Structures and phase transitions of the A 7PSe6 (A = Ag, Cu) argyrodite-type ionic conductors. II. β- and γ-Cu7PSe6

Abstract

The crystal structures of two of the three polymorphic forms of the Cu7PSe6 argyrodite compound are determined by means of single-crystal X-ray diffraction. In the high-temperature form, at 353 K, i.e. 33 K above the first phase transition, γ-Cu7PSe6 crystallizes in cubic symmetry, space group F4¯3m. The full-matrix least-squares refinement of the structure leads to the residual factors R  = 0.0201 and wR = 0.0245 for 31 parameters and 300 observed independent reflections. In the intermediate form, at room temperature, β-Cu7PSe6 crystallizes again in cubic symmetry, but with space group P213. Taking into account a merohedric twinning, the refinement of the β-Cu7PSe6 structure leads to the residual factors R  = 0.0297 and wR  = 0.0317 for 70 parameters and 874 observed, independent reflections. The combination of a Gram–Charlier development of the Debye–Waller factor and a split model for copper cations reveals the possible diffusion paths of the d 10 species in the γ-Cu7PSe6 ionic conducting phase. The partial ordering of the Cu+ d 10element at the phase transition is found in concordance with the highest probability density sites of the high-temperature phase diffusion paths. A comparison between the two Cu7PSe6 and Ag7PSe6 analogues is carried out, stressing the different mobility of Cu+ and Ag+ and their relative stability in low-coordination chalcogenide environments.