PSEUDO-SYMMETRY IN STRUCTURE OF Re₆Se₇Br₄. N. L. Speziali and C. B. Pinheiro, Departamento de Fsica, ICEx - UFMG, Belo Horizonte - Brazil

A number of compounds presenting rhenium octahedral clusters in their structures have been investigated and related to Chevrel phase in molybdenum compounds. The ternary compounds _Re₆X_iY_j (Y=Cl, Br and X=Se,S) crystallize with well defined clusters of Re₆L₈ _(L=X and/or Y), where the Re atoms define an octahedron and are placed in the center of a cube defined by the 8 L atoms. The L atoms are called inner ligands. The link between the clusters is made, in general, via Y elements.

The crystal structure of many compounds in Re₆X_iY_j family have been investigated for different type and number of inter-cluster atoms. The 12 d⁴ state single-covalent-Re-Re bonds envolves 24 of 42 valence electrons (6 Re atoms in 4f¹⁴5d⁵6s² state). The 18 remainder electrons are used in a subtle charge transfer through ionic bonds with halogen and chalcogen atoms. It has been reported that controlling the chalcogen number i and the halogen number j, preserving the number of valence electrons i+2j=18, structures with Re₆L₈ clusters linked by halogen bridges in one, two or three directions of the space can be obtained.

Structure of Re₆Se₇Br₄ single crystal has been studied. In this crystal, Re₆|Se₇Br| clusters link each other via halogen bridges in a three dimensional way. It was evidenced that a monoclinic C12/c1 symmtery is more suitable to describe the structure than the rombohedric R c assumed by other authors. The real monoclinic structure has in fact a pseudo-rhombohedral symmetry: the position of the linking Br atoms can not be described by the R c space group.