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

Abstract

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.

The heat capacities of the above compounds in the solid as well as in the liquid phase were determined by differential scanning calorimetry. A special method - 'step method' developed by SETARAM was applied in these measurements. The heat capacity experimental data were fitted by a polynomial temperature dependence.