STRUCTURES AND PHASE-TRANSITIONS OF THE A(7)PSE(6) (A = AG, CU) ARGYRODITE-TYPE IONIC CONDUCTORS - I - AG7PSE6

The crystal structures of the two polymorphic forms of the argyrodite Ag7PSe6 compound are analysed by means of single-crystal X-ray diffrac tion. Above the phase transition at 453 K leading to the ionic conduct ing phase, gamma-Ag7PSe6 crystallizes in cubic symmetry space group <F (4)over bar 3n>, with a = 10.838 (3) Angstrom, V = 1273.1(12) Angstrom (3) and Z = 4 at 473 K. The refinement of the 473 K structure leads to a reliability factor of R = 0.0326 for 192 independent reflections an d 33 variables. Diffusion paths for silver d(10) ions are evidenced by means of a combination of a Gram-Charlier development of the atomic d isplacement factors and a split model. Below the phase transition beta -Ag7PSe6 crystallizes again in cubic symmetry, but with the space grou p P2(1)3 and a = 10.772 (2) Angstrom, V = 1250.1(6) Angstrom(3) and Z = 4 at room temperature. The refinement of the 293 K structure leads t o a reliability factor of R = 0.0267 for 1125 independent reflections and 68 variables. In the beta-Ag7PSe6 ordered phase the silver cations are found in various sites corresponding to the most pronounced proba bility density locations of the high-temperature diffusion paths. Thos e positions correspond to low coordination (2, 3 and 4) sites, in agre ement with the silver preference for such environments. In addition, t he Ag atoms are found slightly displaced from the true linear, triangu lar or tetrahedral coordination, as expected from second-order Jahn-Te ller effects.