The argyrodite gamma-Ag(9)ALSe(6): A non-metallic filled Laves phase

The crystal structure of the air sensitive argyrodite Ag9AlSe6 is determine d by single-crystal X-ray diffraction at room temperature. Above the phase transition at 287 K gamma -Ag9AlSe6 crystallizes with cubic symmetry, space group F (4) over bar 3m, with a=11.0947(2) Angstrom, V = 1365.68(9) Angstr om (3) and Z = 4. The structure refinement leads to a reliability factor of R = 0.0298 for 271 independent reflections and 45 variables. The selenium substructure of the non-metallic gamma -Ag9AlSe6 is topologically equivalen t to the arrangement of the Mg and Cu atoms in the tetrahedrally close pack ed intermetallic compound MgCu2, a cubic Laves phase. The selenium atoms fo rm a 3D network of tetrahedra partially occupied by Al atoms in an ordered way. The electron density of silver was de scribed by means of a split mode l and a Gram-Charlier development of the atomic displacement factor for one silver position. This approach gives evidence for the diffusion pathways o f silver atoms in the [AlSe6](9-) framework through tetrahedral holes.