Acta Crystallographica Section B

Structural Science

Volume 56, Part 6 (December 2000)

research papers

Acta Cryst. (2000). B56, 972-979 [ doi:10.1107/S0108768100010260 ]

Structures and phase transitions of the A₇PSe₆ (A = Ag, Cu) argyrodite-type ionic conductors. III. -Cu₇PSe₆

E. Gaudin, V. Petricek, F. Boucher, F. Taulelle and M. Evain

Abstract: The crystal structure of the third polymorph of the Cu₇PSe₆ argyrodite compound, [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-temperature form, i.e. below the last phase transition, [alpha] -Cu₇PSe₆ crystallizes in orthorhombic symmetry, space group Pna2₁, with a = 14.3179 (4), b = 7.1112 (2), c = 10.1023 (3) Å, V = 1028.590 (9) Å³ (deduced from powder data, T = 173 K) and Z = 4. Taking into account a twinning by reticular merohedry, the refinement of the [alpha] -Cu₇PSe₆ structure leads 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⁺d¹⁰ element is reached with one twofold-, one threefold- and five fourfold-coordinated Cu atoms. The observation of two phase transitions for Cu₇PSe₆, to be compared with only one for Ag₇PSe₆, is attributed to the d¹⁰ element stability in a low coordination environment, copper being less prone to lower coordination sites than silver, especially at low temperature.

Keywords: Argyrodite; Phase transition; Structure determination; Twinning.