Structure and Stability of the Low-Temperature Modification of Compounds Cs3LnCl6 (Ln = La-Gd)

The crystal structure of the low-temperature modification of Cs3LaCl6 has been determined from X-ray powder diffraction data by the Rietveld method. The monoclinic lattice with a = 27.286(5) Å; b = 8.291(1) Å; c = 13.305(2) Å; β = 99.64(1)° belongs to the Cs3BiCl6-type (space group C2/c). All other compounds Cs3LnCl6 (Ln = Ce-Gd) and the analogous Rb-compounds are isotypic. Emf measurements in a galvanic cell for solid electrolytes for the reactions CsCl + Cs2LnCl5 = Cs3LnCl6 reveal that the compounds with Ln = La, Ce, Pr, 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 0K. The compounds with Ln = Sm, Eu, Gd are formed with a gain in lattice enthalpy and are stable at T = 0K, too. This difference is attributed to the different crystal structures of the neighboring compounds Cs2LnCl5: they crystallize with the K2PrCl5-structure (CN = 7 for Ln3+) for the compounds with Ln = La-Nd, while the other compounds belong to the Cs2DyCl5-type with octahedral surroundings for the Ln3+ ions.