The crystal structure of γ Ag₈GeTe₆, a potential mixed electronic-ionic conductor*

Abstract γ Ag 8 GeTe 6 belongs to space group F [unk]3 m with Z = 4 and a = 11.58 ± 0.02Å. The arrangement of the tellurium atoms is almost precisely that of both the magnesium and copper atoms in the Friauf-Laves cubic phase of MgCu 2 . This arrangement produces 136 tetrahedra per unit cell, of which four are occupied by the Ge atoms. The remainder are divided among two 48-fold, two 16-fold and one 4-fold sets of positions. One of the 16-fold sets of tetrahedra which shares faces with the GeTe 4 tetrahedra is empty. The Ag atoms are distributed over the remaining sets with 29 of the 32 in the unit cell being in the two 48-fold sets and 25 in just one of these. There are many routes whereby the Ag atoms may move through the crystal; in particular the two 48-fold sets themselves form such routes. Not all the Ag atoms are constrained to lie at the near-centers of the tetrahedra; some are closer to particular shared faces. There are substantial differences between the structure reported here and that reported earlier by Rysanek, Laruelle and Katty.