Structure of Mo7S8: A New Binary Sulfide Synthesized by Self Molybdenum Intercalation

doi:10.1016/S0025-5408(97)00191-8

Materials Research Bulletin

Volume 33, Issue 1, January 1998, Pages 43-57

https://doi.org/10.1016/S0025-5408(97)00191-8 Get rights and content

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 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.

Introduction

The Chevrel phases M_xMo₆Ch₈ (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 $3$ . 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 Li_xMo₆S₈, Cu_xMo₆S₈, or Ni_xMo₆S₈. EDS–SEM analysis displays nothing but molybdenum and sulfur. Microprobe analysis gives the Mo_7.17S₈ 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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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.
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Original ArticlesStructure of Mo7S8: A New Binary Sulfide Synthesized by Self Molybdenum Intercalation

Abstract

Introduction

Section snippets

Synthesis and Characterization

Discussion

Acknowledgements

Mater. Res. Bull.

Solid State Comm.

Rev. Chim.

MOLENAn Interactive Intelligent System for Crystal Structure Analysis

Original Articles
Structure of Mo₇S₈: A New Binary Sulfide Synthesized by Self Molybdenum Intercalation