.

VIII. Sulfides of group IB transition metals

Copper and silver react with sulfur at low temperatures. In these sulfides, the metal exists in + 1 oxidation state with a d t~ configuration. Metal atoms tend to occupy the tetrahedral holes of the sulfur packing. Normally, gold does not react with sulfur at low temperatures, although gold sulfides can be prepared by precipitation from solution.

8.1. Copper-sulfur system Copper exists in minerals like covellite (CuS), chalcocite (Cu2S), digenite (Cut.79S-Cu~.76S), djurliete (Cu~.96S) and so on. A phase study of the C u - S system has been reported by Roseboom ~588~ and CookJ 589~Most of the synthetic copper sulfides exist in nature as minerals, but some of the synthetic phases are yet to be discovered. A new copper sulfide mineral with hexagonal symmetry, (CuLs3S),~5s9'~9°~has also been recently reportedJ ~9" The structure of copper sulfides can be described in two types of close packing. In chalcocite, the sulfur atoms are hexagonally close packed while in digenite and in tetragonal Cu~s6S, cubic close pack-

Cu2-x S The mineral digenite has a composition range between Cul.76S to Cu~.79S at room temperatureJ 5~s} On heating to about 78-83°C, depending upon the composition, high digenite is formed. High digenite has an fcc structure, t~) In high digenite, the copper atoms are in tetrahedral holes but displaced by 0.5 A from the center of the holesJ 61°~At 435°C the range of composition includes Cu2S. Morimoto and Kullerud ~6~°) have observed four different superstructures in synthetic digenite of which three are metastable. These polymorphs have superstructures of high digenite. ~6'~'6mA new mineral anilinite (Cu7S4) with an orthorhombic lattice has also been reportedJ 6~3) Digenites are p - t y p e semiconductors. Optical and electrical properties of digenite have been studied, t6°°'614'6~6)Non-stoichiometry produces a shift in the threshold of the transmission towards lower wavelengths and a reduction in the transmission coefficient for wavelengths after the threshold, accompanied by a clear increase in the infrared reflectance coefficient. Djurleite with a composition of Cu~.96S is stable below 93°C and has an orthorhombic structureJ ~ss>

Transition metal sulfides Between 93 ° and 350°C, djurleite produces a mixture of high-temperature digenite and hexagonal chalcocite. A tetragonal Cu,.96S has also been reported, ~9''595'6~7~but it is a metastable phase. Skinner ~9~ synthesized the tetragonal phase under high pressure and observed that a range of solid solutions exist between Cu:S and Cu, 96S. Takedo et al. ~ determined the lattice constants of djurleite and suggested a crystallographic relationship among the various forms on the basis of lattice constants. A paramagnetic phase, Cu3Se2, has been reported, ~6°3'6~9~but Cu~S2 is not known. CuS Copper monosulfide exists as the mineral, covellite. Its structure has been described to consist of two S~ and two S -~ ions; two of the six copper atoms in the unit cell have triangular coordination and the other four have tetrahedral coordinationJ 6~6~ In CuS, copper is in + 1 oxidation state and is diamagneticJ ~ CuS is metallic and superconducting below 1.6 K. ~62~62~ CuS can be prepared hydrothermally at low temperatures. ~6:~ It decomposes above 507°C. CuS has a hexagonal unit cell. ~94~ CuS~ CuS: is prepared under high pressure. ~6~9'63°~It has a pyrite type structure and possesses metallic and superconducting properties (T~ = 1.56 K). ~"°'6n~Copper disulfide shows weak paramagnetism which appears to follow a Curie-Weiss law below 100K with a small localized moment ( ~ txB) and a curie temperature of - 2 5 K. The d-bands in CuS~ are essentially similar to those in CoSz and NiSz although in the latter two compounds the itinerant e l e c t r o n s a r e more strongly correlatedfl ~Superconducting CuS: may have holes in the anion 3p bands as well as 3d bands. 8.2. Sulfides of Ag and Au AgzS Ag/S is the only stable compound in the A g - S system. Up to about 177°C, Ag2S (/3-form) has a monoclinic symmetry and between 177° and 600°C it possesses a body-centered cubic structure of sulfur ions; above 600°C, a face-centered cubic structure is f o r m e d . ~594'632-634) The phase transformations are susceptible to pressure and it seems that at about 25 kbar and 230°C, all these phases coexist. (6"~ Further, the transition temperatures depend upon stoichiometry. A high pressure form of Ag2S has also been reported. ~635"~ In monoclinic /3-Ag2S, the silver atoms are in zig-zag S - A g - S chains with a linear coordination. The other silver atoms bind the chains together, with a tetrahedral coordinationJ 639) a-Ag2S has a disordered structure analogous to c~-AgI. The sulfur ions occupy the body-centered cubic lattice while the Ag ÷ ions in the unit cell are statistically distributed over the sites of the space lattice. ~'641~ In AgCuS, a structure similar to /3-AgzS is found with Cu atoms joining the A g - S - C u c h a i n s . <639)The mineral AgCuS (stromeyerite) is hexagonal above 90°C; above 180°C, it possesses a face-centered cubic structure. ~59')

235

Electrical conductivity of silver sulfide has been studied by various w o r k e r s . ~634'636'637) It exhibits both ionic and electronic conductivity, the conductivity depending on the sulfur content. ~63s~ In a-AgzS (high-temperature phase) conduction is predominantly electronic while in/3-Ag2S (low temperature phase) the proportion of ionic conductivity is considerable. /3-Ag2S is an intrinsic semiconductor with a band gap of 0.9 e V (634'642)and becomes metallic above 180°C; it shows an optical absorption edge at 1.4/~mJ 643'6~) Optical and electrical properties of a-Ag2S have been reported. ~644-652~ a-Ag2S is a metallic phase with Ag * ions participating in the conduction (645.647.653) Optical measurements ~5'6~°~ on Ag:S indicate a gap of 0.87 eV with a conduction band mass of 0.24 me from plasma resonance. Ag2S is a photosensitive material. At wavelengths of about 1 tzm, a photo current maximum has been observed with an activation energy of about 1 eV at 298°C. ~654-656~ Heat-capacity measurements of Ag2S show anomalous behavior between 180° and 350°C due to the order-disorder transition of Ag ÷ i0ns;~651'653)such an anomalous behavior has not been observed in non-stoichiometric Ag2S. Sulfides of gold. Gold does not react with sulfur at low temperatures. Reaction of dry chloroaurates with H2S gives a black product of composition Au2S3. It is likely that the black color of Au2S3 is due to charge transfer between Au(I) and Au(III). Au2S3 is unstable in presence of moisture and at higher temperatures. Gold(I) solutions precipitate Au,_S with H2S, while in the presence of excess polysulfides, AuS~ ions are formed. Reaction of Ag2S with bisthiosulfatoaurates (I), produces Au3AgS2, AgAuS., and amorphous AH2S. (654a) Mossbauer spectra of Au2S2, Au2S3 and Au2S have been recorded, but a complete understanding of the bonding in these compounds is still lacking. ~656~ The spectrum is complex in Au2S3 and Au2S2, but in Au2S only two lines are observed. There have been no studies on crystalline gold sulfides and all preparations seem to yield amorphous compounds. IX. Sulfides of group liB transition metals

These sulfides have been extensively studied as part of the study on II-VI semiconductors and their physics and chemistry have been described by B u b e ~6s7) and Ray. ~58)We shall briefly indicate some of the important features of these sulfides. The sulfides of Zn, Cd and Hg in which the cations exist in divalent state with a filled d-shell do not strictly belong to the transition metal family. It would, however, be appropriate to discuss these sulfides along with the transition metal sulfides in view of their interesting solid-state chemistry which would enable us to make some comparisons. ZnS ZnS exists in several polytypes. Zn +2 with a d'° configuration has a tendency to fill the tetrahedral holes of the close packed anion structure. In zinc blende, the anions form a cubic close packed structure and Zn ÷2ions are placed in the tetrahedral holes. At about 1000°C, Zn +2 ions exist in tetrahedral holes of the hexagonal close packing of the

236

C. N. R. RAO and K. P. R. PISHARODY

anions (wurtzite). Besides these two common polymorphs, many other polytypes are known. ~659-~'These polytypes differ in their packing arrangement of sulfur. For example, a ten-layer packing in 10L polytype can be described as ABC ABC ABAC, fourteen-layer packing in 14L polytype as ABC ABA CB AB CB AC and so on. According to the nomenclature of these polytypes by Ramsdall, ~66" these are described as 10L(8,2), 14L(5,4,2,3) and so on where the numbers in parenthesis denote the number of layers in cyclic order to the left or right to form the unit cell. Thus zinc blende can be described as 2L(1,1) and wurtzite as 3L(3)6. Several such polytypes like 24L(5,3)3, 26L(17,4,2,3), 28L(9,5,5,9), 36L(6,2,2,2)3, etc., have been describedJ ~2) A dislocation model has been proposed to explain the phenomenon of these phase changes. <663'~)At high pressures, a new phase of ZnS with Zn +2 in octahedral holes is obtainedJ 6sS,686) ZnS is a diamagnetic semiconductor/687,6ss) Optical studies on the polytypes indicate that the absorption edge and the birefringence depend on the percentage of hexagonally stacked planes in the unit cellJ 665) Optical properties of ZnS have been extensively investigated because of the luminescent propertiesJ 6s~7~) The optical band gap of 3.5eV shifts to higher values with increasing pressure. Band structure calculations of ZnS indicate that the top of the valence band consists of 3p-like bands which are split into a four-fold F8 and two-fold F7 band due to spin orbit couplingJ 671-675) The conduction band (F6) is mainly slike in character. Reflectance measurements, ~667~'69'676~s°) photoemission, ~67°) electroreflectance ~t) and magneto-optical studies, ~682) Faraday effect measurements ~s-~) and so on have been reported. Reynolds and coworkers ~684)have reviewed the optical properties of all II-VI semiconductors with special emphasis on their excitonic spectra. Other optical and luminescent properties of ZnS have been discussed elsewhere: 657'~5s)ZnS is used as a phosphor with Cu, Ag, Mn, etc., added as impurities to act as activators or coactivators. The luminescent spectrum region of ZnS is extended in the solid solutions of ZnS with CdS or ZnSe. ZnS exhibits photoconducting properties as wellJ 689) CdS CdS has the hexagonal wurtzite type structure at normal conditions of temperature and pressure, but the cubic zinc-blende type is also known in stable form. At high pressures, CdS transforms to NaCI type structureJ ~°-6~) Miller et al. ~93~ have reported a new polymorph with B23 structure at higher pressure and temperature. A triple point for all these three phases is found at 425°C and 16.5 kbar. It has been reported that precipitation from halide solutions invariably gives hexagonal CdS while from nitrate or sulfate solutions yields cubic zinc blende type CdS. ~ Structural changes in CdS occur on electron irradiation: 69~) CdS has interesting semiconducting and photoconducting properties. The mobility of charge carriers is strongly dependent upon temperature due to phonon scattering in addition to polar acoustic modes at low temperaturesJ 6~) Measurements of

photo Hall effect indicate that Hall mobility increases with increasing carrier concentration by photon excitationJ 697) Optical properties of CdS have been studied by various workers and have been discussed in detail elsewhereJ 65s'6s4) In the wurtzite structure, CdS has a band gap of 2.58 eV which decreases with increasing pressureJ 6~) Cardona et al. ~69s) have investigated the reflection spectrum of CdS and observed a close similarity to that of ZnS. Band structure of CdS has been calculated and is similar to that of ZnSJ 6~°~ CdS is a diamagnetic semiconductor) ~7'7°°~ Jayaraman et alJ 7°1) have noted a large drop in resistivity of CdS during the phase changes occurring at higher pressures. CdS shows photoconducting properties and is used in photovoltaic cells. The photosensitivity of CdS can be altered by doping with impurities like iodine, copper, etc. ~57) HgS As in ZnS and CdS, HgS also exists in polymorphic formsJ 7°2) At room temperature and normal atmospheric pressure, the stable a-HgS has the cinnabar structure, whereas the high temperature form, fl-HgS (Metacinnabar), possesses the cubic zinc-blende structure. The lattice of trigonal HgS (a-HgS) consists of spiral chains of S - H g - S - H g with an S-Hg-S angle of 172° and an H g - S - H g angle of 105°. The Hg-S distance is about 2.36 A in the chains and the bonding between the chains are by weak van der Waals forces. The unit cell of a - H g S contains 3 HgS molecules, fl-HgS is black and is obtained by precipitation from mercuric salt solutions; it is converted to a - H g S on heating. The phase transformation between trigonal and cubic forms of HgS takes place between 280 ° and 340°C. ~/°3'7°4) At high pressures, HgS transforms to a distorted NaCl-type structure. ~69°) A new hexagonal form of HgS has also been reported, <7°5) Black (fl) HgS is an n-type degenerate semimetal. Its electron transport properties have been investigatedJ 7~) The Fermi level is 0.1 eV above the bottom of the conduction band. It shows fairly large thermoelectric voltageJ 7°6) Cinnabar is an insulator with electron mobility of the order of 30 cm2/V secJ 7°7~It is photoconducting with a peak conductivity at about 6000,g,.<7°s) Its photoconductivity and luminescence are similar to those of ZnS and C d S J 7°9) Optical properties of black HgS show that its band structure is similar to that of a-Sn, HgSe and HgTe which are zero gap semimetals: 7~°'71"Optical properties of a - H g S have been extensively investigated by various workers: m-7~4) The band gap in a-HgS is about 2.1 eV. There is pronounced dichroism at the fundamental absorption edge: m) Cathode luminescence studies °') reveal strongly polarized emission lines from a-HgS. Infrared studies on both a - and/3-HgS have been reported by Riccius and SiemsenJ 7~6) X. Summary of important data In this section, a brief summary of some of the important data on transition metal sulfides has been presented in the form of tables.

Transition metal sulfides

237

r-f,l

?

E

o

•

0

÷l

.=_ ?,-

g

e..

..I<

c-

i,-,

o

o

;z

o

~

[-

c~

0 o

..~..~ "~ .-.,.~

o

o

0

z

0

<

~2

d

0

o~ ~

go

0 0

~T=" 0 ~"~ ~ . . ~

t',

o

.'0.

~

t'~

'.~ 0 r"~.

,t ._~ "~

c~

..?

.~

li 0

,":,

+ ~..~

II ~ . - ~ ' ~

I

1-,

= a

~

=> i...

g

I

'd .--.

N

~i COl

,~ ~ . ~ o ~,--~

o

o,~

~ o_

~

X

o,,,4

Z

238

C.N.R.

RAO and K. P. R. PISHARODY

t~ f-I t"4

o ,.o

:~

-u--=

ba

o

r ~ ,,,,~ .,~

o,~,

0 '*..,

~ ~.~

o t.9

~ . ~

~

o

~ ~

o

o

o

II

m

=g

m < [-,

~ ' ~

0

r-

~gz< II

~:~

"~-~ A I='% ~

~ ~oo

~. '

'e"u"u°°x NN~'"

¢ o

:~

~

~.~-

•~ e - ~ i ~

~ - u .:L=-~ o~

m

~

r.9

e

E c

r~ oO

>..,

Transition metal sulfides

239

b-

I

"~m4 I

uh~

It

.~.==

{

=

'~

,~

..

~ I

I

<

.,= .,= ~

~,._~

.~ .~ .=~.~.

z

o z <

{.,

_> o,v r3

~

d

%>--

~t'q

© ~

II ~ - ~ ~

e~..~

~

0

~

~o.~

r.~ 0 m < [..

~

O ~

~.~

0

c~ ~ ~ , . ~ X ~ .-~ .~- ~ ~

~/)~I I ~ ~-~ • ~ _~ ~ ~ . _ ~ .~ ~ ~ ~ ,'~ ,. ~ ,

.~ .~ ,~

I ~I ~ '~

. . . . . .

~

._

..~ E ~

"

©o<

H~

.

.=oh= ~,.,.

__~

-~ .=

=o

.=b= =

240

C.N.R.

RAO and K. P. R. P I S H A R O D Y

m

e-.-

O~

0

I

.<

t~

~.,o

o

8~ 1¢3

.4 It O~

>

> ::t. 0

0< .1

r..)

~ it

~ o

.~o.~

~.~ ~

II

o

[-,

~ u

~gx

om

"~ ~

o,o

.o _ . <

.,9,0 ~ ~

tl ..

~

~

o

~'~

x

°~

~'~.

o

i!i

.,.It

[-

"

o

~

,~1 ~

"~-'

.-

e~ 0

N

o ~ ,~

~,

~

N

O~

"~

~

~

N

0"~

N

T r a n s i t i o n m e t a l sulfides

241

r~

I

I

0

I

I

o ~e-,i

H

~J

~2

rj II

0< ¢ o

i~ ~-i

OO

~

~o~ II ~ ....

.~.~

..~?>

~

~._~

~,,_~ .~ ~ ~

•= ~ _ ~ t.

O

e~

~,~

V o L 10, N o . 4 - - - D

e~

.

~

~

~2

~ II ~ ' ~

~._~-<~>~

N Em~

~- . ~

-

.~ "~~ ,~

~ , ~

..

r/3

JPSSC

~

•

242

C.N.R.

,..,

~

R A o and K. P. R. PISHARODY

~

~-

..

~

~

_

"~

eq

r~ .J <

,, ~ _~ ~ ' ~

~

~-~,~

~

~

=

>

~

.~

~

z o

~

~

~

~

< ~Q

K

;>

oo

,:'g O~t~q

i

o II

II

"~

~

'~

;~;.~0~

[-

~• " ~

Z~.

0~¢~5 ~ o o e ~

.°. ."

-- ~

.-~

.

=

.

=o

~,.,.

~o

.c-; ' ~ O

o

~

eo=

--- ~*<

~

~ o~

.~

"-~o oo

~;¢.,.,..

~

~:

-

,. " ~ , . . j o ~

S~,:~ ~ . o ~ ro~ ' ~ ,.0<,_=o .-~

o ,,

=~.~ ~

~,..~

__r~.~r~.

~- ~ ' . z

~

..e~

~ ~eq

~.~.~ ~"o~'~o~o0 .~oo~oo- ~ ~ o~ ~- ~o ~-=~,'~-oo ~s ° ?.

0

E

:>

~>

p.

;>

;>

:>

Transition metal sulfides

243

t'--

,,..., < °u

~g ~., "~ .u

~..~.~

~'~

• --

.~'~

¢~ 0 .,..-.

•~

£.)

a:I "~

°

II

~ ~ o ~-,.., ,,

- ~

~ .~

~

•

.-

,,--~ :~'.~

0

. ~ ,r.~

~-~.-~

~r~

Z>"

-~

~

~o u

~Z

<

~Z.~

<

E

u

~'~ !~.

,d

.~ ~t'~ ~II "~ ~ II II [..-7

~ ~. ~:~ II oot-: ~ ~.

II r~

'*Io II

~I@ *~ .~ qO.

~

I~

~

:~.~

~.~;

.=

~

I= ,i-* ~i. 0

=0~

g

O_ll ~

¢~,i,

v

;>

;>

;>

~°.,~

"~" II o~ ~ II [~ ~,~I*

•

..

Z

~@ ~, Ii o< .~°<

244

C.N.R.

RAO and K. P. R. PISHARODY

m" ¢-,I

7

t"- t'~

7

0

0 0

.~

._= =o

"~

.=

~

> rq

~

.,.~

,~.

'~

?

Ig

il e~ 0

<

~u

~o~ m

II

I1~ ~

m

"~

-.I +~"

-~ ~0< =u °"=

E • E .~,

;.~

I~

I~

~

,

o=

,

"~

.~'~

e~ 0

~.-~

o

o=~

~>

~ = ~'~

•,=

-:, ~ = ~

.=~ ~ ~u~'-

~ I ¢-1

Z

[..

~

" = . ~ :~

,, ~= ~ ~

~-~._~ ~ ~

I~

T r a n s i t i o n metal sulfides

245

~r-,I t-4 ,..~ i'~ oo

--~

-

~

~

"~ ,.~

o

.

~

"r. "~.=.

~

o

o

~

~ I . ~ ~'~'N

~

~ ~ ~..~.-~

~o~ m

j

~, ~

~

~

.

0

~

"5

• -. ~ 0~ "~ "--

c £o.__.

=~[.- o ~ ~ t . - ~

;~,~

~

u~

o

-

I

,..~ ,.,.., i=

o~

~

#

N

.~.

~2~ e~

M

~

,-

II 0 ~, *~

i •-

~

~

I~

=

o.~

~ ~.,

~

~.~

0

~

~

~

~

~

O~

.~ ~

~I~

~

~

~

~

~

~=I~..

~ = ~

.o

~,.~

o~

o~

~

~

=o

[.-

¢

El

~

o

I

246

C.N.R.

RAO and K. P. R. PISHARODY

oo

IN tN

o

C~ ¢) cc~

~<.o

c~

, ~cO = ~

2ge~ <

Z 0

~t~ ~'"

=

~'-

~'

Ec~

l-

Z

<

F-

r. m o

oll cr~

< [,.., ~

~C

o

~1~

r,.) ,,m

cm r-.- e4 ~ . . ~ II

~,~

il ,4

o

o .,-. N II

H

E-

N •.~ ~.

c~

09 [-

[..

,,2

.~

Transition metal sulfides

247

IN

u~

¢"I ~ 1

¢-,I

~

¢M

tN

("q (~1

IN m ~

°_

. ~

2z~

,~

°

ev

~ Z

~

m

:::l.

v

~ ~

V~A

~v ~>-~ I ~.'m

~O~o<

~,

.~

~

~0<~

V Vo~

=, .= -

>

b I~ E . ~

I

~'E

II

~

~~.~

..~ ~

~.<_ .~ ,, ~mZ~- ~

r" 0 ~ - ~

~

-~ II II

•~ ' ~

"S~ <

~

,. •-

~

~ - ~ ~

,.~-~~ __,~ ~ ~

248

C.N.R.

t",l

RAo and K. P. R. P~SHAROD'¢

¢'1

0

~d

~

~,~ --~

=..= ~ 0 "~

0 ~

~.~

~

•~

u

o

,~ II

e~8

-"-

m

T

d ,-1 , ~ 1 "~

~

~

: .-.'.~ ~

x

~ , , < II Z

"'

•

~1 m ¢ ~ , ~

,_~P--..~ ~,~

II

•

~ ~'~

,

,~ ~'.< -~.<

,~ ~;~-~'~ ~ .~,~ .~,--

~

~

~

...~ u II -I-I~ u .~ I ~. , ~ ' ~ •~ ~- ,~ ~ ,~ ~ - ~0 ~

,

~ . ~ ~ ~.~

~,~

~ ~

.~i~

T

°~

e-

0

=b

e~

0

Transition metal sulfides

o

249

i--1 ¢. 1

C~

~d

II

o

~ 0

~

0

T

•~ II

,.

~.~.=

II

.~.~ ~ . o

<

1

.'7.< .. e'.l ~

e ' - ~

~-~ ~ ~

0

r~'~

-.... ~

~

o

r0

~! '° ',n

~I

~

~

~

.,=~ ~

~

0

.'0

"0 r~

u.1

250

C.N.R.

RAo and K. P. R. PISHARODY

t'-.

0 e~ 0

t~7

7

I

" ~ =.~ o ~ - ~

,-1

•

z

• ~, • ~_~

~ " ~~

" ~ ,.~

~ ~ o . ~

[..,

0

~

~

~.~

•

z e.

N ca

-~' ~

°' o

~

ca ,~

~ ~ '~.~ ~

0

~

II

.

"~

.~

~'~ ~.

~ ~ = - ~ - ~

~

~

O~

o~.~ e*-

o~

~'

~

Ca

~

"0

0

0 ~.

~

I~

~

~

•

0

0 0

X

Transition metal sulfides

251

ox r~O

Ox.~-

oo

Ox oot~ I

~'~

0

¢~ 0 ~

~

0

~

0

~-~

' -0~ 0

o

6

x.~ [I ~

~

oo I

~

--~

II

II

~

~o~o~

o +=~

x? e

II

~:~

0

~5

,6,~ ~o

O~

~I~ ~,

,~.-~ ,~--II

ii ~

[[

~

,~.

II

~z •~ ~.~'~= ~.~= ~ ~ ~ ~ ~

.o

~.2

2~

=

,0

z

[]

o

0

252

C.N.R.

R A o and K. P. R. PISHARODY

O~

o 0

6---

,.O

*'+ ,~'~

e,i

= 0

~=

o o

0

° =L) > , o ~

~ ~ ~.~ Q__

I oo

m ~ m

z

Z

~>~

~

~= e~

0

z

o 0

•~ m .N uV~:~

o

m =

II

"-,--,o

oo

o

al

"~" o ' ~ "

=7

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Acknowledgements T h e a u t h o r s are t h a n k f u l to the U.S. N a t i o n a l B u r e a u o f S t a n d a r d s (NBS-G-77) a n d A i r F o r c e Office o f Scientific R e s e a r c h (71-2138) f o r s u p p o r t of this study. O n e of t h e m ( C N R R ) is t h a n k f u l to Jawaharlal Nehru Memorial Fund for the award of a F e l l o w s h i p . T h a n k s are d u e to Mr. O m P r a k a s h f o r his a s s i s t a n c e w i t h l i t e r a t u r e s u r v e y .

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Transition metal sulfides

53. 54. 55. 56. 57.

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261

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262

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