The argyrodites — A new family of tetrahedrally close-packed structures

Numerous members of a new, hitherto unrecognized family of tetrahedrally close-packed structures, termed the argyrodites, have been synthesized and characterized by x-ray methods. Their general formula is: Am+(12−n−x)mn+X2−6−xY−x (A: Cu, Ag, Cd, Hg; B: Ga, Si, Ge, Sn, P, As; X = S, Se, Te; Y = Cl, Br, I; O≤x≤1). They have a common cubic (F4̄3m) HT-structure with a disordered A-cation sublattice. At lower temperatures transitions into various ordered structures occur. Typical structural feature is an anion framework of interpenetrating, centred icosahedra providing ideal and distorted tetrahedral cation sites. Some argyrodites are efficient ionic conductors, others are of interest as non-linear optical materials.