Structures and phase transitions of the A(7)PSe(6) (A = Ag, Cu) argyrodite-type ionic conductors. II. beta- and gamma-Cu7PSe6

The crystal structures of two of the three polymorphic forms of the Cu7PSe6 argyrodite compound are determined by means of single-crystal X-ray diffra ction. In the high-temperature form, at 353 K, i.e. 33 K above the first ph ase transition, gamma-Cu7PSe6 crystallizes in cubic symmetry, space group F (4) over bar 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 roo m temperature, beta-Cu7PSe6 crystallizes again in cubic symmetry, but with space group P2(1)3. Taking into account a merohedric twinning, the refineme nt of the beta-Cu7PSe6 structure leads to the residual factors R = 0.0297 a nd 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 o f the d(10) species in the gamma-Cu7PSe6 ionic conducting phase. The partia l ordering of the Cu+ d(10) element at the phase transition is found in con cordance with the highest probability density sites of the high-temperature phase diffusion paths. A comparison between the two Cu7PSe6 and Ag7PSe6 an alogues is carried out, stressing the different mobility of Cu+ and Ag+ and their relative stability in low-coordination chalcogenide environments.