Structures and phase transitions of the A7PSe6 (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 diffraction. Above the phase transition at 453 K leading to the ionic conducting 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 and 33 variables. Diffusion paths for silver d(10) ions are evidenced by means of a combination of a Gram-Charlier development of the atomic displacement factors and a split model. Below the phase transition beta-Ag7PSe6 crystallizes again in cubic symmetry, but with the space group 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 to 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 probability density locations of the high-temperature diffusion paths. Those positions correspond to low coordination (2, 3 and 4) sites, in agreement with the silver preference for such environments. In addition, the Ag atoms are found slightly displaced from the true linear, triangular or tetrahedral coordination, as expected from second-order Jahn-Teller effects.