The argyrodite γ-Ag9AlSe6: a non-metallic filled Laves phase

Abstract

Abstract The crystal structure of the air sensitive argyrodite Ag9AlSe6 is determined by single-crystal X-ray diffraction at room temperature. Above the phase transition at 287 K γ-Ag9AlSe6 crystallizes with cubic symmetry, space group F4̅3m, with a = 11.0947(2) Å, V = 1365.68 (9) Å3 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 γ-Ag9AlSe6 is topologically equivalent to the arrangement of the Mg and Cu atoms in the tetrahedrally close packed intermetallic compound MgCu2, a cubic Laves phase. The selenium atoms form a 3D network of tetrahedra partially occupied by Al atoms in an ordered way. The electron density of silver was described by means of a split model and a Gram-Charlier development of the atomic displacement factor for one silver position. This approach gives evidence for the diffusion pathways of silver atoms in the [AlSe6]9- framework through tetrahedral holes.