Unusual nature of lanthanide chloride–alkali metal chloride M3LnCl6 compounds in the solid state

Thermodynamic and transport properties of M3LnCl6 compounds (M = K, Rb, Cs; Ln = La, Ce, Pr, Nd) were determined experimentally. The results obtained clearly show that the M3LnCl6 compounds can be divided into two groups, with compounds having only high temperature modification of cubic, elpasolite-type, crystal structure, in the first one, and compounds having both low-temperature (monoclinic, Cs3BiCl6-type) and high-temperature (cubic, elpasolite-type) modifications in the second one. Within the two groups, the thermodynamic and transport properties of M3LnCl6 compounds are well correlated with their crystal structure. These results suggest different order–disorder mechanisms of the alkali metal cation sublattice whereas the LnCl6 octahedra, forming anionic sublattice, retain their normal lattice positions.