Thermochemical studies on the systems ABr-LaBr 3 ( A ≡ Na, K, Rb, Cs)

Abstract

The pseudo-binary systems ABr-LaBr 3 ( A ≡ Na- Cs) were investigated by means of differential thermal analysis and X-ray diffraction of crystal powders. The system NaBr LaBr 3 is eutectic: in all other systems the compounds A 2LaBr 5 (K 2PrCl 5-type) exist. Additional compounds A 3LaBr 6, stable only at high temperature, are formed with RbBr and CsBr. They have the elpasolite-structure; modifications, metastable at ambient temperature, crystallize into the K 3MoCl 6-type. A further high-temperature compound, CsLa 2Br 7, has the same structure as RbDy 2Cl 7. 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 LaCl 3 system. The main result is that, except for the halide ion, the A 2LaX 5 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.