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Ta1.09Fe2.39Te4, a new non-stoichiometric ternary tantalum telluride
Journal of Alloys and Compounds, 204 (1994) 215-221 JALCOM 933
215
Tal.o9Fe2.39Te4, a new non-stoichiometric ternary tantalum telluride J6rg Neuhausen
and Wolfgang Tremel*
Institut fiir Anorganische Chemic und Ana~ytische Chemic der Johannes Gutenberg-Universiti~t, J.-J.-Becherweg 24, 55128 Mainz (Germany)
(Received August 7, 1993)
Abstract
prepared by chemical transport from the elements in sealed silica tubes in a temperature adient from 700 to 600 °C. It crystallizes in the monoclinic space group P2/m with a =6.162(2) /~, b=7.852(3) , c = 7.250(3) ~,/3 = 95.32(3) and Z = 2. Its structure can be derived from a hexagonal close packing of tellurium atoms with tantalum and iron atoms in octahedral voids and additional iron atoms in tetrahedral voids. The structure is closely related to the structures of MM'Te2 (M-=-Nb, Ta; M'~-Fe, Co, Ni) and MxFeyTe2 (M=-Nb, x=0.89, y=0.93; M - T a , x=0.77, y=0.90). Tax.o9Fe2.39Te4 was
~
1. I n t r o d u c t i o n
Ever-increasing attention in modern solid state chemistry has been focused on transition metal compounds with incompletely filled d-bands. A wide range of physical phenomena are associated with partially filled conduction bands. Early transition metal chalcogenides are one class of compounds where most of these phenomena are encountered. Charge density waves [1], superconductivity [2] and anisotropic electrical and optical properties [3] have been reported for many of these materials. Whereas sulphides and selenides have been studied extensively, only the last few years have seen an increasing interest in telluride chemistry. A number of binary' and ternary compounds such as Ta6Te5 [4], TaM'2Te2 ( M ' = C o , Ni) [5], Ta4SiTe4 [6], MM'Te2 ( M = N b , Ta; M ' - F e , Co, Ni) [5a, 7], M~FeyTe2 (M - Nb, x = 0.89, y = 0.93; M -= Ta, x = 0.77, y = 0.90) [8] and Ta4Pd3Te16 [9] have been characterized. All have low dimensional structures, many of them containing extended networks of metal clusters. The high stability of the metal-rich ternary phases can be related to the high affinity between electron-rich and electron-deficient metals, whereas the electronic factors responsible for the striking structural differences between the tellurides and the sulphides or selenides are not well understood. While Te-Te interactions might be relevant for the stability of some particular compounds, e.g. MM'Te5 ( M - N b , Ta; M ' - N i , Pd, Pt) [10], in general neither packing considerations nor the results of band structure calculations allow for a reliable differentiation between *Author to whom correspondence should be addressed. 0925-8388/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0925-8388(93 )00933-P
tellurides and their lighter homologues. Detailed knowledge of these factors, however, is important for an understanding of the structural principles, electronic properties and chemical reactivity in this class of compounds. Recently we synthesized and characterized a number of new ternary and quaternary early transition metal chalcogenides. In this paper we report the synthesis and structure of Tal.ogFe2.39Te4.
2. S y n t h e s i s
Tal.o9Fe2.39Te4 was prepared by heating the elements tantalum powder (Starck, 99.8%), iron powder (Merck 99.99%) and tellurium powder (Merck 99.9%) - in the ratio Ta:Fe:Te = 2:3.5:5 in a silica tube (length approximately 15 cm) sealed under vacuum (about 10 -5 Torr). A small amount of TeBr4 (approximately 1 mg cm -3) was used as a transport agent. The ampoule was placed in a two-zone furnace with a temperature gradient from 700 to 600 °C with the charge at the hot end. After 15 days, large rectangular plate-like crystals showing metallic lustre were obtained at the cold end of the tube. The samples were examined by X-ray powder diffraction using a vacuum Guinier camera (Enraf-Nonius FR 552) with monochromated Cu Kal radiation and quartz as an internal standard. The powder patterns obtained could not be assigned to any of the known phases in the system Ta/Fe/Te. Minor Fel+xTe [11] impurities could be detected in some samples by Xray powder diffraction. Energy-dispersive analysis of Xrays in a scanning electron microscope (Philips PSEM -
216
J. Neuhausen, W. Tremel / Tal.ogFez39Te . a new tantalum telluride
500 equipped with a Kevex analyser) yielded the approximate stoichiometry Ta:Fe:Te= 1:2:4. A chemical analysis performed on several randomly selected crystals using an electron microprobe (Cameca Camebax) uniformly afforded the composition Tal.09(e)Fe2.390)Te4.00(1). Chemical analysis performed on 100 mg of crystalline sample (Analytische Laboratorien G.m.b.H., Gummersbach) yielded the composition Tao.98(2~Fe2.sT(S)Tea.00(2). From our experimental evidence we cannot conclude whether these differences arise from a phase width or from a contamination with Fe~ +xTe. The results of structure refinement (see below) are in best agreement with the data obtained by electron microprobe analysis. However, the occupancies of some metal atoms obtained from structure refinement are highly correlated and their standard deviations are large. Therefore it is questionable to favour one composition over the other on the basis of structure refinements. Since the electron microprobe analysis was performed on selected single-crystal specimens, we believe that the results of this investigation are most likely to be correct. Hence we assume the stoichiometry of the phase to be Ta~.o9Fe2.39Te4.
3. Crystal structure determination
Single-crystal studies were carried out on a Siemens P4 four-circle diffractometer equipped with a graphite monochromator (Mo Ka radiation, A= 0.71073 ~ ) and a scintillation counter. Tal.09Fe239Te4 exhibits monoclinic symmetry. No systematic extinctions were observed, leading to the possible space groups P2/m (No. 10), P2 (No. 3) and Pm (No. 6). Lattice parameters were obtained by leastsquares refinement of the setting angles of 24 centred reflections in the range 18°<20<40 °. Intensity data were collected using the 0-20 technique. The intensities of two check reflections measured every 98 scans showed no significant deviations during data collection. Conventional atomic scattering factors were used and anomalous dispersion corrections were applied [12]. The processed data were empirically corrected for absorption (XEMP) and symmetry-equivalent reflections were averaged. Details concerning the data collection are given in Table 1. The structure was solved and refined using the SHELXTL-PLUS programme system (Siemens Analytical X-ray Instruments). The distribution of normalized structure factors indicated a centrosymmetric structure with space group P2/m. Successful structure solution in this space group proved this choice to be correct. Direct methods yielded the positions of three Te and four metal sites. Following chemical reasoning, the octahedrally coordinated sites were assigned to Ta and
the tetrahedral sites were assigned to Fe. The structure was refined with full-matrix least-squares methods minimizing the function E w(Fo2-Fc2) 2. Refinement including anisotropic thermal parameters for the seven atoms resulted in values of R=0.122 and Rw=0.167 with unreasonably high values of the thermal parameters for one tetrahedrally and both octahedrally coordinated metal atoms. Refinement of the occupation parameters of the corresponding metal atoms resulted in values of R ---0.0341 and Rw = 0.0352 with occupancies of 0.90(2) and 0.42(4) for the two Ta sites and 0.71(2) for the Fe site (additional refinement of the occupancy of the second tetrahedral site confirmed the full occupation of this site by iron). In contradiction to the analytical data, this results in a total composition Tax.32Fel.71Te4. Inspection of the interatomic distances revealed significantly shorter metal-tellurium distances for one octahedral metal site, which led to the assumption of a mixed occupation of this position by Fe and Ta. In fact, refinement of a model with both Fe and Ta on this octahedral site (hereafter referred to as the M(2) site) using common anisotropic thermal parameters for both atom types including a variable site occupancy resulted in a composition Taa.10(6)Fe2.36ol)Te4.oo with residuals o f R = 0.0339 and Rw = 0.0350, an improvement which is highly significant based on Hamilton's test [131. Analysis of ]E w(Fo2-F¢2) 2 as a function of Miller indices, Fo2, (sin 0)/A and setting angles revealed no unusual trends. No indications of superstructure formation due to an ordering of the vacant sites and/or the atoms on the mixed metal site have been found so far. However, short-range ordering cannot be excluded. Final atomic and thermal parameters are given in Tables 2 and 3 and important interatomic distances and angles are compiled in Tables 4 and 5. Further information on the crystal structure analysis is available on request from the Fachinformationszentrum Karlsruhe, Gesellschaft fiJr Wissenschaftlich Technische Information m.b.H., 76344 Eggenstein-Leopoldshafen, Germany by quoting the depository number CSD-57842, the names of the authors and the journal citation.
4. Results and discussion
Tal.o9Fe2.39Te4 crystallizes in a layered structure. Figure l(a) shows a perspective view of this structure along [010] and Fig. l(b) shows one layer projected along [001]. The shortest intralayer Te-Te distances are 3.664(2)/~, whereas the shortest interlayer Te-Te distances are 3.706(2) ~, indicating the presence of only weak van der Waals interactions between adjacent layers.
217
J. Neuhausen, I4<. Tremel / Tal.ogFez39Te, a new tantalum telluride
T A B L E 1. Crystal data, intensity collection and structure refinement parameters for TaLogFez39Te4 Chemical formula Formula mass Data collection temperature Crystal size Crystal system Space group Cell constants
Tal.09Fe2.39Te4 841.11 g mol -x 298 K 0.3×0.1 ×0.03 mm 3 Monoclinic P2/m (No. 10) a =6.162(2) /~. b = 7.852(3) ,~ c = 7.250(3) /~ /3=95.32(3) ° 349.27 A3 2 7.998 g cm -3 38.15 mm -x Siemens P4 Mo Ka Graphite, horizontal 4°-60 ° 0--20 1.4 ° + K a splitting Variable, 2.9-29.3 deg min 2179 1096 1020 0.039 0.0339 0.0350 2.37 1/oa(F) 49 - 2 . 6 , 3.1 e ,~-3 + scan, XEMP 0.502, 0.896
Cell volume Z Density (calculated) Absorption coefficient Diffractometer Radiation Monochromator 20 range Scan type Scan range Scan speed Measured reflections I n d e p e n d e n t reflections Observed reflections I > 2 ~ / ) Rint R~ Rw~ Goodness of fit ( G O F ) ~ Weighting scheme N u m b e r of parameters refined Largest difference peaks (min., max.) Absorption correction Min., max. transmission
"R = EIIFol- IF
Site
x
y
z
U~q~
SOF
Ta(1) M(2) Fe(1) Fe(2) Te(1) Te(2) Te(3)
2k 2/ 2m 2n 2n 4o 2m
0 ½ 0.1402(4) 0.3405(3) 0.2662(1) 0.2296(1) 0.2685(1)
0.29398(8) 0.1961(3) 0 ½ ½ 0.24633(8) 0
½ ½ 0.3823(4) 0.3899(3) 0.7358(1) 0.1841(1) 0.7152(1)
0.0138(2) 0.0159(6) 0.0125(7) 0.0107(4) 0.0139(2) 0.0156(2) 0.0225(3)
0.452(2) Fe, 0.32(5); Ta, 0.10(2) 0.357(4) 0.5 0.5 1.0 0.5
~Equivalent isotropic U defined as one-third of the trace of the orthogonalized Uij tensor.
The structure of Ta~.o9Fe2.39Te4 contains seven crystallographically independent sites, namely Ta(1), M(2), Fe(1), Fe(2), Te(1), Te(2) and Te(3). The three tellurium atoms form a distorted h.c.p, arrangement where approximately one-half of the octahedral voids and approximately one-quarter of the tetrahedral voids are filled with metal atoms. Ta(1) is situated in a distorted octahedral void with an occupancy of 90.4(4)% (dTa(1)_Te(l) = 2.777(1) ~ (2 × ), dTa(1)_Te(2 ) = 2.826(1) (2 X ), dTa(1)_Te(3 ) = 3.166(2) ~ (2 × )), while the distorted
octahedral site M(2) is occupied by both Ta and Fe with occupancies of 20(4)% and 64(10)% respectively (dM(z)_Te(3) = 2.693(2) /~, (2o><), dM(z)_Te(2) = 2.733(1) ~ (2 × ), dM(2)-Te(l)= 3.339(2) A (2 × )). The bond distances show that the tellurium coordination of these metal atoms can be regarded as 4 + 2 rather than octahedral. Finally, Fe(1) and Fe(2) are found in distorted tetrahedral voids with occupancies of 71.4(8)% and 100% respectively (dveo>Te(2)= 2.501(2) dw(l)_veo)=2.469(3) ~, dFco)_wO) = 2.551 (3)(2 ×/~',
218
J. Neuhausen, W. Tremel / Tal.ogFez39Te . a new tantalum telluride
TABLE 3. Anisotropic thermal parameters (/~2) for TaLogFe2.39Te4a Atom
U11
/-]22
U33
U~2
U13
U23
Ta(1) M(2) Fe(1) Fe(2) Te(1) Te(2) Te(3)
0.0146(3) 0.015(1) 0.013(1) 0.0114(7) 0.0115(3) 0.0168(3) 0.0196(4)
0.0116(3) 0.009(1) 0.007(1) 0.0061(8) 0.0158(4) 0.0133(3) 0.0187(5)
0.0159(3) 0.022(1) 0.018(1) 0.0151(8) 0.0148(4) 0.0169(3) 0.0306(5)
0 0 0 0 0 -0.0022(2) 0
0.0048(2) -0.0047(5) 0.0025(9) 0.0035(6) 0.0025(3) 0.0023(2) 0.0107(4)
0 0 0 0 0 -0.0035(2) 0
aThe anisotropic temperature factor has the form exp[ ~ 2"rr2(h2a*eUll + k2b*2U22 q- 12c'2U33 + 2hka*b* U12 + 21db*c* U23 + 2hla*c*U13)]. TABLE 4. Important interatomic distances (/~) in Tal.09Fe2.a9Te4 Ta(1)-Te(1) (2×) Ta(1)-Te(2) (2×) Ta(1)-Te(3) (2 ×) Ta(1)-Fe(1) (2×) Ta(1)-Ve(2) (2 × ) Ta(1)-Ta(1) Ta(1)-M(2) (2×) Fe(1)-Te(2) (2 ×) Fe(1)-Te(3) Fe(1)-Te(3) Fe(1)-Fe(1) Te(2)-Te(2) Te(2)-Te(3)
2.777(1) M(2)-Te(1) (2)<) 3.339(2) 2.826(1) M(E)-Te(2) (2×) 2.733(1) 3.166(2) M(2)-Te(3) (2 × ) 2.693(2) 2.634(2) M(2)-Fe(1) (2×) 2.768(3) 2.823(2) M(2)-Fe(2) (2 × ) 2.675(2) 3.235(2) M(E)-M(2) 3.079(5) 3.176(1) 2.501(2) Fe(E)-Te(1) 2.597(2) 2.469(3) Fe(2)-Te(1) 2.667(2) 2.551(3) Fe(2)-Te(2) (2 x) 2.544(2) 2.538(5) Fe(2)-Fe(2) 2.415(3) 3.706(2) (shortest interlayer Te-Te contact) 3.664(2) (shortest intralayer Te-Te contact)
dr<2)-T<2) = 2.544(2) A ~ ) (2 × ), dF<2r-Te(1)= 2.597(2) /~, dve(z)-T
tances in MM'Te2-type compounds (M=Nb, Ta; M ' - F e , Co, Ni) [7, 8]. Whereas the average metal-tellurium distances are almost equal for the two octahedral sites (dTa(1)-Te= 2.923 ~, dM(2)-T~= 2.922/~), the M(2) site shows a much larger deviation from the mean value compared with Ta(1). The shortest M(2)-Te distances (2.693(1) ~) are still longer than those observed for octahedrally coordinated iron in Fe2Te3 (2.62 /~ [lla]) and the octahedral sites in Fel+~Te (2.61 /~ [11]) and FeTe2 (2.51, 2.56 and 2.64 ~ [11, 15]), but they are significantly shorter than the octahedral Ta-Te distances in the related ternary MM'Te2 phases. We believe that this is caused by a simultaneous occupancy of the M(2) site by iron and tantalum atoms. It might seem strange to assume a mixed occupancy of one site by two kinds of metal atoms as different as Fe and Ta considering their atomic radii. However, in TaFel.~4Te3 [14] iron atoms are found in a distorted square pyramidal environment (which may be considered as a truncated octahedron) with Fe-Te distances as large as 2.76(1) /~. Even in species containing tetrahedrally coordinated iron such as [Fe4Ten(SC6Hs)4]3 [16] large Fe-Te distances (2.691/~) are observed. This shows that the Fe-Te distances occurring for the M(2) site in Tal.o9Fe2.agTe4 are not generally unreasonable. The 4 + 2 coordination of the M(2) site, which is illustrated in Fig. l(b), can be regarded as a transition between octahedral and tetrahedral coordination. In
TABLE 5. Selected bond angles (deg) in Tal.o9Fe2.39Te4 Te(1)-Ta(1)-Te(1) Te(1)-Ta(1)-Te(2) (2 × ) Te(1)-Ta(1)-Te(2) (2×) Te(1)-Ta(1)-Te(3) (2 × ) Te(1)-Ta(1)-Te(3) (2 × ) Te(2)-Ta(1)-Te(2) Te(2)-Ta(1)-Te(3) (2 × ) Te(2)-Ta(1)-Te(3) (2×) Te(3)-Ta(1)-Te(3) Te(2)-Fe(1)-Te(2) Te(2)-Fe(1)-Te(3) (2x) Te(2)-Fe(1)-Te(3) (2 × ) Te(3)-Fe(1)-Te(3)
108.7(1) 105.3(1) 83.7(1) 82.5(1) 168.6(1) 164.8(1) 91.4(1) 77.4(1) 86.4(1) 101.3(1) 119.6(1) 96.0(1) 119.3(1)
Te(1)-M(2)-Te(1) Te(1)-M(2)-Te(2) (2 × ) Te(1)-M(2)-Te(2) (2×) Te(1)-M(2)-Te(3) (2×) Te(1)-M(2)-Te(3) (2×) Te(2)-M(2)-Te(2) Te(2)-M(2)-Te(3) (2 × ) Te(2)-M(2)-Te(3) (2 ×) Te(3)-M(2)-Te(3) Te(1)-Fe(2)-Te(1) Te(1)-Fe(2)-Te(2) (2 ×) Te(1)-Fe(2)-Te(2) (2 X ) Te(2)-Fe(2)-Te(2)
88.8(1) 93.9(1) 74.1(1) 80.5(1) 169.1(1) 163.4(1) 84.9(1) 104.7(1) 110.3(1) 125.3(1) 120.4(1) 90.2(1) 103.1(1)
219
J. Neuhausen, W. Tremel / TalogFez3gTe, a new tantalum telluride
)
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© C
×
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o
(.1 ,,b
re(2)
)
/x3,
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(b) Fig. 1. (a) Perspective view of the Ta1.09Fe2.39Te4 structure along [010]: large shaded circles, Te; medium hatched circles, Ta(1); medium open circles, M(2); small shaded circles, Fe. (b) Projection of one layer along [001]: large shaded circles, Te; medium hatched circles, Ta(1); medium open circles, M(2); small shaded circles, Fe. The 4 + 2 coordination of Ta(1) and M(2) is illustrated by solid and dashed bonds.
addition, with respect to the interatomic distances it must be pointed out that X-ray diffraction gives only an averaged picture of the complete lattice, i.e. we find an average position of two locally different metal sites where the individual positions cannot be resolved. In fact, a refinement of a model with a split position for Fe and Ta atoms on the M(2) site was not successful owing to high correlation of the positional parameters of the two atoms. Finally, a model with iron confined to the tetrahedral sites is clearly ruled out by the results of chemical analysis. The structural relationship with the NiAs and CdI2 structure is illustrated in Fig. 2. Removal of every second metal layer parallel to the (100) plane of the
Fig. 2. Derivation of the Tal.09Fe2.39Te4 structure from the NiAs structure type. The NiAs structure is shown projected along [001]. Large circles symbolize non-metal atoms, medium circles metal atoms. The NiAs cell is indicated by solid lines. Removal of metal layers parallel to bc with respect to the NiAs cell (marked by medium open circles) leads to a (fictitious) MTez structure. Displacement of the M atoms in directions indicated by arrows and filling of the tetrahedral sites marked by small circles with Fe atoms lead to the structure of Tal.09Fe2.39Te4 (filled arrows, z = 0 with respect to the NiAs cell; filled circles, z = +~; open arrows, z=½; open circles, z=½+~; Ta(1) is located at z = 0 with respect to the N i A s cell; M(2) is found at z =½). The unit cell of TaLogFe2.39Te4 is indicated by dashed lines.
NiAs structure leads to a (fictitious) MTe2 structure, where in Tal.09Fe2.39Te4 approximately one-half of the M sites are occupied by Ta and the other half occupied by both Ta and Fe. Displacement of the metal atoms in the direction indicated by arrows and partial occupation of the tetrahedral voids marked by points with Fe atoms result in the structure Tal.o9Fea.39Te4 . The displacement from the centre of the MTe6 octahedra leads to a 4 + 2 coordination around the M atoms. The occupation of both octahedral and tetrahedral voids within a layer results in rhomboid-like clusters as described for the closely related structures of MM'Te2 ( M - N b , Ta; M ' - F e , Co, Ni) [7] and M, FeyTe2 (M = Nb, x = 0.89, y = 0.93; M = Ta, x = 0.77, y = 0.90) [8]. In fact, neglecting the mixed occupancy for one of the octahedral sites and the non-stoichiometry, the structure of Tal.o9Fe2.39Te4 represents a monoclinic variant of the orthorhombic MM'T%-type structures ( M - N b , Ta; M ' - F e , Ni; space group Pmna). The
220
J. Neuhausen, W. Tremel / Tai.ooFezsgTe ~ a new tantalum telluride
=b a
e ~ ~ ) e
- ~ F e\~' (2)
I~= ~ - J ~
~
~
~l-ell;
/
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\\ / ~ % ~ # 11
//// ~"
// "'
/
~÷+.
\
//~
#~'
/
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- -
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\
I
\
\\
//
\
~, ~ - - ~ ' ~ e
%% // // \\\\\\ // XX //
'\ xx
#:'~
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II
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Fig. 3. Projection of the metal sublattice of TaL09Fe2.39T% along [0011: large hatched circles, Ta(1); large shaded circles, M(2); small shaded circles, Fe. The presence of rhomboid-like clusters is emphasized.
corresponding cobalt compounds MCoTez (M - Nb, Ta) show only a different layer stacking. In comparison with the orthorhombic variants, where only one type of M2M'2 cluster is found, the monoclinic distortion leads to the occurrence of two crystallographically independent rhomboid-like clusters, one being formed by two Ta(1) atoms and two Fe(1) atoms (d~e(1)-ve(n = 2.538(5) •~, dTam_F
as well. Metal-metal bonding is assumed to be responsible for the preference of the iron atoms for the octahedral and tetrahedral sites within the layers instead of the tetrahedral sites between the layers. In contrast with the CdI2 structure where the layers are stacked in the [001] direction corresponding to the original NiAs cell, in Tal.o9Fe2.39Te 4 the stacking direction is parallel to the (100) plane. The stabilization of an MTe2 structure (M = Nb, Ta) following this stacking scheme by inclusion of iron group metals in tetrahedral sites within a layer has been related to the interaction of electron-rich and electron-deficient metals [5a, 7b,d, 8]. In fact, this structure type permits the maximum number of contacts between electron-rich and electron-deficient metals, which is consistent with the high affinity between such metals as proposed by the generalized Lewis acid-base concept of Brewer and Wengert [21]. These effects are assumed to be important for the electronic stability of Tal.09Fe2.a9Te4 as well. Nevertheless, there is a clear tendency when comparing the MM'Tez-type compounds: while the Ni and Co compounds are stoichiometric, the Fe compounds show occupancies less than 100% for the metal sites, indicating a reduced affinity between the 3d metal and tantalum (niobium) compared with the Ni and Co compounds. Regarding the electron concentration on the 3d metals, this is in agreement with Brewer and Wengert's concept, iron being a weaker Lewis base than cobalt or nickel. In Tal.o9Fe2.39Te 4 we even observe (based on our model with a mixed metal site) the formation of rhomboidlike clusters containing four Fe atoms or three Fe atoms
J. Neuhausen, W. Tremel / Tal.ooFe239Te, a new tantalum telluride
and one Ta atom, giving rise to an increased number of Fe-Fe contacts at the expense of Ta-Fe interactions. This points towards a reduced affinity between tantalum and iron compared with its more electron-rich neighbours in the periodic table as well. Efforts to understand the electronic factors responsible for this behaviour as well as measurements of the electrical and magnetic properties are in progress. Detailed results of these investigations and of M6ssbauer as well as photoelectron spectroscopy will be be reported in a separate paper [22].
Acknowledgments This work has been supported by a grant from the Bundesministerium fiir Forschung und Technologie (contract numbers BMFT-05 439GXB3 and 05 5UMGAB). Further support came from the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie. We are grateful to Dr. G. H6fer (Heraeus Quarzschmelze) and Dr. J. Peters (H.C. Starck Co.) for generous gifts of silica tubes and tantalum powder. We are also indebted to Professor G. Henkel (Universit~t-GH-Duisburg) for access to the four-circle diffractometer.
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221
4 M. Conrad and B. Harbrecht, Chemiedozententagung 1992, Heidelberg, Abstr. p. 35. 5 (a) W. Tremel, J. Chem. Soc., Chem. Commun., (1991) 1405; (b) W. Tremel, Angew. Chem., 104 (1992)230;Angew. Chem. Int. Edn. Engl., 31 (1992) 217. 6 M.E. Badding and F.J. DiSalvo, Inorg. Chem., 29 (1990) 3952. 7 (a) J.L. Huang, S.X. Liu and B.Q. Huang, Sci. China, Set. B, 34 (1991) 666; (b) W. Tremel, Angew. Chem., 103 (1991) 900; Angew. Chem. Int. Edn. Engl., 30 (1991) 840; (c) J. Li, M.E. Badding and F.J. DiSalvo, Inorg. Chem., 31 (1992) 1050; (d) W. Tremel, Habilitationsschrift, Universit~it Mtinster, 1992. 8 J. Neuhausen, K.-L. Storck, E. Potthoff and W. Tremel, Z. Naturf B, 47 (1992) 1203. 9 A. Mar and J.A. Ibers, J. Chem. Soc., Dalton Trans., (1991) 639. 10 (a) E.W. Liimatta and J.A. Ibers, J. Solid State Chem., 71 (1987) 384; (b) E.W. Liimatta and J.A. Ibers, J. Solid State Chem., 77 (1988) 141; (c) E.W. Liimatta and J.A. Ibers, J. Solid State Chem., 78 (1989) 7; (d) A. Mar and J.A. Ibers, Z Solid State Chem., 92 (1991) 352. 11 (a) F. Gr~nvold, H. Haraldsen and J. Vihovde, Acta Chem. Scand., 8 (1954) 1927; (b) D. Fruchart, P. Convert, P. Wolfers, R. Madar, J.P. Senateur and R. Fruchart, Mater. Res. Bull., 10 (1975) 169; (c) J. Leciejewicz, Acta Chem. Scand., 17
(1963) 2593. 12 J.A. Ibers and W.C. Hamilton (eds.), International Tables for X-ray Crystallography, Vol. IV, Kynoch, Birmingham, 1974. 13 W.C. Hamilton, Acta Crystallogr., 18 (1965) 502. 14 (a) J. Neuhausen, E. Potthoff, W. Tremel, J. Ensling and P. Gtitlich, Z. Naturf B, 48 (1993) 797; (b) M.E. Badding, J. Li, F.J. DiSalvo, W. Zhou and P.P. Edwards, J. Solid State Chem., 100 (1992) 313; (c) S.X. Liu, G.L. Cai and J.L. Huang, Acta Crystallogr. C, 49 (1993) 4. 15 G. Brostigen and A. Kjekshus, Acta Chem. Scand., 24 (1970) 1925. 16 P. Barbaro, A. Bencini, I. Bertini, F. Briganti and S. Midollini, J. Am. Chem. Soc., 112 (1990) 7238. 17 S.C. Lee and R.H. Holm, 3. Am. Chem. Soc., 112 (1990) 9654. 18 CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 65th edn., 1984. 19 W. Simon, A. Wilk, B. Krebs and G. Henkel, Angew. Chem., 99 (1987) 1039;Angew. Chem. Int. Edn. Engl., 26 (1987) 1009. 20 (a) M.L. Steigerwald, T. Siegrist, S.M. Stuczynski and Y.-U. Kwon, J. Am. Chem. Soc., 114 (1992) 3155; (b) F. Cecconi, C.A. Ghilardi, S. Midollini and A. Orlandini, 9'. Chem. Soc., Chem. Commun., (1992) 910. 21 L. Brewer and P. Wengert, Metall. Trans., 4 (1973) 83, 2674. 22 J. Neuhausen, R.K. Kremer, W. Tremel, J. Ensling, P. Giitlich, C. Felser-Wenz and G. Hohlneicher, J. Mater. Chem., in preparation.
215
Tal.o9Fe2.39Te4, a new non-stoichiometric ternary tantalum telluride J6rg Neuhausen
and Wolfgang Tremel*
Institut fiir Anorganische Chemic und Ana~ytische Chemic der Johannes Gutenberg-Universiti~t, J.-J.-Becherweg 24, 55128 Mainz (Germany)
(Received August 7, 1993)
Abstract
prepared by chemical transport from the elements in sealed silica tubes in a temperature adient from 700 to 600 °C. It crystallizes in the monoclinic space group P2/m with a =6.162(2) /~, b=7.852(3) , c = 7.250(3) ~,/3 = 95.32(3) and Z = 2. Its structure can be derived from a hexagonal close packing of tellurium atoms with tantalum and iron atoms in octahedral voids and additional iron atoms in tetrahedral voids. The structure is closely related to the structures of MM'Te2 (M-=-Nb, Ta; M'~-Fe, Co, Ni) and MxFeyTe2 (M=-Nb, x=0.89, y=0.93; M - T a , x=0.77, y=0.90). Tax.o9Fe2.39Te4 was
~
1. I n t r o d u c t i o n
Ever-increasing attention in modern solid state chemistry has been focused on transition metal compounds with incompletely filled d-bands. A wide range of physical phenomena are associated with partially filled conduction bands. Early transition metal chalcogenides are one class of compounds where most of these phenomena are encountered. Charge density waves [1], superconductivity [2] and anisotropic electrical and optical properties [3] have been reported for many of these materials. Whereas sulphides and selenides have been studied extensively, only the last few years have seen an increasing interest in telluride chemistry. A number of binary' and ternary compounds such as Ta6Te5 [4], TaM'2Te2 ( M ' = C o , Ni) [5], Ta4SiTe4 [6], MM'Te2 ( M = N b , Ta; M ' - F e , Co, Ni) [5a, 7], M~FeyTe2 (M - Nb, x = 0.89, y = 0.93; M -= Ta, x = 0.77, y = 0.90) [8] and Ta4Pd3Te16 [9] have been characterized. All have low dimensional structures, many of them containing extended networks of metal clusters. The high stability of the metal-rich ternary phases can be related to the high affinity between electron-rich and electron-deficient metals, whereas the electronic factors responsible for the striking structural differences between the tellurides and the sulphides or selenides are not well understood. While Te-Te interactions might be relevant for the stability of some particular compounds, e.g. MM'Te5 ( M - N b , Ta; M ' - N i , Pd, Pt) [10], in general neither packing considerations nor the results of band structure calculations allow for a reliable differentiation between *Author to whom correspondence should be addressed. 0925-8388/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0925-8388(93 )00933-P
tellurides and their lighter homologues. Detailed knowledge of these factors, however, is important for an understanding of the structural principles, electronic properties and chemical reactivity in this class of compounds. Recently we synthesized and characterized a number of new ternary and quaternary early transition metal chalcogenides. In this paper we report the synthesis and structure of Tal.ogFe2.39Te4.
2. S y n t h e s i s
Tal.o9Fe2.39Te4 was prepared by heating the elements tantalum powder (Starck, 99.8%), iron powder (Merck 99.99%) and tellurium powder (Merck 99.9%) - in the ratio Ta:Fe:Te = 2:3.5:5 in a silica tube (length approximately 15 cm) sealed under vacuum (about 10 -5 Torr). A small amount of TeBr4 (approximately 1 mg cm -3) was used as a transport agent. The ampoule was placed in a two-zone furnace with a temperature gradient from 700 to 600 °C with the charge at the hot end. After 15 days, large rectangular plate-like crystals showing metallic lustre were obtained at the cold end of the tube. The samples were examined by X-ray powder diffraction using a vacuum Guinier camera (Enraf-Nonius FR 552) with monochromated Cu Kal radiation and quartz as an internal standard. The powder patterns obtained could not be assigned to any of the known phases in the system Ta/Fe/Te. Minor Fel+xTe [11] impurities could be detected in some samples by Xray powder diffraction. Energy-dispersive analysis of Xrays in a scanning electron microscope (Philips PSEM -
216
J. Neuhausen, W. Tremel / Tal.ogFez39Te . a new tantalum telluride
500 equipped with a Kevex analyser) yielded the approximate stoichiometry Ta:Fe:Te= 1:2:4. A chemical analysis performed on several randomly selected crystals using an electron microprobe (Cameca Camebax) uniformly afforded the composition Tal.09(e)Fe2.390)Te4.00(1). Chemical analysis performed on 100 mg of crystalline sample (Analytische Laboratorien G.m.b.H., Gummersbach) yielded the composition Tao.98(2~Fe2.sT(S)Tea.00(2). From our experimental evidence we cannot conclude whether these differences arise from a phase width or from a contamination with Fe~ +xTe. The results of structure refinement (see below) are in best agreement with the data obtained by electron microprobe analysis. However, the occupancies of some metal atoms obtained from structure refinement are highly correlated and their standard deviations are large. Therefore it is questionable to favour one composition over the other on the basis of structure refinements. Since the electron microprobe analysis was performed on selected single-crystal specimens, we believe that the results of this investigation are most likely to be correct. Hence we assume the stoichiometry of the phase to be Ta~.o9Fe2.39Te4.
3. Crystal structure determination
Single-crystal studies were carried out on a Siemens P4 four-circle diffractometer equipped with a graphite monochromator (Mo Ka radiation, A= 0.71073 ~ ) and a scintillation counter. Tal.09Fe239Te4 exhibits monoclinic symmetry. No systematic extinctions were observed, leading to the possible space groups P2/m (No. 10), P2 (No. 3) and Pm (No. 6). Lattice parameters were obtained by leastsquares refinement of the setting angles of 24 centred reflections in the range 18°<20<40 °. Intensity data were collected using the 0-20 technique. The intensities of two check reflections measured every 98 scans showed no significant deviations during data collection. Conventional atomic scattering factors were used and anomalous dispersion corrections were applied [12]. The processed data were empirically corrected for absorption (XEMP) and symmetry-equivalent reflections were averaged. Details concerning the data collection are given in Table 1. The structure was solved and refined using the SHELXTL-PLUS programme system (Siemens Analytical X-ray Instruments). The distribution of normalized structure factors indicated a centrosymmetric structure with space group P2/m. Successful structure solution in this space group proved this choice to be correct. Direct methods yielded the positions of three Te and four metal sites. Following chemical reasoning, the octahedrally coordinated sites were assigned to Ta and
the tetrahedral sites were assigned to Fe. The structure was refined with full-matrix least-squares methods minimizing the function E w(Fo2-Fc2) 2. Refinement including anisotropic thermal parameters for the seven atoms resulted in values of R=0.122 and Rw=0.167 with unreasonably high values of the thermal parameters for one tetrahedrally and both octahedrally coordinated metal atoms. Refinement of the occupation parameters of the corresponding metal atoms resulted in values of R ---0.0341 and Rw = 0.0352 with occupancies of 0.90(2) and 0.42(4) for the two Ta sites and 0.71(2) for the Fe site (additional refinement of the occupancy of the second tetrahedral site confirmed the full occupation of this site by iron). In contradiction to the analytical data, this results in a total composition Tax.32Fel.71Te4. Inspection of the interatomic distances revealed significantly shorter metal-tellurium distances for one octahedral metal site, which led to the assumption of a mixed occupation of this position by Fe and Ta. In fact, refinement of a model with both Fe and Ta on this octahedral site (hereafter referred to as the M(2) site) using common anisotropic thermal parameters for both atom types including a variable site occupancy resulted in a composition Taa.10(6)Fe2.36ol)Te4.oo with residuals o f R = 0.0339 and Rw = 0.0350, an improvement which is highly significant based on Hamilton's test [131. Analysis of ]E w(Fo2-F¢2) 2 as a function of Miller indices, Fo2, (sin 0)/A and setting angles revealed no unusual trends. No indications of superstructure formation due to an ordering of the vacant sites and/or the atoms on the mixed metal site have been found so far. However, short-range ordering cannot be excluded. Final atomic and thermal parameters are given in Tables 2 and 3 and important interatomic distances and angles are compiled in Tables 4 and 5. Further information on the crystal structure analysis is available on request from the Fachinformationszentrum Karlsruhe, Gesellschaft fiJr Wissenschaftlich Technische Information m.b.H., 76344 Eggenstein-Leopoldshafen, Germany by quoting the depository number CSD-57842, the names of the authors and the journal citation.
4. Results and discussion
Tal.o9Fe2.39Te4 crystallizes in a layered structure. Figure l(a) shows a perspective view of this structure along [010] and Fig. l(b) shows one layer projected along [001]. The shortest intralayer Te-Te distances are 3.664(2)/~, whereas the shortest interlayer Te-Te distances are 3.706(2) ~, indicating the presence of only weak van der Waals interactions between adjacent layers.
217
J. Neuhausen, I4<. Tremel / Tal.ogFez39Te, a new tantalum telluride
T A B L E 1. Crystal data, intensity collection and structure refinement parameters for TaLogFez39Te4 Chemical formula Formula mass Data collection temperature Crystal size Crystal system Space group Cell constants
Tal.09Fe2.39Te4 841.11 g mol -x 298 K 0.3×0.1 ×0.03 mm 3 Monoclinic P2/m (No. 10) a =6.162(2) /~. b = 7.852(3) ,~ c = 7.250(3) /~ /3=95.32(3) ° 349.27 A3 2 7.998 g cm -3 38.15 mm -x Siemens P4 Mo Ka Graphite, horizontal 4°-60 ° 0--20 1.4 ° + K a splitting Variable, 2.9-29.3 deg min 2179 1096 1020 0.039 0.0339 0.0350 2.37 1/oa(F) 49 - 2 . 6 , 3.1 e ,~-3 + scan, XEMP 0.502, 0.896
Cell volume Z Density (calculated) Absorption coefficient Diffractometer Radiation Monochromator 20 range Scan type Scan range Scan speed Measured reflections I n d e p e n d e n t reflections Observed reflections I > 2 ~ / ) Rint R~ Rw~ Goodness of fit ( G O F ) ~ Weighting scheme N u m b e r of parameters refined Largest difference peaks (min., max.) Absorption correction Min., max. transmission
"R = EIIFol- IF
Site
x
y
z
U~q~
SOF
Ta(1) M(2) Fe(1) Fe(2) Te(1) Te(2) Te(3)
2k 2/ 2m 2n 2n 4o 2m
0 ½ 0.1402(4) 0.3405(3) 0.2662(1) 0.2296(1) 0.2685(1)
0.29398(8) 0.1961(3) 0 ½ ½ 0.24633(8) 0
½ ½ 0.3823(4) 0.3899(3) 0.7358(1) 0.1841(1) 0.7152(1)
0.0138(2) 0.0159(6) 0.0125(7) 0.0107(4) 0.0139(2) 0.0156(2) 0.0225(3)
0.452(2) Fe, 0.32(5); Ta, 0.10(2) 0.357(4) 0.5 0.5 1.0 0.5
~Equivalent isotropic U defined as one-third of the trace of the orthogonalized Uij tensor.
The structure of Ta~.o9Fe2.39Te4 contains seven crystallographically independent sites, namely Ta(1), M(2), Fe(1), Fe(2), Te(1), Te(2) and Te(3). The three tellurium atoms form a distorted h.c.p, arrangement where approximately one-half of the octahedral voids and approximately one-quarter of the tetrahedral voids are filled with metal atoms. Ta(1) is situated in a distorted octahedral void with an occupancy of 90.4(4)% (dTa(1)_Te(l) = 2.777(1) ~ (2 × ), dTa(1)_Te(2 ) = 2.826(1) (2 X ), dTa(1)_Te(3 ) = 3.166(2) ~ (2 × )), while the distorted
octahedral site M(2) is occupied by both Ta and Fe with occupancies of 20(4)% and 64(10)% respectively (dM(z)_Te(3) = 2.693(2) /~, (2o><), dM(z)_Te(2) = 2.733(1) ~ (2 × ), dM(2)-Te(l)= 3.339(2) A (2 × )). The bond distances show that the tellurium coordination of these metal atoms can be regarded as 4 + 2 rather than octahedral. Finally, Fe(1) and Fe(2) are found in distorted tetrahedral voids with occupancies of 71.4(8)% and 100% respectively (dveo>Te(2)= 2.501(2) dw(l)_veo)=2.469(3) ~, dFco)_wO) = 2.551 (3)(2 ×/~',
218
J. Neuhausen, W. Tremel / Tal.ogFez39Te . a new tantalum telluride
TABLE 3. Anisotropic thermal parameters (/~2) for TaLogFe2.39Te4a Atom
U11
/-]22
U33
U~2
U13
U23
Ta(1) M(2) Fe(1) Fe(2) Te(1) Te(2) Te(3)
0.0146(3) 0.015(1) 0.013(1) 0.0114(7) 0.0115(3) 0.0168(3) 0.0196(4)
0.0116(3) 0.009(1) 0.007(1) 0.0061(8) 0.0158(4) 0.0133(3) 0.0187(5)
0.0159(3) 0.022(1) 0.018(1) 0.0151(8) 0.0148(4) 0.0169(3) 0.0306(5)
0 0 0 0 0 -0.0022(2) 0
0.0048(2) -0.0047(5) 0.0025(9) 0.0035(6) 0.0025(3) 0.0023(2) 0.0107(4)
0 0 0 0 0 -0.0035(2) 0
aThe anisotropic temperature factor has the form exp[ ~ 2"rr2(h2a*eUll + k2b*2U22 q- 12c'2U33 + 2hka*b* U12 + 21db*c* U23 + 2hla*c*U13)]. TABLE 4. Important interatomic distances (/~) in Tal.09Fe2.a9Te4 Ta(1)-Te(1) (2×) Ta(1)-Te(2) (2×) Ta(1)-Te(3) (2 ×) Ta(1)-Fe(1) (2×) Ta(1)-Ve(2) (2 × ) Ta(1)-Ta(1) Ta(1)-M(2) (2×) Fe(1)-Te(2) (2 ×) Fe(1)-Te(3) Fe(1)-Te(3) Fe(1)-Fe(1) Te(2)-Te(2) Te(2)-Te(3)
2.777(1) M(2)-Te(1) (2)<) 3.339(2) 2.826(1) M(E)-Te(2) (2×) 2.733(1) 3.166(2) M(2)-Te(3) (2 × ) 2.693(2) 2.634(2) M(2)-Fe(1) (2×) 2.768(3) 2.823(2) M(2)-Fe(2) (2 × ) 2.675(2) 3.235(2) M(E)-M(2) 3.079(5) 3.176(1) 2.501(2) Fe(E)-Te(1) 2.597(2) 2.469(3) Fe(2)-Te(1) 2.667(2) 2.551(3) Fe(2)-Te(2) (2 x) 2.544(2) 2.538(5) Fe(2)-Fe(2) 2.415(3) 3.706(2) (shortest interlayer Te-Te contact) 3.664(2) (shortest intralayer Te-Te contact)
dr<2)-T<2) = 2.544(2) A ~ ) (2 × ), dF<2r-Te(1)= 2.597(2) /~, dve(z)-T
tances in MM'Te2-type compounds (M=Nb, Ta; M ' - F e , Co, Ni) [7, 8]. Whereas the average metal-tellurium distances are almost equal for the two octahedral sites (dTa(1)-Te= 2.923 ~, dM(2)-T~= 2.922/~), the M(2) site shows a much larger deviation from the mean value compared with Ta(1). The shortest M(2)-Te distances (2.693(1) ~) are still longer than those observed for octahedrally coordinated iron in Fe2Te3 (2.62 /~ [lla]) and the octahedral sites in Fel+~Te (2.61 /~ [11]) and FeTe2 (2.51, 2.56 and 2.64 ~ [11, 15]), but they are significantly shorter than the octahedral Ta-Te distances in the related ternary MM'Te2 phases. We believe that this is caused by a simultaneous occupancy of the M(2) site by iron and tantalum atoms. It might seem strange to assume a mixed occupancy of one site by two kinds of metal atoms as different as Fe and Ta considering their atomic radii. However, in TaFel.~4Te3 [14] iron atoms are found in a distorted square pyramidal environment (which may be considered as a truncated octahedron) with Fe-Te distances as large as 2.76(1) /~. Even in species containing tetrahedrally coordinated iron such as [Fe4Ten(SC6Hs)4]3 [16] large Fe-Te distances (2.691/~) are observed. This shows that the Fe-Te distances occurring for the M(2) site in Tal.o9Fe2.agTe4 are not generally unreasonable. The 4 + 2 coordination of the M(2) site, which is illustrated in Fig. l(b), can be regarded as a transition between octahedral and tetrahedral coordination. In
TABLE 5. Selected bond angles (deg) in Tal.o9Fe2.39Te4 Te(1)-Ta(1)-Te(1) Te(1)-Ta(1)-Te(2) (2 × ) Te(1)-Ta(1)-Te(2) (2×) Te(1)-Ta(1)-Te(3) (2 × ) Te(1)-Ta(1)-Te(3) (2 × ) Te(2)-Ta(1)-Te(2) Te(2)-Ta(1)-Te(3) (2 × ) Te(2)-Ta(1)-Te(3) (2×) Te(3)-Ta(1)-Te(3) Te(2)-Fe(1)-Te(2) Te(2)-Fe(1)-Te(3) (2x) Te(2)-Fe(1)-Te(3) (2 × ) Te(3)-Fe(1)-Te(3)
108.7(1) 105.3(1) 83.7(1) 82.5(1) 168.6(1) 164.8(1) 91.4(1) 77.4(1) 86.4(1) 101.3(1) 119.6(1) 96.0(1) 119.3(1)
Te(1)-M(2)-Te(1) Te(1)-M(2)-Te(2) (2 × ) Te(1)-M(2)-Te(2) (2×) Te(1)-M(2)-Te(3) (2×) Te(1)-M(2)-Te(3) (2×) Te(2)-M(2)-Te(2) Te(2)-M(2)-Te(3) (2 × ) Te(2)-M(2)-Te(3) (2 ×) Te(3)-M(2)-Te(3) Te(1)-Fe(2)-Te(1) Te(1)-Fe(2)-Te(2) (2 ×) Te(1)-Fe(2)-Te(2) (2 X ) Te(2)-Fe(2)-Te(2)
88.8(1) 93.9(1) 74.1(1) 80.5(1) 169.1(1) 163.4(1) 84.9(1) 104.7(1) 110.3(1) 125.3(1) 120.4(1) 90.2(1) 103.1(1)
219
J. Neuhausen, W. Tremel / TalogFez3gTe, a new tantalum telluride
)
©
© C
×
©
o
(.1 ,,b
re(2)
)
/x3,
e
-'A
(b) Fig. 1. (a) Perspective view of the Ta1.09Fe2.39Te4 structure along [010]: large shaded circles, Te; medium hatched circles, Ta(1); medium open circles, M(2); small shaded circles, Fe. (b) Projection of one layer along [001]: large shaded circles, Te; medium hatched circles, Ta(1); medium open circles, M(2); small shaded circles, Fe. The 4 + 2 coordination of Ta(1) and M(2) is illustrated by solid and dashed bonds.
addition, with respect to the interatomic distances it must be pointed out that X-ray diffraction gives only an averaged picture of the complete lattice, i.e. we find an average position of two locally different metal sites where the individual positions cannot be resolved. In fact, a refinement of a model with a split position for Fe and Ta atoms on the M(2) site was not successful owing to high correlation of the positional parameters of the two atoms. Finally, a model with iron confined to the tetrahedral sites is clearly ruled out by the results of chemical analysis. The structural relationship with the NiAs and CdI2 structure is illustrated in Fig. 2. Removal of every second metal layer parallel to the (100) plane of the
Fig. 2. Derivation of the Tal.09Fe2.39Te4 structure from the NiAs structure type. The NiAs structure is shown projected along [001]. Large circles symbolize non-metal atoms, medium circles metal atoms. The NiAs cell is indicated by solid lines. Removal of metal layers parallel to bc with respect to the NiAs cell (marked by medium open circles) leads to a (fictitious) MTez structure. Displacement of the M atoms in directions indicated by arrows and filling of the tetrahedral sites marked by small circles with Fe atoms lead to the structure of Tal.09Fe2.39Te4 (filled arrows, z = 0 with respect to the NiAs cell; filled circles, z = +~; open arrows, z=½; open circles, z=½+~; Ta(1) is located at z = 0 with respect to the N i A s cell; M(2) is found at z =½). The unit cell of TaLogFe2.39Te4 is indicated by dashed lines.
NiAs structure leads to a (fictitious) MTe2 structure, where in Tal.09Fe2.39Te4 approximately one-half of the M sites are occupied by Ta and the other half occupied by both Ta and Fe. Displacement of the metal atoms in the direction indicated by arrows and partial occupation of the tetrahedral voids marked by points with Fe atoms result in the structure Tal.o9Fea.39Te4 . The displacement from the centre of the MTe6 octahedra leads to a 4 + 2 coordination around the M atoms. The occupation of both octahedral and tetrahedral voids within a layer results in rhomboid-like clusters as described for the closely related structures of MM'Te2 ( M - N b , Ta; M ' - F e , Co, Ni) [7] and M, FeyTe2 (M = Nb, x = 0.89, y = 0.93; M = Ta, x = 0.77, y = 0.90) [8]. In fact, neglecting the mixed occupancy for one of the octahedral sites and the non-stoichiometry, the structure of Tal.o9Fe2.39Te4 represents a monoclinic variant of the orthorhombic MM'T%-type structures ( M - N b , Ta; M ' - F e , Ni; space group Pmna). The
220
J. Neuhausen, W. Tremel / Tai.ooFezsgTe ~ a new tantalum telluride
=b a
e ~ ~ ) e
- ~ F e\~' (2)
I~= ~ - J ~
~
~
~l-ell;
/
\" xx
\\ / ~ % ~ # 11
//// ~"
// "'
/
~÷+.
\
//~
#~'
/
\\/
- -
%
\
I
\
\\
//
\
~, ~ - - ~ ' ~ e
%% // // \\\\\\ // XX //
'\ xx
#:'~
%
II
\\
#////
Fig. 3. Projection of the metal sublattice of TaL09Fe2.39T% along [0011: large hatched circles, Ta(1); large shaded circles, M(2); small shaded circles, Fe. The presence of rhomboid-like clusters is emphasized.
corresponding cobalt compounds MCoTez (M - Nb, Ta) show only a different layer stacking. In comparison with the orthorhombic variants, where only one type of M2M'2 cluster is found, the monoclinic distortion leads to the occurrence of two crystallographically independent rhomboid-like clusters, one being formed by two Ta(1) atoms and two Fe(1) atoms (d~e(1)-ve(n = 2.538(5) •~, dTam_F
as well. Metal-metal bonding is assumed to be responsible for the preference of the iron atoms for the octahedral and tetrahedral sites within the layers instead of the tetrahedral sites between the layers. In contrast with the CdI2 structure where the layers are stacked in the [001] direction corresponding to the original NiAs cell, in Tal.o9Fe2.39Te 4 the stacking direction is parallel to the (100) plane. The stabilization of an MTe2 structure (M = Nb, Ta) following this stacking scheme by inclusion of iron group metals in tetrahedral sites within a layer has been related to the interaction of electron-rich and electron-deficient metals [5a, 7b,d, 8]. In fact, this structure type permits the maximum number of contacts between electron-rich and electron-deficient metals, which is consistent with the high affinity between such metals as proposed by the generalized Lewis acid-base concept of Brewer and Wengert [21]. These effects are assumed to be important for the electronic stability of Tal.09Fe2.a9Te4 as well. Nevertheless, there is a clear tendency when comparing the MM'Tez-type compounds: while the Ni and Co compounds are stoichiometric, the Fe compounds show occupancies less than 100% for the metal sites, indicating a reduced affinity between the 3d metal and tantalum (niobium) compared with the Ni and Co compounds. Regarding the electron concentration on the 3d metals, this is in agreement with Brewer and Wengert's concept, iron being a weaker Lewis base than cobalt or nickel. In Tal.o9Fe2.39Te 4 we even observe (based on our model with a mixed metal site) the formation of rhomboidlike clusters containing four Fe atoms or three Fe atoms
J. Neuhausen, W. Tremel / Tal.ooFe239Te, a new tantalum telluride
and one Ta atom, giving rise to an increased number of Fe-Fe contacts at the expense of Ta-Fe interactions. This points towards a reduced affinity between tantalum and iron compared with its more electron-rich neighbours in the periodic table as well. Efforts to understand the electronic factors responsible for this behaviour as well as measurements of the electrical and magnetic properties are in progress. Detailed results of these investigations and of M6ssbauer as well as photoelectron spectroscopy will be be reported in a separate paper [22].
Acknowledgments This work has been supported by a grant from the Bundesministerium fiir Forschung und Technologie (contract numbers BMFT-05 439GXB3 and 05 5UMGAB). Further support came from the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie. We are grateful to Dr. G. H6fer (Heraeus Quarzschmelze) and Dr. J. Peters (H.C. Starck Co.) for generous gifts of silica tubes and tantalum powder. We are also indebted to Professor G. Henkel (Universit~t-GH-Duisburg) for access to the four-circle diffractometer.
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