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The phase relations and homogeneity range of the iron chevrel compound FexMo6S8−y
Journal of the fess-~ornrno~
53
Metals, 113 (1985) 53 - 63
THE PHASE RELATIONS AND HOMOGENEITY IRON CHEVREL COMPOUND Fe,Mo&& _ y
RANGE OF THE
H. WADA, M. ONODA, H. NOZAKI and I. KAWADA National Institute for Research Gun, Ibaraki 305 {Japnn) (Received August 23,1984;
in Inorganic Materials, 1 -I Namiki, Sakura-Mura, Niihari-
in revised form November 9,1984)
Summary
The phase relations of Chevrel phase compounds in the Fe-MO-S system were investigated by performing experiments in sealed silica tubes at 1000 “C. The results of these experiments enabled the phase diagram of the Fe-MO-S system to be constructed. The presence of the following ternary phase compounds was confirmed by X-ray powder diffraction examination of the quenched samples: monoclinic FeMo&, rhombohedral Fe,Mo&_, and triclinic FeMo&. The iron Chevrel compound Fe,Mo&._, coexists with molybdenum and MO& at low iron contents and with FeMosS, at higher iron contents. With respect to the homogeneity range it has been found that the iron and sulphur contents of rhombohedral FexMo&_y lie in the ranges 1.15
1. Introduction
Ternary molybdenum sulphides with the general formula M~Mo~Ss (M = Cu, Ni, Co, Fe, Mn, Sn, Pb, rare earth elements, etc.) have received much attention in recent years. because of their interesting structures and physical properties [I]. Compounds of this type, which are usually called Chevrel phases, contain building blocks with the formula MO&~ and generally crystallize in the hexagonal-rhombohedral structure [ 21. However, some of these compounds with a small M cation (e.g. iron or copper) exhibit triclinic deformations which occur as a function of the metal content and the temperature [ 3, 41. With respect to the synthesis of rhombohedral phase compounds, it has been reported that single phases with the exact composition MMo&s are often very difficult to obtain owing to the coexistence of secondary phases such as molybdenum and molybdenum sulphides [ 51. The preparation conditions and the homogeneity range of some of the Chevrel sulphides at 0022-5088/85/$3.30
@ Elsevier Sequoia/Printed
in The Netherlands
64
temperatures above 1000 “C have been studied extensively by several investigators [ 6 - lo]. The experimental results for lead, tin and copper compounds indicate that there is a rather large deviation of the [MO] : [ S] ratio from the 6:8 stoichiometry toward the molybdenum-rich side. Furthermore, it has been clarified that this kind of non-stoichiomet~ has a si~ificant effect on the superconducting transition temperature l’, and the cryst~o~aphi~ parameters. With respect to the relation between the structure and the composition of Chevrel phase sulphides, it is still not known whether the non-stoichiometry is accommodated by sulphur vacancies or by excess molybdenum in the lattice. In the case of lead compounds the former model has been supported by Marezio et al. [II] and Hauck [6], but Guillevic et al. [ 12}, Chevrel [ 131 and Fhikiger et al. [ 143 have favoured the latter model. In addition, Hinks et af. [15] have recently suggested that the oxygen impurity is responsible for the variation of T, and c/a in SnMo&s and PbMo,$s, and have proposed an oxygen-containing point defect model on the basis of a structural analysis of smples by means of the neutron diffraction method. This problem has not yet been satisfactorily resolved. Precise data on the non-stoichiometry of Chevrel phase sulphides containing other metals are still absent. More detailed studies of the preparation and crystal chemistry are required to enable defect types of Chevrel compounds to be characterized. The system Fe,Mo6Ss was first studied by Chevrel et at. [3] who found two types of compounds with rhombohedral and triclinic structures. Subsequent single-crystal X-ray analyses and measurements of the electrical resistivity and Mossbauer effect of these two compounds have yielded much information on their lattice and electronic properties [ 16 - 191. However, little is known about their exact composition and the precise shape of their phase boundaries. In addition, the phase relations are not well understood because of the presence of a structural phase transfo~ation which depends on temperature and composition. It is therefore of particular importance not only to obtain as much information as possible on the non-stoichiometry of iron Chevrel compounds but also to establish the phase diagrams of the Fe-MO-S system from a thermodynamic viewpoint. In this paper we report the stability relation of iron molybdenum sulphides, the phase diagrams of the Fe-MO-S system at 1000 “C and the homogeneity range of iron Chevrel sulphides. The relation between the crystal chemistry and the non-stoichiometry of rhombohedral Fe,Mo,Ss_ Y compounds is also described.
2. Experimental
details
Molybdenum (purity, 99.9%), iron (purity, 99.99%) and sulphur (purity, 99.9999%) powders were used as the starting materials for the preparation of the ternary sulphides. MO& and FeS were prepared from the constituent elements at 1100 “C and ‘700 “C respectively and were employed
55
as source materials in all the experiments. The composition of the binary sulphides and the molybdenum powder was determined by means of the weight-loss method on oxidation to MOO, and FezOs. The oxygen impurity in the molybdenum metal was estimated to be less than 0.2 wt.%. Stoichiometric amounts of the elements and sulphides were weighed, mixed in an agate mortar, pressed into pellets and sealed in an evacuated silica tube under a pressure of less than 10e3 Torr. An Al,Oa crucible was used as the sample container inside the tube to avoid the reaction of the silica with the pellets at higher temperatures. Prior to the equilibrium study of the Fe-MO-S system, the stability relations of the compounds were examined to obtain the information on the reaction kinetics needed to determine whether the equilibrium state had been attained. For this purpose, samples with the bulk composition Fe1.25M06S7.75 were heated at various temperatures between 500 and 1050 “C for 1 week and were checked by means of X-ray examination. In the equilibrium studies the heat treatment was carried out at 1000 “C for 24 h and then the tube was quenched with water. Samples thus obtained were identified using an X-ray diffractometer with nickel-filtered Cu Ko radiation. Pure silicon was used as the internal standard for measurement of the lattice parameters of the Chevrel phase. The lattice parameters were calculated using the least-squares program RSLCS of UNICS. Samples were also studied using reflected-light microscopy but the phases were too fine grained; therefore electron diffraction and scanning electron microscope methods were employed to examine the phase relations. In addition, the presence of ferromagnetic free iron was effectively checked using a bar magnet. The density was measured by means of Archimedes displacement of a sample in CCIB. 3. Results and discussion 3.1. The stability relations of iron molybdenum sulphides Figure 1 shows the experimental results obtained for the stability relations of compounds with bulk composition Fe1.25M06S7.7s at various temperatures. The relative abundance of the phases observed was estimated qualitatively from comparison of the intensities of the main X-ray reflection peaks, i.e. (002) for hexagonal 2H-MoSz, (110) for cubic molybdenum, (100) for rhombohedral Fe,MogSs_Y and (002) for monoclinic FeMo,S+ In the run at 500 “C no ternary sulphide phase was observed in the X-ray powder pattern after heating mixtures of the source materials for 1 week. In the run at 550 ‘C, however, FeMo& was easily formed as a first ternary phase according to the reaction $ MO + + MoS, + FeS -
FeMo&
(1)
This phase was stable in the temperature range 500 - 600 “C and coexisted with residual excess molybdenum and MO&. However, it is particularly
56
25
600
700 800 Temperature
900 PC)
1000
Fig. 1. Stability relation of an iron Fe~.&!?o&7.75 vs. temperature.
molybdenum
sulphide
with
bulk
composition
interesting that a small rhombohedral (100) peak of the iron Chevrel phase was observed at the tail position of the broad (002) reflection of 2H-MO& on X-ray patterns of the sample treated at 600 “C. This means that the lower limit of stability of iron Chevrel phase sulphide may be located near 600 “C, which is very close to the decomposition temperature (594 f 5 “C) of Cu,Mo&s_, [20], The iron Chevrel phase was dominant over FeMo,S4 in the run at 700 “C, and most of FeMo&& disappeared at SO0 “C. These results indicate that the reaction 4+Y xFeMo,S, + -MO+ 2
S-4x-y 2
MoSz = Fe,Mo&-,
(2)
where x = 1.25 and y = 0.25, takes place in the system as a second step. At temperatures above 800 “C only iron Chevrel sulphide was obtained as a single phase. With respect to the reaction kinetics, it was found that heat treatment for 10 min at 1000 “C enables the initial source mixtures to transform completely into the iron Chevrel phase; no evidence of further phase changes was observed on subsequent prolonged heating. It follows from this result that the equilibrium state of compounds in the Fe-MO-S system is attained in relatively short times at higher temperatures such as 1000 “C. In equilibrium studies of the Fe-MO-S system, therefore, we considered that heating times of about 1 day were adequate for investigations of the phase relations at 1000 “C!. 3.2. The phase diagrams of the Fe-MO-S system at 1000 “C Tentative phase diagrams of the Fe-MO-S system at 1000 “C were constructed on the basis of our experimental results and from the binary MO-S, MO-Fe and Fe-S phase diagrams [ 211. The phase relations and the compositional limits are summarized in Figs. 2 and 3. The boundaries between the phase assemblages were determined mainly by X-ray diffraction of quenched samples with various compositions. In the Fe-S system hexagonal Fe,_ %S and the liquid phase exist as binary phases at 1000 ‘C. The position of the latter is situated at about
57
at.% MO
Fig. 2. Phase diagram of the Fe-Mo-S system at 1000 “C (Ch, Chevrel phase ): 0, experimental points. The positions of most of the tie lines in the divariant regions are conjectural.
I
I
75
8.0
8.5 8-Y
-
Fig. 3. The single-phase field and phase relations of Fe,MosSs_, at 1000 “C: A, FeMo&L+; 0, Fe,Mo6Ss_y; - - -, tie lines between Fe,Mo&_Y and FeMo&.
44 at.% S on the FeS-Fe boundary. In the MO-Fe system cubic molybdenum, rhombohedral Fe&lo? and the cubic a-Fe-M0 and y-Fe phases exist at 1000 “C. In the MO-S system hexagonal MoSz and monoclinic Mo,Ss exist at 1000 “C [ 221. The former has a stoichiometric composition very close to an [S]/[Mo] ratio of 2, but the latter seems to be a non-stoichiometric compound rich in molybdenum metal. The phase limits of MO& observed in the present work range from MoSie4s6 to Mo&,~~. With respect to the
58
non-stoichiometry of MO&, our results are in good agreement with those reported by Morimoto and KulIerud [ 231 and Suzuki et al. [24]. In addition to the binary phases described above, we confirmed the presence of the following ternary phase compounds in the Fe-MO-S system: monoclinic FeMo&, rhombohedral FeXMo6S8_y and triclinic FeMo&. The phase relations of these compounds are shown in detail in Fig. 3. A few remarks should be made here regarding the phase relation between FexMo&&, and FeMosS,. Chevrel et al. [3] first reported that the crystal structure of Fe,Mo,S, is rhombohedral for small values of X, but becomes triclinic when x is increased at room temperature. Our results are consistent in this regard except for the presence of a sulphur vacancy. However, it should be noted that some of the phase relations in Fig. 3 exhibit the nonequilibrium state because of the coexistence of four solid phases (MO&, FeMo,S4, FexMo6S8_y and FeMo&,). This difficulty may arise from the non-quenchability of the high temperature phase. According to Yvon et al. [17] triclinic FeMo&, transforms into the rhombohedral modification at temperatures above 200 “C owing to the order-disorder transition of the iron atoms. Since the quenched sample of FeMosS, reveals only the triclinic phase, it is possible that this order-disorder transition is reversible. Hence the high temperature phase may not be quenchable. With respect to the real phase relations, the possibility exists of forming a solid solution of the rhombohedral phase in the range Fe,Mo$s_, - FeMo& at high temperatures because both end members have the same structure. As shown in Fig. 2, we assumed the existence of such a solid solution phase at 1000 “C. Features of the overall phase relations seem to favour this assumption. Therefore it is reasonable to assume that the triclinic phase FeMo&, forms within its solid solution field owing to a phase separation on quenching to room temperature. The solubility of a third component in the binary phase is found to be relatively small. For example, in the case of single-crystal FeS coexisting with the triclinic phase electron probe microanalyses revealed that the molybdenum content was less than 1.2 wt.%. 3.3. The homogeneity range of iron Chevrel sulphide The homogeneity range of the iron Chevrel phase is shown in Fig. 3, where the formula FexMo6S8_y is used to describe the rhombohedral phase because we chose a perfect molybdenum sublattice corresponding to an Mo6 cluster. In the graphical representations our experimental data are plotted as functions of the iron content x and the sulphur content 8 - y. The limiting compositions of iron and sulphur in Fe,Mo$s_ y vary in the ranges 1.15 < x < 1.35 and 7.70 =G8 -y < 7.90 respectively. A two-phase field exists in the region between FexMo6Ss_y and FeMo&. At low iron contents Fe,Mo6Ss_y coexists with MO& and molybdenum, and at high iron contents it coexists with FeMo&. In contrast, FeMo& coexists with iron, FeS, FeMo,S,, FesMo, and molybdenum. MO& is in equilibrium with compounds in the region between Fei.,s Mo&J,.~~ and Fe1.5aMo6S,.9,. It is particularly
59
interesting that the single-phase region of the rhombohedral phase does not include the stoichiometric composition FeMo& which suggests the presence of a large number of sulphur defects. 3.4. The X-ray diffraction patterns of the samples X-ray diffractogram traces of Fe,Mo,S,.s with x ranging from 1.20 to 2.00 are reproduced in Fig. 4. As has been reported by Chevrel et al. [3], the lattice distortion of iron Chevrel phases with the rhombohedral structure (space group, R3) increases with increasing iron content. The X-ray powder patterns of FerV4sMo6S7.sshow evidence of the presence of small peaks in addition to those characteristic of the rhombohedral phase. All of them are assigned to reflections of the triclinic phase (space group, Pi) such as (liO), (iio), (2io), (021) and (221). These reflection intensities gradually increase with increasing iron content. The splitting of the (111) reflection observed for compositions with 3c= 1.55 and x = 1.60 is characteristic of the coexistence of rhombohedral and triclinic phases. FexMOgs7.8
x=1.20
1.45 1.55 1.60
2.00 20
30
40 Degrees
50 2 6 ECUb)
60
Fig. 4. X-ray powder diffraction patterns of Fe,MosS,.s.
3.5. Dependence of the lattice parameters sulphides on composition
and density
of the iron Chew-e1
The hexagonal lattice parameters of some of the iron Chevrel sulphides prepared are shown as a function of composition in Fig. 5. The lattice constants a and c and the unit cell volume V increase linearly with increasing iron content within the single-phase field. However, it should be noted that they do not become constant in the two-phase field Rn + Ta (Rn, rhombohedral; Ta, triclinic), but decrease with increasing iron content of the bulk
60 X 1.0
in
FexMo&+Y
1.2
1.6
1.4
aH&
I 9.57
9.55
10.28
9.53
VHtWI
1.080
813
811
1,078
809
807
7
8
.S
10
at.96 Fe Fig. 5. Variation of the lattice parameters a, c, c/a and V of the hexagonal-rhombohedral compound Fe,Mo&_, with the iron content x: 0, single-phase field for Fe,Mo&.s; O, single-phase field for FeXMo&.,; 0, two-phase field for Fe,Mo&.s; m, two-phase field for Fe,Mo&.s; RH, rhombohedral; TR, triclinic.
composition Fe,Mo6S,.s. This behaviour can be explained by considering the change in the compositions of the two coexisting phases. As shown in Fig. 5, the tie line connecting the equilibrium composition between the RN and Ta phases can be drawn such that it traverses the composition line FexMo&s, and hence the position of the rhombohe~l phase is shifted to the sulphurpoor side when the bulk iron content increases. Although perfect consistency is not always observed, the trend in the change in lattice parameters in the Rn f TR field is in good agreement with that corresponding to the change of composition from FexMo&s to Fe,Mo,S,,. The phase limit of the rhombohedral iron Chevrel sulphide lies in the range 7.7 - 9 at.% Fe. The volume of the unit cell changes by about 0.5% within this composition range. The variation in the corresponding rhombohedral parameters uR with the angle Q!is shown in Fig. 6. The lattice parameter ca increases from 6.478 to 6.494 as the iron content varies from 8 to 9 at.% whereas the angle cy decreases from 94.626’ to 94.603’. The characteristics of this variation are similar to those of Pbl.O,MO&-~ reported by Hauck [6]. In the rhombohedral structure of the iron Chevrel phase, the iron atoms are distributed over 12 available sites which are composed of six equivalent
61
‘\
\,
94.60 94.61 94.62 94.63 *(Ol
Fig. 6. Relation
between
the rhombohedral
lattice
length aR and the angle (Y.
inner sites M, and six equivalent outer sites M2. The degree of occupation of the iron atom obviously varies depending on the site and also as a function of the overall iron content. With respect to the composition dependence of the lattice parameters it can be assumed that the increase in the average iron occupancy on these sites enhances the repulsive coulombic Fe-Fe and FeMO interactions, causing an enlargement of the lattice dimension. By comparing the change in o with that in the lattice parameters a, c and c/a, it can be inferred that repulsive force of interaction along the a direction is slightly reduced relative to that along the c direction as the iron content of the compound is increased. Density measurements were carried out to determine the defect type of the rhombohedral iron Chevrel phase sulphide. The lattice parameters and densities of several samples are listed in Table 1. Scanning electron micrographs of powder samples of the corresponding rhombohedral phase Fe 1.25M~6S,.7are shown in Fig. 7. The morphology of the fine grains is not clear, but the samples seem to be homogeneous. The experimental results indicate that the measured density is in good agreement (within less than 1%) with the value calculated from the sulphur vacancy model. Vacancies can be introduced in the special position of sulphur sites on the g-fold axis, as suggested by Marezio et al. [ll]. However, more detailed studies of the structure and measurements of the densities of large single crystals are necessary for a better understanding of the defect type of iron Chevrel sulphides. TABLE Lattice
1 parameters
and densities
of several iron Chevrel
Compound
a (8)
c (A)
F%zoM0&.7o Fel.zsM%S7.7o Fe1.3&k&.7o
9.524 f 4 9.531 f 3 9.540 * 2
10.276 10.288 10.292
+ 4 + 3 f 2
sulphides
d obs k cmP3)
d cak (g cmP3)
5.48 5.49 5.52
5.49 5.49 5.52
62
Fig. 7. Scanning electron micrographs of powder samples of rhombohedral Fe1_25M06S7.7.
4. Conclusions The phase relations and the homogeneity range of iron Chevrel phase sulphides have been clarified and a tentative phase diagram has been established for the Fe-MO-S system at 1000 “C. The presence of the following ternary compounds has been confirmed from X-ray phase ident~ication of the quenched samples: monoclinic FeMo,S,, rhombohedral Fe,Mo&_, and triclinic FeMo$$. The iron Chevrel phase compound Fe,Mo&!&_, coexists with molybdenum and MO& at low iron contents and with FeMosS, at high iron contents, The single-phase region of Fe,Mo&$_, is found to be situated at 1.15 < x G 1.35 and 7.70 < 3 -y G 7.90. .C~sta~o~aphic studies have shown that the lattice parameters a, c and c/a increase with increasing iron content. The results of density measurements suggest strongly that the nonstoichiometry of the rhombohedral iron Chevrel phase arises from a sulphur vacancy rather than from excess molybdenum in the lattice. Acknowledgments The authors wish to thank Mr. K. Kosuda for performing the electron probe microanalysis and Dr. H. Kanda for the scanning electron microscopy observations.
63
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Q. Fischer,AppZ. Phys., 16 (1978) 1. K. Yvon, Curr. Top. Mater. Sci., 3 (1979) 53 - 129. R. Chevrel, M. Sergent and J. Prigent, J. Solid State Chem., 3 (1971) 515. R. Fliikiger, R. Baillif, J. Muller and K. Yvon, J. Less-Common Met., 72 (1980) 193. M. Sergent, R. Chewel, C. Rossel and 8. Fischer, J. Less-Common Met., 58 (1978 179. J. Hauck, Mater. Res. Bull., 12 (1977) 1015. H. A. Wagner and H. C. Freyhardt, J. Phys. Chem. Solids, 43 (1982) 177. K. Y. Cheungand B. C. H. Steele, Mater. Res. Bull., 15 (1980) 1717. S. Yamamoto, K. Matsui, M. Wakihara and M. Taniguchi, Mater. Res. Bull., 18 (1983 1311. G. Krabbes and H. Oppermann, 2. Anorg. AZZg.Chem., 481 (1981) 13. M. Marezio, P. D. Dernier, J. P. Remeika, E. Corenzwit and B. T. Matthias, Mater. Res. BUZZ.,8 (1973) 657. J. Guillevic, H. LeStrat and D. Grandjean, Acta Crystallogr., Sect. B, 32 (1976) 1342. R. Chevrel, Thesis B112, University of Rennes, 1974. R. Fliikiger, R. Baillif and E. Walker, Mater. Res. Bull., 13 (1978) 743. D. Hinks, J. Jorgensen and H. Li,Phys. Reu. Lett., 51 (1983) 1911. J. Guillevic, 0. Bars and D. Grandjean,Acta Crystallogr., Sect. B, 32 (1976) 1338. K. Yvon, R. Chevrel and M. Sergent, Acta Crystallogr., Sect. B 36 (1980) 685. A. C. Lawson, Mater. Res. Bull., 7 (1972) 773. F. Y. Fradin, B. D. Dunlap, G. K. Shenoy and C. W. Kimball, in M. B. Maple and 8. Fischer (eds.), Top. Curr. Phys., 34 (1982) 201. B. Grover and G. H. Moh, Neues Jahrb. Miner. Mona&h., (1969) 529. F. A. Shunk, Constitution ofBinary Alloys, McGraw-Hill, New York, 1969. G. H. Moh, Top. Curr. Chem., 76 (1978) 108. N. Morimoto and G. Kullerud, Carnegie Inst. Washington Yearb., 60 (1961) 143. Y. Suzuki, T. Uchida, M. Wakihara and M. Taniguchi, Mater. Res. Bull., 16 (1981) 1085.
53
Metals, 113 (1985) 53 - 63
THE PHASE RELATIONS AND HOMOGENEITY IRON CHEVREL COMPOUND Fe,Mo&& _ y
RANGE OF THE
H. WADA, M. ONODA, H. NOZAKI and I. KAWADA National Institute for Research Gun, Ibaraki 305 {Japnn) (Received August 23,1984;
in Inorganic Materials, 1 -I Namiki, Sakura-Mura, Niihari-
in revised form November 9,1984)
Summary
The phase relations of Chevrel phase compounds in the Fe-MO-S system were investigated by performing experiments in sealed silica tubes at 1000 “C. The results of these experiments enabled the phase diagram of the Fe-MO-S system to be constructed. The presence of the following ternary phase compounds was confirmed by X-ray powder diffraction examination of the quenched samples: monoclinic FeMo&, rhombohedral Fe,Mo&_, and triclinic FeMo&. The iron Chevrel compound Fe,Mo&._, coexists with molybdenum and MO& at low iron contents and with FeMosS, at higher iron contents. With respect to the homogeneity range it has been found that the iron and sulphur contents of rhombohedral FexMo&_y lie in the ranges 1.15
1. Introduction
Ternary molybdenum sulphides with the general formula M~Mo~Ss (M = Cu, Ni, Co, Fe, Mn, Sn, Pb, rare earth elements, etc.) have received much attention in recent years. because of their interesting structures and physical properties [I]. Compounds of this type, which are usually called Chevrel phases, contain building blocks with the formula MO&~ and generally crystallize in the hexagonal-rhombohedral structure [ 21. However, some of these compounds with a small M cation (e.g. iron or copper) exhibit triclinic deformations which occur as a function of the metal content and the temperature [ 3, 41. With respect to the synthesis of rhombohedral phase compounds, it has been reported that single phases with the exact composition MMo&s are often very difficult to obtain owing to the coexistence of secondary phases such as molybdenum and molybdenum sulphides [ 51. The preparation conditions and the homogeneity range of some of the Chevrel sulphides at 0022-5088/85/$3.30
@ Elsevier Sequoia/Printed
in The Netherlands
64
temperatures above 1000 “C have been studied extensively by several investigators [ 6 - lo]. The experimental results for lead, tin and copper compounds indicate that there is a rather large deviation of the [MO] : [ S] ratio from the 6:8 stoichiometry toward the molybdenum-rich side. Furthermore, it has been clarified that this kind of non-stoichiomet~ has a si~ificant effect on the superconducting transition temperature l’, and the cryst~o~aphi~ parameters. With respect to the relation between the structure and the composition of Chevrel phase sulphides, it is still not known whether the non-stoichiometry is accommodated by sulphur vacancies or by excess molybdenum in the lattice. In the case of lead compounds the former model has been supported by Marezio et al. [II] and Hauck [6], but Guillevic et al. [ 12}, Chevrel [ 131 and Fhikiger et al. [ 143 have favoured the latter model. In addition, Hinks et af. [15] have recently suggested that the oxygen impurity is responsible for the variation of T, and c/a in SnMo&s and PbMo,$s, and have proposed an oxygen-containing point defect model on the basis of a structural analysis of smples by means of the neutron diffraction method. This problem has not yet been satisfactorily resolved. Precise data on the non-stoichiometry of Chevrel phase sulphides containing other metals are still absent. More detailed studies of the preparation and crystal chemistry are required to enable defect types of Chevrel compounds to be characterized. The system Fe,Mo6Ss was first studied by Chevrel et at. [3] who found two types of compounds with rhombohedral and triclinic structures. Subsequent single-crystal X-ray analyses and measurements of the electrical resistivity and Mossbauer effect of these two compounds have yielded much information on their lattice and electronic properties [ 16 - 191. However, little is known about their exact composition and the precise shape of their phase boundaries. In addition, the phase relations are not well understood because of the presence of a structural phase transfo~ation which depends on temperature and composition. It is therefore of particular importance not only to obtain as much information as possible on the non-stoichiometry of iron Chevrel compounds but also to establish the phase diagrams of the Fe-MO-S system from a thermodynamic viewpoint. In this paper we report the stability relation of iron molybdenum sulphides, the phase diagrams of the Fe-MO-S system at 1000 “C and the homogeneity range of iron Chevrel sulphides. The relation between the crystal chemistry and the non-stoichiometry of rhombohedral Fe,Mo,Ss_ Y compounds is also described.
2. Experimental
details
Molybdenum (purity, 99.9%), iron (purity, 99.99%) and sulphur (purity, 99.9999%) powders were used as the starting materials for the preparation of the ternary sulphides. MO& and FeS were prepared from the constituent elements at 1100 “C and ‘700 “C respectively and were employed
55
as source materials in all the experiments. The composition of the binary sulphides and the molybdenum powder was determined by means of the weight-loss method on oxidation to MOO, and FezOs. The oxygen impurity in the molybdenum metal was estimated to be less than 0.2 wt.%. Stoichiometric amounts of the elements and sulphides were weighed, mixed in an agate mortar, pressed into pellets and sealed in an evacuated silica tube under a pressure of less than 10e3 Torr. An Al,Oa crucible was used as the sample container inside the tube to avoid the reaction of the silica with the pellets at higher temperatures. Prior to the equilibrium study of the Fe-MO-S system, the stability relations of the compounds were examined to obtain the information on the reaction kinetics needed to determine whether the equilibrium state had been attained. For this purpose, samples with the bulk composition Fe1.25M06S7.75 were heated at various temperatures between 500 and 1050 “C for 1 week and were checked by means of X-ray examination. In the equilibrium studies the heat treatment was carried out at 1000 “C for 24 h and then the tube was quenched with water. Samples thus obtained were identified using an X-ray diffractometer with nickel-filtered Cu Ko radiation. Pure silicon was used as the internal standard for measurement of the lattice parameters of the Chevrel phase. The lattice parameters were calculated using the least-squares program RSLCS of UNICS. Samples were also studied using reflected-light microscopy but the phases were too fine grained; therefore electron diffraction and scanning electron microscope methods were employed to examine the phase relations. In addition, the presence of ferromagnetic free iron was effectively checked using a bar magnet. The density was measured by means of Archimedes displacement of a sample in CCIB. 3. Results and discussion 3.1. The stability relations of iron molybdenum sulphides Figure 1 shows the experimental results obtained for the stability relations of compounds with bulk composition Fe1.25M06S7.7s at various temperatures. The relative abundance of the phases observed was estimated qualitatively from comparison of the intensities of the main X-ray reflection peaks, i.e. (002) for hexagonal 2H-MoSz, (110) for cubic molybdenum, (100) for rhombohedral Fe,MogSs_Y and (002) for monoclinic FeMo,S+ In the run at 500 “C no ternary sulphide phase was observed in the X-ray powder pattern after heating mixtures of the source materials for 1 week. In the run at 550 ‘C, however, FeMo& was easily formed as a first ternary phase according to the reaction $ MO + + MoS, + FeS -
FeMo&
(1)
This phase was stable in the temperature range 500 - 600 “C and coexisted with residual excess molybdenum and MO&. However, it is particularly
56
25
600
700 800 Temperature
900 PC)
1000
Fig. 1. Stability relation of an iron Fe~.&!?o&7.75 vs. temperature.
molybdenum
sulphide
with
bulk
composition
interesting that a small rhombohedral (100) peak of the iron Chevrel phase was observed at the tail position of the broad (002) reflection of 2H-MO& on X-ray patterns of the sample treated at 600 “C. This means that the lower limit of stability of iron Chevrel phase sulphide may be located near 600 “C, which is very close to the decomposition temperature (594 f 5 “C) of Cu,Mo&s_, [20], The iron Chevrel phase was dominant over FeMo,S4 in the run at 700 “C, and most of FeMo&& disappeared at SO0 “C. These results indicate that the reaction 4+Y xFeMo,S, + -MO+ 2
S-4x-y 2
MoSz = Fe,Mo&-,
(2)
where x = 1.25 and y = 0.25, takes place in the system as a second step. At temperatures above 800 “C only iron Chevrel sulphide was obtained as a single phase. With respect to the reaction kinetics, it was found that heat treatment for 10 min at 1000 “C enables the initial source mixtures to transform completely into the iron Chevrel phase; no evidence of further phase changes was observed on subsequent prolonged heating. It follows from this result that the equilibrium state of compounds in the Fe-MO-S system is attained in relatively short times at higher temperatures such as 1000 “C. In equilibrium studies of the Fe-MO-S system, therefore, we considered that heating times of about 1 day were adequate for investigations of the phase relations at 1000 “C!. 3.2. The phase diagrams of the Fe-MO-S system at 1000 “C Tentative phase diagrams of the Fe-MO-S system at 1000 “C were constructed on the basis of our experimental results and from the binary MO-S, MO-Fe and Fe-S phase diagrams [ 211. The phase relations and the compositional limits are summarized in Figs. 2 and 3. The boundaries between the phase assemblages were determined mainly by X-ray diffraction of quenched samples with various compositions. In the Fe-S system hexagonal Fe,_ %S and the liquid phase exist as binary phases at 1000 ‘C. The position of the latter is situated at about
57
at.% MO
Fig. 2. Phase diagram of the Fe-Mo-S system at 1000 “C (Ch, Chevrel phase ): 0, experimental points. The positions of most of the tie lines in the divariant regions are conjectural.
I
I
75
8.0
8.5 8-Y
-
Fig. 3. The single-phase field and phase relations of Fe,MosSs_, at 1000 “C: A, FeMo&L+; 0, Fe,Mo6Ss_y; - - -, tie lines between Fe,Mo&_Y and FeMo&.
44 at.% S on the FeS-Fe boundary. In the MO-Fe system cubic molybdenum, rhombohedral Fe&lo? and the cubic a-Fe-M0 and y-Fe phases exist at 1000 “C. In the MO-S system hexagonal MoSz and monoclinic Mo,Ss exist at 1000 “C [ 221. The former has a stoichiometric composition very close to an [S]/[Mo] ratio of 2, but the latter seems to be a non-stoichiometric compound rich in molybdenum metal. The phase limits of MO& observed in the present work range from MoSie4s6 to Mo&,~~. With respect to the
58
non-stoichiometry of MO&, our results are in good agreement with those reported by Morimoto and KulIerud [ 231 and Suzuki et al. [24]. In addition to the binary phases described above, we confirmed the presence of the following ternary phase compounds in the Fe-MO-S system: monoclinic FeMo&, rhombohedral FeXMo6S8_y and triclinic FeMo&. The phase relations of these compounds are shown in detail in Fig. 3. A few remarks should be made here regarding the phase relation between FexMo&&, and FeMosS,. Chevrel et al. [3] first reported that the crystal structure of Fe,Mo,S, is rhombohedral for small values of X, but becomes triclinic when x is increased at room temperature. Our results are consistent in this regard except for the presence of a sulphur vacancy. However, it should be noted that some of the phase relations in Fig. 3 exhibit the nonequilibrium state because of the coexistence of four solid phases (MO&, FeMo,S4, FexMo6S8_y and FeMo&,). This difficulty may arise from the non-quenchability of the high temperature phase. According to Yvon et al. [17] triclinic FeMo&, transforms into the rhombohedral modification at temperatures above 200 “C owing to the order-disorder transition of the iron atoms. Since the quenched sample of FeMosS, reveals only the triclinic phase, it is possible that this order-disorder transition is reversible. Hence the high temperature phase may not be quenchable. With respect to the real phase relations, the possibility exists of forming a solid solution of the rhombohedral phase in the range Fe,Mo$s_, - FeMo& at high temperatures because both end members have the same structure. As shown in Fig. 2, we assumed the existence of such a solid solution phase at 1000 “C. Features of the overall phase relations seem to favour this assumption. Therefore it is reasonable to assume that the triclinic phase FeMo&, forms within its solid solution field owing to a phase separation on quenching to room temperature. The solubility of a third component in the binary phase is found to be relatively small. For example, in the case of single-crystal FeS coexisting with the triclinic phase electron probe microanalyses revealed that the molybdenum content was less than 1.2 wt.%. 3.3. The homogeneity range of iron Chevrel sulphide The homogeneity range of the iron Chevrel phase is shown in Fig. 3, where the formula FexMo6S8_y is used to describe the rhombohedral phase because we chose a perfect molybdenum sublattice corresponding to an Mo6 cluster. In the graphical representations our experimental data are plotted as functions of the iron content x and the sulphur content 8 - y. The limiting compositions of iron and sulphur in Fe,Mo$s_ y vary in the ranges 1.15 < x < 1.35 and 7.70 =G8 -y < 7.90 respectively. A two-phase field exists in the region between FexMo6Ss_y and FeMo&. At low iron contents Fe,Mo6Ss_y coexists with MO& and molybdenum, and at high iron contents it coexists with FeMo&. In contrast, FeMo& coexists with iron, FeS, FeMo,S,, FesMo, and molybdenum. MO& is in equilibrium with compounds in the region between Fei.,s Mo&J,.~~ and Fe1.5aMo6S,.9,. It is particularly
59
interesting that the single-phase region of the rhombohedral phase does not include the stoichiometric composition FeMo& which suggests the presence of a large number of sulphur defects. 3.4. The X-ray diffraction patterns of the samples X-ray diffractogram traces of Fe,Mo,S,.s with x ranging from 1.20 to 2.00 are reproduced in Fig. 4. As has been reported by Chevrel et al. [3], the lattice distortion of iron Chevrel phases with the rhombohedral structure (space group, R3) increases with increasing iron content. The X-ray powder patterns of FerV4sMo6S7.sshow evidence of the presence of small peaks in addition to those characteristic of the rhombohedral phase. All of them are assigned to reflections of the triclinic phase (space group, Pi) such as (liO), (iio), (2io), (021) and (221). These reflection intensities gradually increase with increasing iron content. The splitting of the (111) reflection observed for compositions with 3c= 1.55 and x = 1.60 is characteristic of the coexistence of rhombohedral and triclinic phases. FexMOgs7.8
x=1.20
1.45 1.55 1.60
2.00 20
30
40 Degrees
50 2 6 ECUb)
60
Fig. 4. X-ray powder diffraction patterns of Fe,MosS,.s.
3.5. Dependence of the lattice parameters sulphides on composition
and density
of the iron Chew-e1
The hexagonal lattice parameters of some of the iron Chevrel sulphides prepared are shown as a function of composition in Fig. 5. The lattice constants a and c and the unit cell volume V increase linearly with increasing iron content within the single-phase field. However, it should be noted that they do not become constant in the two-phase field Rn + Ta (Rn, rhombohedral; Ta, triclinic), but decrease with increasing iron content of the bulk
60 X 1.0
in
FexMo&+Y
1.2
1.6
1.4
aH&
I 9.57
9.55
10.28
9.53
VHtWI
1.080
813
811
1,078
809
807
7
8
.S
10
at.96 Fe Fig. 5. Variation of the lattice parameters a, c, c/a and V of the hexagonal-rhombohedral compound Fe,Mo&_, with the iron content x: 0, single-phase field for Fe,Mo&.s; O, single-phase field for FeXMo&.,; 0, two-phase field for Fe,Mo&.s; m, two-phase field for Fe,Mo&.s; RH, rhombohedral; TR, triclinic.
composition Fe,Mo6S,.s. This behaviour can be explained by considering the change in the compositions of the two coexisting phases. As shown in Fig. 5, the tie line connecting the equilibrium composition between the RN and Ta phases can be drawn such that it traverses the composition line FexMo&s, and hence the position of the rhombohe~l phase is shifted to the sulphurpoor side when the bulk iron content increases. Although perfect consistency is not always observed, the trend in the change in lattice parameters in the Rn f TR field is in good agreement with that corresponding to the change of composition from FexMo&s to Fe,Mo,S,,. The phase limit of the rhombohedral iron Chevrel sulphide lies in the range 7.7 - 9 at.% Fe. The volume of the unit cell changes by about 0.5% within this composition range. The variation in the corresponding rhombohedral parameters uR with the angle Q!is shown in Fig. 6. The lattice parameter ca increases from 6.478 to 6.494 as the iron content varies from 8 to 9 at.% whereas the angle cy decreases from 94.626’ to 94.603’. The characteristics of this variation are similar to those of Pbl.O,MO&-~ reported by Hauck [6]. In the rhombohedral structure of the iron Chevrel phase, the iron atoms are distributed over 12 available sites which are composed of six equivalent
61
‘\
\,
94.60 94.61 94.62 94.63 *(Ol
Fig. 6. Relation
between
the rhombohedral
lattice
length aR and the angle (Y.
inner sites M, and six equivalent outer sites M2. The degree of occupation of the iron atom obviously varies depending on the site and also as a function of the overall iron content. With respect to the composition dependence of the lattice parameters it can be assumed that the increase in the average iron occupancy on these sites enhances the repulsive coulombic Fe-Fe and FeMO interactions, causing an enlargement of the lattice dimension. By comparing the change in o with that in the lattice parameters a, c and c/a, it can be inferred that repulsive force of interaction along the a direction is slightly reduced relative to that along the c direction as the iron content of the compound is increased. Density measurements were carried out to determine the defect type of the rhombohedral iron Chevrel phase sulphide. The lattice parameters and densities of several samples are listed in Table 1. Scanning electron micrographs of powder samples of the corresponding rhombohedral phase Fe 1.25M~6S,.7are shown in Fig. 7. The morphology of the fine grains is not clear, but the samples seem to be homogeneous. The experimental results indicate that the measured density is in good agreement (within less than 1%) with the value calculated from the sulphur vacancy model. Vacancies can be introduced in the special position of sulphur sites on the g-fold axis, as suggested by Marezio et al. [ll]. However, more detailed studies of the structure and measurements of the densities of large single crystals are necessary for a better understanding of the defect type of iron Chevrel sulphides. TABLE Lattice
1 parameters
and densities
of several iron Chevrel
Compound
a (8)
c (A)
F%zoM0&.7o Fel.zsM%S7.7o Fe1.3&k&.7o
9.524 f 4 9.531 f 3 9.540 * 2
10.276 10.288 10.292
+ 4 + 3 f 2
sulphides
d obs k cmP3)
d cak (g cmP3)
5.48 5.49 5.52
5.49 5.49 5.52
62
Fig. 7. Scanning electron micrographs of powder samples of rhombohedral Fe1_25M06S7.7.
4. Conclusions The phase relations and the homogeneity range of iron Chevrel phase sulphides have been clarified and a tentative phase diagram has been established for the Fe-MO-S system at 1000 “C. The presence of the following ternary compounds has been confirmed from X-ray phase ident~ication of the quenched samples: monoclinic FeMo,S,, rhombohedral Fe,Mo&_, and triclinic FeMo$$. The iron Chevrel phase compound Fe,Mo&!&_, coexists with molybdenum and MO& at low iron contents and with FeMosS, at high iron contents, The single-phase region of Fe,Mo&$_, is found to be situated at 1.15 < x G 1.35 and 7.70 < 3 -y G 7.90. .C~sta~o~aphic studies have shown that the lattice parameters a, c and c/a increase with increasing iron content. The results of density measurements suggest strongly that the nonstoichiometry of the rhombohedral iron Chevrel phase arises from a sulphur vacancy rather than from excess molybdenum in the lattice. Acknowledgments The authors wish to thank Mr. K. Kosuda for performing the electron probe microanalysis and Dr. H. Kanda for the scanning electron microscopy observations.
63
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