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

The crystal structure of the third polymorph of the Cu7PSe6 argyrodite comp ound, alpha -Cu7PSe6, heptacopper phosphorus hexaselenide, is determined by means of single-crystal diffraction from twinned crystals and X-ray powder diffraction, with the help of extensive NMR measurements. In the low-tempe rature form, i.e. below the last phase transition, alpha -Cu7PSe6 crystalli zes in orthorhombic symmetry, space group Pna2(1), with a = 14.3179 (4), b = 7.1112 (2), c = 10.1023 (3) Angstrom, V = 1028.590 (9) Angstrom (3) (dedu ced from powder data, T = 173 K) and Z = 4. Taking into account a twinning by reticular merohedry, the refinement of the alpha -Cu7PSe6 structure lead s to the residual factors R = 0.0466 and wR = 0.0486 for 127 parameters and 3714 observed, independent reflections (single-crystal data, T = 173 K). A full localization of the Cu(+)d(10) element is reached with one twofold-, one threefold- and five fourfold-coordinated Cu atoms. The observation of t wo phase transitions for Cu7PSe6, to be compared with only one for Ag7PSe6, is attributed to the d(10) element stability in a low coordination environ ment, copper being less prone to lower coordination sites than silver, espe cially at low temperature.