The Chevrel phase In4.07Mo15S11.87Se7.13 with mixed chalcogenide occupancy

The single-crystal of the title compound, indium pentadecamolybdenum nonadeca(sulfide/selenide), was obtained by solid state reaction with an S/Se mixture. It adopts the structure type of In3Mo15Se19 and In3.7Mo15S19, which are non-substituted Chevrel phases in the space group P63/m. The Mo, one S/Se and two In sites have point symmetry m.. and two S/Se and one In atoms are in 3.. sites. This compound contains isolated Mo6 and Mo9 clusters. The shapes of clusters are octahedral and confacial bioctahedral, respectively, face-capped by chalcogen atoms over each triangle face. The Mo—X bonds (X = S, Se) play an important role for the constitution of the framework. The Mo—X distances of 2.479 (2)–2.6687 (9) Å are within the ranges of average values of Mo—S and Mo—Se distances. The In atoms located on sites with m.. symmetry are partially occupied.

The Chevrel phase In 4.07 Mo 15 S 11.87 Se 7.13 with mixed chalcogenide occupancy Y.-S. Lee and S. K. Kang

Comment
The classical Chevrel phases, containing blocks of Mo 6 X 8 , have been in interest for both structural respects and application to rechargeable batteries (Suresh, et al., 2008;Aurbach, et al., 2007). The new Chevrel phases In x Mo 15 Se 19 (x=2.9 and 3.3) also have been studied by X-ray single-crystal method (Grüttner et al., 1979). These were the first compound having a transition metal cluster with the isolated Mo 6 and Mo 9 clusters. The Mo 9 cluster has the shape of a confacial bioctahedron resulting from the condensation of two octahedral Mo 6 clusters. Both clusters are surrounded by face-capping Se atoms to form Mo 6 Se 8 and Mo 9 Se 11 cluster units, and they are interconnected through Mo-Se bonds to build the three dimensional framework ( Fig. 1). On our continuous studies to develop new materials for rechargeable batteries, herein, we report the single-crystal structure of the mixed chalcogenide compound In 4.07 Mo 15 S 11.87 Se 7.13 (1). We have investigated the effect of the partial substitution of Se by S atoms in the related Chevrel phase, hoping that the building blocks of Chevrel phase would not be changed.
The crystal structure of the title compound in a unit cell is shown in Fig. 1. The framework is composed of Mo 6 X 8 and Mo 9 X 11 cluster units (X=Se/S) that are interconnected through Mo-X bonds. The Mo 6 cluster forms the octahedral geometry with Mo-Mo bonds between the six Mo atoms, and the eight faces on the octahedron share a chalcogen atom to create the Mo 6 X 8 building block (Fig. 2). The Mo 9 cluster is formed by one dimensional trans-face sharing of two Mo 6 octahedron, and surrounded by eleven face-capping chalcogen atoms. The Mo-Mo bond distance related through the threefold axis in the Mo 6 clusters is 2.6728 (11) Å. And the Mo-Mo distances within the Mo 9 clusters are in the range of 2.6415 (10) -2.7540 (8) Å which are within the normal range of the other Chevrel phases (Grüttner et al., 1979;Salloum, Gautier et al., 2004;. The amount of substitution of Se atoms by S atoms are dependent on the atomic positions with the range of 34% (for X3 atom) -86% (for X1 atom). The higher the S atom occupation, the shorter Mo-X bond distances are.

Experimental
The title compound was prepared from powder elemental indium (99.999 at.%), molybdenum (99.999 at.%), sulfur (99.98 at.%), and selenium (99.99 at.%) from Aldrich products in the slightly off-stoichiometric 5:15:12:7 ratio. The reaction mixture was sealed under a nitrogen atmosphere in a silica tube and heated at 1343 K for 72 h and cooled to room temperature at the rate of 10 K/h to obtain black single crystals for X-ray studies.

Refinement
The crystal structure of the title compound was solved and refined starting from the atomic coordinates reported for In~3Mo 15 Se 19 compound (Grüttner et al., 1979). In the first stage of the refinement, the positions of all atoms but In3 were supplementary materials sup-2 obtained reasonably. The remaining In3 atom was located in subsequent difference Fourier syntheses. The maximum and minimum residual electron density peaks were located at 1.07 and 0.46 Å, respectively, from the In1 atom. Fig. 1. View of the crystal structure of the title compound along [110], with displacement ellipsoids at the 80% probability level.   (7) 0.0136 (7) 0.0073 (6) −0.0028 (5) −0.0045 (5) S5