THERMOCHEMICAL STUDIES ON THE SYSTEMS ABR-LABR3 (A = NA, K, RB, CS)

The pseudo-binary systems ABr-LaBr3 (A = Na-Cs) were investigated by means of differential thermal analysis and X-ray diffraction of crystal powders. The system NaBr/LaBr3 is eutectic: in all other systems the compounds A2LaBr5 (K2PrCl5-type) exist. Additional compounds A3LaBr6, stable only at high temperature, are formed with RbBr and CsBr. They have the elpasolite-structure; modifications, metastable at ambient temperature, crystallize into the K3MoCl6-type. A further high-temperature compound, CsLa2Br7, has the same structure as RbDy2Cl7. The thermodynamic functions of the ternary bromides were determined using solution calorimetry and e.m.f. vs. T measurements in galvanic cells for solid electrolytes. These were then compared with those of the LaCl3 system. The main result is that, except for the halide ion, the A2LaX5 compounds are formed from adjacent compounds in the systems with a resulting gain in lattice enthalpy. All other compounds have less favourable structures: a loss of lattice enthalpy is compensated for by a gain in entropy at sufficiently high temperatures.