Structure D 2000Unusual Nature of Lanthanide Chloride-Alkali Metal Chloride M 3 LnCl 6 Compounds in the Solid State. -As revealed by heat capacity and electrical conductivity measurements, the compounds M3LnCl6 (M: K, Rb, Cs; Ln: La, Ce, Pr, Nd) can be divided into two groups: compounds having only the high temperature modification of cubic elpasolite-type structure and compounds having both low-temperature (monoclinic, Cs 3 BiCl 6 -type) and high-temperature (cubic, elpasolite-type) modifications. Within the two groups, the thermodynamic and transport properties of the compounds are well correlated to their crystal structure. Different order-disorder mechanisms of the alkali metal cation sublattice are suggested, whereas the LnCl6 octahedra, forming the anionic sublattice, retain their normal lattice positions. -(GAUNE-ESCARD*, M.; RYCERZ, L.; J. Alloys Compd. 408-412 (2006) 76-79; Ec. Polytech., Mecanique Energ., Technopole Chateau-Gombert, F-13453 Marseille, Fr.; Eng.) -W. Pewestorf 18-008