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Enthalpy of Phase Transitions and Heat Capacity of Stoichiometric Compounds in LaBr3-MBr Systems (M=K,Rb, Cs)
Authors:Rycerz  L  Gaune-Escard  M
Institution:(1) Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland;(2) IUSTI, CNRS-UMR 139, Universitè de Provence, Technopole de Chateau Gombert, 13453 Marseille Cedex 13, France
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. This revised version was published online in July 2006 with corrections to the Cover Date.
Keywords:alkali metal bromides  enthalpy of formation  heat capacity  lanthanum bromide
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