Thermal motion of the univalent metal ions in KCr5S8-type chalcogenides, ternary chromium selenides M xCr5Se8 (M = Rb, Cs)

Abstract

The nonstoichiometric compounds Rb0.75Cr5Se8 and Cs0.76Cr5Se8 crystallise in the monoclinic space group C2/m (Z = 2) being isostructural to KCr5S8 (Rb0.75Cr5Se8: a = 1877.0(8) pm, b = 361.2(2) pm, c = 902.1(9) pm, b = 104.23(6)°, Rf = 0.0399, 826 unique reflections; Cs0.76Cr5Se8: a = 1885.3(9) pm, b = 362.7(2) pm, c = 910.9(3) pm, b = 104.27(4)°, Rf = 0.0524, 1280 unique reflections). The structures are build up by edge and face shared CrSe6 octahedra forming one-dimensional channels parallel to the monoclinic b axis. In these channels, the alkali metal ions are located. Many compounds of the KCr5S8-structure type display significant enlargement of the U11 or the U22 displacement parameters of the univalent metal ions. A model that can explain the enlargement of these parameters is presented. It is based on the relative size of the MI lattice sites including repulsions and M+ ion deficiency phenomena.