Enthalpy of Phase Transitions and Heat Capacity of Stoichiometric Compounds in LaBr3-MBr Systems (M=K, Rb, Cs)

Molar enthalpies of solid-solid and solid-liquid phase transitions of the LaBr3, K2LaBr5, Rb2LaBr5, Rb3LaBr6 and Cs3LaBr6 compounds were determined by differential scanning calorimetry. K2LaBr5 and Rb2LaBr5 exist at ambient temperature and melt congruently at 875 and 864 K, respectively, with corresponding enthalpies of 81.5 and 77.2 kJ mol-1. Rb3LaBr6 and Cs3LaBr6 are the only 3:1 compounds existing in the investigated systems. The first one forms from RbBr and Rb2LaBr5 at 700 K with an enthalpy of 44.0 kJ mol-1 and melts congruently at 940 K with an enthalpy of 46.7 kJ mol-1. The second one exists at room temperature, undergoes a solid-solid phase transition at 725 K with an enthalpy of 9.0 kJ mol-1 and melts congruently at 1013 K with an enthalpy of 57.6 kJ mol-1. Two other compounds existing in the CsBr-based systems (Cs2LaBr5 and CsLa2Br7) decompose peritectically at 765 and 828 K, respectively.