The crystal structure of the low-temperature modification of (\mathrm{Cs}{3} \mathrm{LaCl}{6}) has been determined from (\mathrm{X}) ray powder diffraction data by the Rietveld method. The monoclinic lattice with (a=27.286(5) \AA); (b=8.291(1) \AA ; c=13.305(2) \AA ; \beta=99.64(1)^{\circ}) belongs to the (\mathrm{Cs}{3} \mathrm{BiCl}{6})-type (space group (C 2 / c) ). All other compounds (\mathrm{Cs}{3} L n \mathrm{Cl}{6}) ( (\left.L n=\mathrm{Ce}-\mathrm{Gd}\right)) and the analogous (\mathrm{Rb})-compounds are isotypic. Emf measurements in a galvanic cell for solid electrolytes for the reactions (\mathrm{CsCl}+\mathrm{Cs}{2} L n \mathrm{Cl}{5}=\mathrm{Cs}{3} L n \mathrm{Cl}{6}) reveal that the compounds with (L n=\mathrm{La}, \mathrm{Ce}, \mathrm{Pr}, \mathrm{Nd}) are formed with a loss of lattice enthalpy, compensated by a considerable gain in entropy; they therefore are stable only at temperatures higher than (0 \mathrm{~K}). The compounds with (L n=\mathrm{Sm}, \mathrm{Eu}, \mathrm{Gd}) are formed with a gain in lattice enthalpy and are stable at (T=0 \mathrm{~K}), too. This difference is attributed to the different crystal structures of the neighboring compounds (\mathrm{Cs}{2} \mathrm{LnCl}{5}) : they crystallize with the (\mathrm{K}{2} \mathrm{PrCl}{5})-structure (\left(\mathrm{CN}=7\right.) for (\mathrm{Ln}^{3+}) ) for the compounds with (L n=\mathrm{La}-\mathrm{Nd}), while the other compounds belong to the (\mathrm{Cs}_{2} \mathrm{DyCl})-type with octahedral surroundings for the (\mathrm{Ln}^{3+}) ions. ( 1993 Academic Press. Inc.