Acta Crystallographica Section B

Structural Science

Volume 56, Part 3 (June 2000)

research papers

 

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

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

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, -Cu₇PSe₆ crystallizes in cubic symmetry, space group F43m. 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, -Cu₇PSe₆ crystallizes again in cubic symmetry, but with space group P2₁3. Taking into account a merohedric twinning, the refinement of the -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 d¹⁰ species in the -Cu₇PSe₆ ionic conducting phase. The partial ordering of the Cu⁺ d¹⁰element 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.

Keywords: Phase transitions; Argyrodite; Ionic conductors; Polymorphism.

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