THERMOCHEMICAL INVESTIGATIONS ON CHLORO-ELPASOLITES OF LANTHANUM

Elpasolites A2BLaCl6 with alkali metal chlorides (LiCl excepted) have been prepared from the melts. They are Cs2RbLaCl6 (dimorphouS), Cs2KLaCl6 (trimorphous), Cs2NaLaCl6 and Rb2KLaCl6 which is stable only at T greater-than-or-equal-to 748 K. According to X-ray diffraction measurements of crystal powders all compounds have modifications with the cubic elpasolite structure, space group Fm3m. The room-temperature modification of Cs2KLaCl6 is slightly distorted to a tetragonal structure (subgroup P4/mnc), while low-temperature Cs2RbLaCl6 is more strongly distorted to a monoclinic modification (Cs3BiCl6 type; subgroup C2/c). The transition enthalpies are 0.92 kJ mol-1 and 5.79 kJ mol-1, respectively. The enthalpies for the formation from the binary components ACl, BCl and LaCl3 reveal that CS2NaLaCl6 is the compound formed with the greatest gain in lattice enthalpy. This correlates with the fact, that its ''tolerance factor'', derived by Babel from the perovskites, is next to 1.0.