Structure of Mo7S8: A new binary sulfide synthesized by self molybdenum intercalation

Article abstract of DOI:10.1016/S0025-5408(97)00191-8

A new Mo₇S₈ binary sulfide structure and its crystal growth are reported. It is the first example of Mo atoms in the channels of the Chevrel phases. The single crystal structure determination indicates extra molybdenum atoms located in the chalcogen site that is centered at the rhombohedral unit-cell origin. The nature and valence state of these extra atoms are discussed through chemical and physical properties. 1998 Elsevier Science Ltd.

Full text of DOI:10.1016/S0025-5408(97)00191-8

Materials Research Bulletin, Vol. 33, No. 1, pp. 43–57, 1998
Copyright © 1998 Elsevier Science Ltd
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PII S0025-5408(97)00191-8
STRUCTURE OF Mo7S8: A NEW BINARY SULFIDE SYNTHESIZED
BY SELF MOLYBDENUM INTERCALATION
Ste´phanie Belin,* Roger Chevrel, and Marcel Sergent
Laboratoire de Chimie du Solide et Inorganique Mole´culaire (LCSIM), UMR CNRS
6511, Universite´ de Rennes-1, Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France
(Refereed)
(Received May 29, 1997; Accepted May 29, 1997)
ABSTRACT
A new Mo7S8 binary sulfide structure and its crystal growth are reported. It is
the first example of Mo atoms in the channels of the Chevrel phases. The
single crystal structure determination indicates extra molybdenum atoms lo-
cated in the chalcogen site that is centered at the rhombohedral unit-cell
origin. The nature and valence state of these extra atoms are discussed through
chemical and physical properties. © 1998 Elsevier Science Ltd
KEYWORDS: A. chalcogenides, B. crystal growth, C. X-ray diffraction, D.
crystal structure
INTRODUCTION
The Chevrel phases MxMo6Ch8 (M ϭ metal ion; Ch ϭ S, Se, or Te) are a large class of
materials exhibiting interesting properties such as superconductivity with high critical mag-
netic fields, coexistence of magnetic order and superconducting state, and cationic mobility
[1,2]. Sulfides and selenides are mainly represented in this family, of which most compounds
៮
crystallize in the rhombohedral–hexagonal system with space group R3. Their structure is
based on a tri-dimensional framework of quasi-rigid Mo6Ch8 units with orthogonal secant
tunnels between units where many vacant chalcogenide sites are located that are able to
receive cations Mnϩ. The nature, size, and the amount x of cations present in these phases
control their general properties.
*To whom correspondence should be addressed.
43

44
S. BELIN et al.
Vol. 33, No. 1
TABLE 1
Parameters of Binary Chevrel Phases and PbMo6S8
Mo6Te8 [1]
Mo6Se8 [1]
␣-Mo6S8 [3]
␤-Mo6S8*
PbMo6S8 [1]
aR (Å)
(°)
aH (Å)
cH (Å)
VH (Å3)
7.10
92.60
10.20
11.65
1050
6.658
91.58
9.454
11.209
884
6.432
91.34
9.20
10.88
797
6.449
89.47
9.078
11.273
804
6.544
89.48
9.212
11.437
840
␣
R
*This work.
This family of compounds has three binaries Mo6Te8, Mo6Se8, and ␣-Mo6S8 (Table 1).
The latter is a metastable compound only obtained through a deintercalation method using a
ternary compound such as Cu2Mo6S8 or Ni2Mo6S8 [2,3].
In 1974, Chevrel et al. [3] noted that the binary ␣-Mo6S8 is transformed into a “new
sulfide,” which was called ␤-Mo6S8 [2]), at about 470°C with the occurrence of MoS2. As
shown in Table 1, the interesting feature of this sulfide is essentially its rhombohedral angle,
which is lower than 90° and close to the one of PbMo6S8.
In this paper, we focus our interest on the ␤-Mo6S8 binary, the structure of which was
unknown.
SYNTHESIS AND CHARACTERIZATION
Until now, the only way to synthesize ␤-Mo6S8 has been by thermal decomposition of
␣-Mo6S8 [3]. We have performed this method as follows. We have to synthesize a ternary
compound that is easy to deintercalate by dilute hydrochloric acid [2,3]; we have chosen the
Ni2Mo6S8 Chevrel phase. This compound shows an hexagonal unit cell volume very similar
to the volume of ␣-Mo6S8 and ␤-Mo6S8, allowing minimization of the crystallographic
distortions related to this synthesis process. In the first stage, we prepare Ni2Mo6S8 com-
pound from a stoichiometric mixture of the starting materials, MoS2, Mo, and Ni powders,
which are pressed into a pellet and placed into a degassed silica tube sealed under vacuum
and heated at 800°C for 1 week. In the second stage, the microcrystalline Ni2Mo6S8 powder
is placed into dilute hydrochloric acid for 3 days to remove Ni2ϩ cations from the host
structure. Afterwards the powder is washed in distilled water, then in alcohol and ether,
before being dried under vacuum. The third stage lies in the transformation of ␣-Mo6S8 into
␤-Mo6S8. This reaction takes place in a silica tube kept under secondary dynamic vacuum
and heated at 550°C for 6 h.
The X-ray powder diffraction pattern of the ␤-Mo6S8 final compound is given in Figure 1.
It is indexed in the hexagonal system as ␣-Mo6S8 (cf. Table 2 and Fig. 1) and is always
obtained in the presence of MoS2. The amount of MoS2 binary is minimized when the sulfur
partial pressure decreases, for instance, under a dynamic vacuum or a reducing gas flow.
Of course, we have tried to synthesize ␤-Mo6S8 by other methods. These methods were
based on the fact that ␤-Mo6S8 has parameters close to the ones of the large cation Chevrel
phases such as PbMo6S8 (Table 3) and, thus, a very similar X-ray powder diffraction pattern
(Fig. 1). We have succeeded in deintercalating Chevrel phases with large cations such as
SnMo6S8, PbMo6S8, or LuMo6S8 [4]. Potel et al. [2] showed that InMo6S8 could be

Vol. 33, No. 1
MOLYBDENUM SULFIDE
45
FIG. 1
Powder diffraction pattern of ␣-Mo6S8, ␤-Mo6S8, and PbMo6S8 (with silicium standard) (␭
Cu K␣1).
deintercalated under a flow of HCl gas because of the volatility of InCl3 chloride. In the same
way, we have deintercalated Sn, Lu, and Pb from the host structure of the respective ternary
compounds. Every deintercalation is successful, but the binary ␣-Mo6S8 is always obtained
[4]. So far, the only way to synthesize ␤-Mo6S8 is the three-stage process previously
presented.
The crystal growth was carried out in the same way. At first, we easily made single crystals
of Ni2Mo6S8 by transport reaction from stoichiometric mixtures of the starting materials
MoS2, Mo, and Ni powders. The mixture, pressed into a pellet, was sealed with a MoCl5
transport agent in a silica tube that had been degassed. The silica tube was then placed into
a horizontal furnace and heated to about 1200°C for 2 weeks with a temperature gradient of
50°C. We isolated truncated apex cubic-shaped single crystals with volumes of about 1 or 1.5
mm3.
The Ni2ϩ cations were removed using dilute hydrochloric acid (ϳ50% vol HCl) for 2
weeks, according to the following reaction in aqueous solution:
Ni2Mo6S8 ϩ 4 HCl 3 ␣-Mo6S8 ϩ 2 NiCl2 ϩ 2 H21
After cleaning, the purity of crystals was checked on a scanning electron microscope
equipped with an energy-dispersive X-ray spectrometer (EDS). The hydrochloric acid leach-
ing leads to traces of nickel lower than 100 ppm [3]. The single crystals have the ␣-Mo6S8
composition.

46
S. BELIN et al.
Vol. 33, No. 1
TABLE 2
X-ray Powder Diffraction Data of
Mo7S8, So-called ␤-Mo6S8
hkl (hex)
dobs (Å)
dcal (Å)
I/I0
101
012
110
003
021
113
211
104
122
024
220
015
131
214
312
223
006
042
125
321
116
232
107
315
051
324
226
217
511
6.4786
4.5933
4.5428
3.7687
3.7151
2.9004
2.8763
2.6614
2.6330
2.2931
2.2700
2.1730
2.1415
2.0499
2.0359
1.9455
1.8863
1.8588
1.8009
1.7829
1.7418
1.7200
1.5805
1.5704
1.5590
1.5212
1.4503
1.4197
1.4022
6.4485
4.5809
4.5389
3.7578
3.7117
2.8945
2.8733
2.6530
2.6286
2.2905
2.2695
2.1673
2.1408
2.0449
2.0336
1.9427
1.8789
1.8558
1.7961
1.7809
1.7360
1.7178
1.5777
1.5674
1.5573
1.5192
1.4473
1.4159
1.4011
76
23
18
12
7
5
29
17
68
10
17
25
45
36
22
100
14
1
5
4
2
21
8
5
6
9
12
3
10
These crystals are then put into an Al2O3 crucible inside a silica tube and heated at 700°C
for 48 hours under secondary dynamic vacuum. Finally, small single crystals of ␤-Mo6S8 are
obtained. These single crystals are very small because they arise from two successive
reactions at the crystalline state, which cause some damage to the crystals (Fig. 2). The ␣R
rhombohedral angle alters from 94°68 for Ni2Mo6S8 to 91°34 for ␣-Mo6S8 and then to 89°37
for ␤-Mo6S8. These changes generate important strains, which induce the shattering of the
single crystals.
Crystal Structure Analysis and Refinement of ␤-Mo6S8
Experimental determination. A single crystal 0.07 ϫ 0.047 ϫ 0.047 mm3 with a regular
parallelepipedic shape was chosen in order to undertake the crystal structure determination.
X-ray diffraction intensities were collected on a Nonius CAD-4 four circle diffractometer

TABLE 3
Structural Parameters, Valence Electron Concentration (VEC), and Tc Superconducting Critical Temperature
of Different Chevrel Phases
Mo6S8
[1]
AgMo6S8
[1]
Pd1.6Mo6S8
[5]
Cu1.8Mo6S8
[1]
PbMo6S8
[1]
SnMo6S8
[1]
InMo6S8
[1]
Mo7S8
(Mo–Mo)⌬
intra⌬ (Å)
(MoD–Mo⌬)
inter⌬ (Å)
(Mo–Mo)
intercluster (Å)
Mo–X
intercluster (Å)
X2c–X2c diagonals
via Mo6 (Å)
X2c–X2c diagonals
via origin (Å)
aH (Å)
2.698
2.862
3.084
2.425
6.155
2.705
2.804
3.154
2.459
6.016
2.704
2.795
3.169
2.472
6.046
2.683
2.752
3.237
2.494
6.118
2.665
2.742
3.164
2.536
5.907
2.678
2.726
3.267
2.559
5.865
2.688
2.737
3.232
2.550
5.881
2.705
2.725
3.258
2.529
5.94
4.724
9.20
10.88
4.814
9.32
10.83
815
4.695
9.344
10.741
812
4.132
9.584
10.250
815
5.366
9.078
11.273
5.572
9.212
11.437
840
5.482
9.206
11.363
834
4.74
9.46
10.68
828
cH (Å)
VH (Å3)
797
804
␣
(°)
91.34
6.432
20
91.97
6.48
21
92.37
6.475
ϳ21
94.93
6.503
Ͻ22
89.47
6.449
22
89.48
6.544
22
89.70
6.527
22
93.02
6.52
23
R
aR (Å)
VEC/Mo6
Tc (K)
1.8
7.5
10.8
-
14
14

48
S. BELIN et al.
Vol. 33, No. 1
FIG. 2
Photograph of the ␤-Mo6S8 “single” crystal recorded for the structural determination.
under the experimental conditions given in Table 4. All calculations were performed using
the MOLEN program system [6], and a spherical absorption correction was applied.
The crystal structure of ␤-Mo6S8 is assumed to be similar to the one of ␣-Mo6S8 [2]. We
have refined the atomic positions: the Mo(1) molybdenum and S(1) sulfur atoms are in 6f
general positions and the S(2) sulfur atoms in 2c special positions. Under these conditions,
we are not able to refine the structure completely; in fact, reliability factors are still high. The
differential map of electronic density reveals a heavy peak in 6f positions located in the origin
site. These sixfold peaks mean that there are atoms delocalized in this site. We attribute these
electrons to molybdenum atoms. This conclusion has been confirmed by other experimental
chemical features.
A good reliability factor is obtained despite the mosaic structure of the “pseudo” single
crystal (Table 4 and Fig. 2). Actually, many hkl Bragg reflections have large FWHM (full
width at half maximum), showing the poor quality of the single crystal. Many reflections
have been omitted with the criterion I Ͻ 3␴(I). The final atomic coordinates, occupancy, and
thermal parameters of Mo1.18Mo6S8 are summarized in Table 5
Crystal structure description. The crystal structure of Chevrel phases has been described
in the literature as a stacking of Mo6S8 units (Fig. 3) [1]. The distances and angles of
␤-Mo6S8 and ␣-Mo6S8 are reported in Table 6. Per unit, each Mo6S8 building block is a
slightly distorted cube, where the sulfur atoms sit at the corner. In ␤-Mo6S8, the six Mo(1)
atoms form a distorted octahedral cluster, therefore, there are two kinds of distances: the
(Mo(1)–Mo(1))⌬ intracluster (or intertriangle) distances (2.665 Å) between two molybdenum
atoms belonging to the same triangular plane perpendicular to the ternary axis, and the
Mo(1)⌬–Mo(1)⌬ intracluster distances (2.742 Å) between two Mo atoms belonging to two
different triangular planes. Moreover, each Mo atom of the Mo6 cluster is linked to another
Mo atom belonging to a neighboring Mo cluster forming the intercluster distance (3.164 Å).
The Mo atom is located slightly outside the middle of the face of the S8 pseudo-cube and is
in a square-based pyramidal sulfur site.
This particular stacking of Mo6S8 units leaves vacant three types of cavities in the sulfur
atom network (Fig. 3). S8 cavities 1 and 2 are empty in the binary ␣-Mo6S8; they are filled

Vol. 33, No. 1
MOLYBDENUM SULFIDE
49
TABLE 4
Crystal Data and Experimental Parameters for the Intensity
Data Collection of Mo1.18Mo6S8
I. Crystal data
Formula: Mo1.18Mo6S8
Crystal system: rhombohedral
M ϭ 945.36 g/mol
៮
space group: R3 (no. 148)
Parameters: aR ϭ 6.449(1) Å, ␣R ϭ 89.48(2)°
aH ϭ 9.078(2) Å, cH ϭ 11.273(4)°
Unit cell refined from 25 reflections (9° Ͻ ␪ Ͻ 20°)
VR ϭ 268.2(1) Å3
VH ϭ 804.6(3) Å3
␳
ϭ 5.855 g/cm3
Z ϭ 1 (rhombohedral)
cal
Crystal size: 0.07 ϫ 0.047 ϫ 0.047 mm3
Linear absorption factor: 91.946 mmϪ1
II. Data collections
Temperature: 295 K
Diffractometer: Enraf Nonius CAD-4
Monochromator: graphite
Wavelength: Mo K␣
Scan mode: ␻-2␪
Scan width: 1.20 ϩ 0.35 tg ␪
Ϫ9 Ͻ h Ͻ 9; 0 Ͻ k Ͻ 9; 0 Ͻ l Ͻ 9
420 measured reflections
␪
ϭ 30
max
366 independent reflections (with I Ͼ ␴(I))
Rint ϭ 0.061
III. Structure determination
Lorentz polarization and crystal corrections
Spherical absorption correction: SAC
Refinement on F
246 independent reflections with I Ͼ 3 ␴(I)
Refined parameters: 33
Unweighted agreement factor R ϭ 0.062
Ponderation factor: 0.07
Weighted agreement factor R␻ ϭ 0.068
2
2
2 2
␻ ϭ 4f0 /[␴(F0 ) ϩ (0.07 F0 ) ]
(⌬/␴)max Ͻ 0.01
Validity of refinement S ϭ 1.105
⌬␳
ϭ 2.29 e/Å3
⌬
ϭ Ϫ1.305 e/Å3
max
rmin
Extinction correction: ER [7]
Extinction coefficient: 1.24 ϫ 10Ϫ7
Atomic scattering factor from Internal Tables for X-ray Crystallography [8]
in Cu4Mo6S8 and only site 1 is filled in PbMo6S8 or SnMo6S8. Cavity 3 is not vacant because
of the occurrence of the Mo–Mo intercluster weak bondings.
In ␤-Mo6S8, only site 1 is occupied by Mo(2) atoms located on a sixfold equivalent
crystallographic position grouped around the inversion center (origin) with an occupancy of
0.196. The six equivalent positions of Mo(2) atoms describe a distorted octahedron (Fig. 4)
with the distances “(Mo(2)–Mo(2))⌬” ϭ 1.686 Å and “Mo(2)⌬–Mo(2)⌬” ϭ 1.442 Å (Table
6). Mo(2) atoms are in a squared planar sulfur site. The distance from origin to sulfur atoms
is different with S2 atoms (Org-S2 ϭ 2.683 Å) and the S(1) atom (Org-S1 ϭ 3.022 Å) in the
quasi-cubic S8 origin site.
DISCUSSION
The nature of the cations located in cavity 1 of the structure has been determined by some
chemical properties of this compound. First, ␤-Mo6S8 via ␣-Mo6S8 can be obtained from

TABLE 5
Positional, Occupancy, and Thermal Parameters of Mo1.18Mo6S8
Atom
Site
x
y
z
Beq (Å2)
b11
b22
b33
b12
b13
b23
Mo(1)
S(1)
S(2)
6f
6f
2c
6f
0.2225(3)
0.1281(9)
0.2379(9)
0.9512(15)
0.4150(3)
0.7504(10)
0.2379(9)
0.0275(15)
0.5604(3)
0.3773(10)
0.2379(9)
0.1629(16)
1.04(3)
1.4(1)
1.49(5)
1.0(2)
0.0061(3)
0.005(1)
0.009(1)
0.004(2)
0.0064(4)
0.011(1)
b11
0.0062(3)
0.010(1)
b11
0.0006(7)
0.000(2)
Ϫ0.000(2)
0.0012(7)
0.002(2)
b12
0.0001(7)
0.002(2)
b12
Mo(2)
0.003(2)
0.011(2) Ϫ0.003(3) Ϫ0.008(3)
0.003(3)
The thermal factor is given by exp {Ϫ(b11h2 ϩ b22k2 ϩ b33l2 ϩ b12hk ϩ b13hl ϩ b23kl)} ␶ (Mo(2)) ϭ 0.196 [6].

Vol. 33, No. 1
MOLYBDENUM SULFIDE
51
FIG. 3
Projection of the Chevrel phase framework on (11–20) plane in a hexagonal system. View of
the three types of cavities.
different ternary starting compounds such as LixMo6S8, CuxMo6S8, or NixMo6S8. EDS–SEM
analysis displays nothing but molybdenum and sulfur. Microprobe analysis gives the
Mo7.17S8 formula. Moreover, ␤-Mo6S8 decomposes above 810°C into molybdenum sulfide
and molybdenum, according to X-ray studies.
The DSC analysis of ␣-Mo6S8 under vacuum from room temperature to 900°C shows that
the transformation ␣ 3 ␤ is exothermic and occurs at about 560°C because of the temper-
ature rate (Fig. 5). We can see from the DSC curve that ␤-Mo6S8 is stable until 810°C, then
it undergoes an exothermic degradation reaction.

52
S. BELIN et al.
Vol. 33, No. 1
TABLE 6
Distances (Å) and Angles (°) of Mo1.18Mo6S8 (␤-Mo6S8)
and ␣-Mo6S8
Mo1.18Mo6S8
␣-Mo6S8 [3]
Mo6S8 cubic cluster unit
Intratriangle (Mo(1)–Mo(1))⌬
Intertriangle Mo(1)⌬–Mo(1)⌬
Intercluster Mo(1)–Mo(1)
Mo(1)–S(2)
2 ϫ 2.665(3)
2 ϫ 2.742(3)
3.164(3)
2 ϫ 2.698
2 ϫ 2.862
3.082
2.382(4)
2.439
S(1)
2.423(6)
2.426
S(1)
2.433(6)
2.460
S(1)
2.527(7)
2.431
Intercluster Mo(1)–S(1)
S(2)–S(1)
S(1)–S(1)
S(1)–S(2)–S(1) angle
2.536(6)
2.425
3 ϫ 3.497(7)
2 ϫ 3.384(8)
89°52
3 ϫ 3.550
2 ϫ 3.310
85°94
S8 origin cubic site
S(2)–S(1)
S(1)–S(1)
Org–S(2)
Org–S(1)
S(1)–S(2)–S(1) angle
(Mo(2)–Mo(2))⌬
Mo(2)⌬–Mo(2)⌬
Mo(2)–Mo(2)
Mo(2)–S(2)
S(1)
3 ϫ 3.343(9)
2 ϫ 3.451(9)
2 ϫ 2.683
6 ϫ 3.022
95°85
2 ϫ 1.686(14)
2 ϫ 1.442(14)
2.219(14)
3 ϫ 3.339
2 ϫ 3.538
2 ϫ 2.362
6 ϫ 3.153
102°37
2.361(11)
2.412(12)
S(1)
2.520(12)
S(1)
2.600(12)
Mo(1)–Mo(2)
Mo(2)
3.237(10)
3.537(10)
S(2)–S(1) (square-faced diagonal)
in site 2
3 ϫ 3.410(6)
3 ϫ 3.447
S–S chains
S(1)–S(2)–S(1) angle
S(1)–S(1)–S(1) angle
141°05
141°31
138°01
139°86
␤-Mo6S8 sulfide is stable in aqueous hydrochloric acid, but molybdenum atoms Mo(2)
located in cavity 1 can be removed from the host structure by using a flow of HCl gas heated
at 600°C for 6 h; the binary ␣-Mo6S8 is then obtained anew [4].
Some cationic exchange reactions have been carried out on ␤-Mo6S8. We have intercalated
Cu or Ni elements in ␤-Mo6S8 powder (with MoS2 impurity) at 430°C for several days with
intermediate rehomogenization [9]. The X-ray powder diffraction pattern of the mixture of
phases obtained shows elemental Mo, a CuxMo6S8 or NixMo6S8 phase and MoS2. Therefore,
it is possible to deintercalate Mo atoms contained in ␤-Mo6S8 with smaller elements such as
Cu and Ni.

Vol. 33, No. 1
MOLYBDENUM SULFIDE
53
FIG. 4
Perspective view of the S8 origin cube (site 1) of some Chevrel phases.
Formulation. In view of these results, we claim that the atoms occurring in the channels are
Mo atoms. ␤-Mo6S8 is a pseudoternary compound with two types of Mo atoms: Mo(1) atoms
forming an octahedral cluster Mo6 and Mo(2) atoms in site 1 of the Chevrel phase structure.
The refined occupancy of Mo(2) is 0.196, meaning 1.18 Mo(2) atoms per Mo6S8 formula. We
think that the Mo(2) occupancy can vary slightly around this value, explaining why the cH
parameter changes (11.28 Å Յ cH Յ 11.32 Å) between different batches of ␤-Mo6S8 powder.

54
S. BELIN et al.
Vol. 33, No. 1
FIG. 5
DSC analysis on the ␣-Mo6S8 heated to 1000°C.
Therefore, ␤-Mo6S8 can be commonly called Mo7S8. Mo7S8 does not arise from a phase
transition of ␣-Mo6S8 but from a self-molybdenum intercalation reaction:
␣Mo6S8
2 ␣Mo6S8 ϩ 2 Mo
3
3
2 Mo ϩ 4 MoS2
2 MoMo6S8
3 ␣Mo6S8
3
2 MoMo6S8 ϩ 4 MoS2
A part of ␣-Mo6S8 decomposes into Mo and MoS2, and, immediately, the nascent Mo atoms
reduce the remaining ␣-Mo6S8 into the pseudobinary Mo7S8. It should be noted that the
molar weight of Mo7S8 is much heavier than that of MoS2; therefore, the weight mixture is
roughly 1/4 MoS2 and 3/4 Mo7S8.
To our knowledge, this is the first time that such a metal self-intercalation reaction has
been observed in solid state chemistry.
Delocalization of Mo(2) atoms. Mo(2) atoms are located in six equivalent positions form-
ing a distorted octahedron (Fig. 4) in site 1.
Comparison between the “mother” and “daughter” structures. It is very interesting to
investigate the structural changes in the chalcogen network that the self-molybdenum inter-
calation has generated in the ␣-Mo6S8 “mother” precursor binary. One of the great differ-
ences is the S8 origin cube called site 1 in both structures. It is very compressed along the
ternary axis in ␣-Mo6S8; the diagonal represented by the S(2)–S(2) distance is (2 ϫ Org-S2)
4.724 Å against 5.366 Å in ␤-Mo6S8 (Table 3). This is reflected in the opening of the
S(1)–S(2)–S(1) angle of the S8 origin cube. The S(1)–S(2)–S(1) angle in ␣-Mo6S8 is very

Vol. 33, No. 1
MOLYBDENUM SULFIDE
55
FIG. 6
The delocalization of the cation M as a function of the ␣R rhombohedral angle in MxMo6S8.
open, 102°37, whereas, it is 95°85 in ␤-Mo6S8 (Table 6), which is close to 94°34 found in
SnMo6S8. The S(2)–S(1) distances of the S8 origin cube are similar (3.339 Å vs. 3.343 Å),
whereas, the S(1)–S(1) distances are different (3.538 Å for ␣-Mo6S8 and 3.451 Å for
␤-Mo6S8). We also can point out that the shortening of the S(2)–S(2) cube diagonal via the
Mo6 cluster is 5.907 Å in ␤-Mo6S8 vs. 6.155 Å in ␣-Mo6S8.
Comparison of Mo7S8 and other ternaries. In Figure 4, we have represented different S8
origin cubes of well known Chevrel phases showing different models of occupancy of site 1.
By comparing the Org-M distances, we can easily see that Mo(2) atoms in Mo7S8 in site 1
are more delocalized than indium atoms in InMo6S8, but less than Cu atoms in Cu1.8Mo6S8
or Pd atoms in Pd1.6Mo6S8, and the pseudo cube S8 is similar to that of SnMo6S8.
Another interesting feature is that Mo7S8 does not respect the model of cationic delocal-
ization suggested by Yvon (Fig. 6) [12]. In Mo7S8, the rhombohedral angle is equal to 89°48.
This model expects that the cation Mo(2) is localized at the origin of site 1 for such an ␣R
rhombohedral angle, as in PbMo6S8 or SnMo6S8 compounds. Actually, the delocalization of
Mo(2) is 1.109 Å from the origin of site 1. The curve predicts a rhombohedral angle larger
than 94°. Mo7S8 is not the only exception to this model: the model does not fit Pd1.6Mo6S8
either [5]. This apparent disagreement can be explained by the existence of metal–metal
bonding between the atoms contained in the channels of both compounds.
Nature of the Mo(2) atoms. As we have already described, Mo(2) atoms are located on
sixfold general positions, yielding a distorted octahedron (Fig. 4), but the occupancy on these
six equivalent positions (6f) is 0.196. This means that there is more than one Mo atom per
origin site, that is, there are two Mo(2) atoms in some unit cells. These two Mo(2) atoms are
2.22 Å apart on the diagonal of the octahedron. This distance is close to a multiple bond. For
instance, quadruple bonds occur in the divalent molybdenum(II) complex compounds
K4[Mo2Cl8]⅐2H2O and Mo2(CH3CO2)4; they contain a pair of Mo atoms tightly bonded
together (Mo–Mo ϭ 2.14 Å and 2.11 Å, respectively) [10]. In these (Mo2)4ϩ species

56
S. BELIN et al.
Vol. 33, No. 1
FIG. 7
Environment of molydenum atoms in MoMo6S8 and K4[Mo2Cl8].
compounds, all four electrons on each molybdenum atom are used to form a Mo–Mo
quadruple bond (␴ ϩ 2␲a ϩ ␦). The Mo atoms are in a squared planar environment of L
ligands as for Mo7S8 (Fig. 7). These results allow us to assume that there is fourfold bonding
between two Mo(2) atoms; therefore, we can write Mo7S8 as (MoII–MoII)4ϩ1/2(Mo6S8)2Ϫ
.
The (Mo2)4ϩ species inside the channels of the Chevrel phase structure form a kind of
[Mo2L8] molecule like [Mo2Cl8]4Ϫ in K4Mo2Cl8⅐2H2O (Fig. 7).
Charge transfer. Because of the divalent valence state of MoII (2) atoms, the Mo6S8 unit in
␤-Mo6S8 has a Ϫ2 charge corresponding to a VEC (valence electron concentration) per Mo6
cluster close to 22 electrons. In Chevrel phases, we have a direct relationship between VEC
and (Mo⌬–Mo⌬) intracluster distances [11].
The significant Mo6 cluster distances of Mo7S8 and some other Chevrel phases are listed
in Table 3. Mo7S8 and ␣-Mo6S8 (VEC ϭ 20 eϪ) present quite different Mo6 clusters. The
comparison of Mo6 cluster distances of Mo7S8 and some other ternary Chevrel phases,
especially those of PbMo6S8, shows a VEC near 22 eϪ per cluster for Mo7S8 (Table 3). This
is in agreement with the quadruple bond Mo(2)–Mo(2), which presumes that the valence state
of Mo(2) is ϩ2. Chevrel phases with 22 eϪ per cluster often exhibit interesting supercon-
ducting properties (PbMo6S8: Tc ϭ 15 K, Bc2 (0 K) ϭ 60 tesla [12]). We have found no
superconducting properties for Mo7S8 compound by inductive measurements down to 1.6 K,
when ␣-Mo6S8 is a superconductor (Tc ϭ 1.8 K). We have also carried out EPR and DC
susceptibility (SQUID) measurements. We have not seen odd electrons (Curie-Weiss law) of
the Mo(2) atoms by these methods, and Mo7S8 shows a TIP (Pauli paramagnetism) about
0.7 ϫ 10Ϫ7 uem CGS, which supports our hypothesis of a Mo(2)–Mo(2) quadruple bond.
Such a feature can probably affect the electronic band structure of the model used for the
Chevrel phases [13]. The Mo(2) dumbbells have 4d molecular orbitals that can take part in
the electronic band structure at the Fermi level and, thus, prevent superconductivity.
Another assumption could be that Mo7S8 undergoes an electronic phase transition at low
temperature as in MMo6S8 (M ϭ Eu, Ba, Sr) compounds (VEC ϭ 22eϪ); the half-filled Eg
band splits into two subbands [14].

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MOLYBDENUM SULFIDE
57
These probable modifications of the Fermi level can explain the loss of superconductivity
in this pseudobinary compound with 22 electrons.
ACKNOWLEDGMENT
We are grateful to M. Potel, C. Perrin, and A. Perrin for assistance in the single crystal
structure determination.
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