Original ArticlesStructure of Mo7S8: A New Binary Sulfide Synthesized by Self Molybdenum Intercalation
Introduction
The Chevrel phases MxMo6Ch8 (M = metal ion; Ch = S, Se, or Te) are a large class of materials exhibiting interesting properties such as superconductivity with high critical magnetic 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 R. 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 Mn+. The nature, size, and the amount x of cations present in these phases control their general properties.
This family of compounds has three binaries Mo6Te8, Mo6Se8, and α-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 α-Mo6S8 is transformed into a “new sulfide,” which was called β-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 β-Mo6S8 binary, the structure of which was unknown.
Section snippets
Synthesis and Characterization
Until now, the only way to synthesize β-Mo6S8 has been by thermal decomposition of α-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 α-Mo6S8 and β-Mo6S8, allowing minimization of the crystallographic distortions related to this synthesis process. In the first stage,
Discussion
The nature of the cations located in cavity 1 of the structure has been determined by some chemical properties of this compound. First, β-Mo6S8 via α-Mo6S8 can be obtained from 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, β-Mo6S8 decomposes above 810°C into molybdenum sulfide and molybdenum, according to X-ray studies.
The DSC analysis of α-Mo6S8
Acknowledgements
We are grateful to M. Potel, C. Perrin, and A. Perrin for assistance in the single crystal structure determination.
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2008, Comptes Rendus ChimieCitation Excerpt :Its structure is characterized by a Mo6S8 motif containing octahedral Mo6 clusters (Mo–Mo = 2.69 and 2.86 Å) inserted into a distorted cube of sulfur atoms. The related sulfide obtained by self-intercalation of molybdenum into Mo6S8 host is Mo1.18Mo6S8 or Mo7.18S8 [11]. Mo6S8 is a generic host for different derivatives, so-called Chevrel phases which can be considered as products of intercalation of different chemical species (particularly bivalent cations such as Pb2+) between the Mo6S8 units.
Electrochemical and Chemical Behavior of Extra Molybdenum Atoms into the Chevrel Phase Host Network
1999, Journal of Solid State Chemistry