Enthalpies of Phase Transitions and Heat Capacity of TbCl3 and Compounds Formed in TbCl3–MCl Systems (M=K, Rb, Cs)

Abstract

The molar enthalpies of the solid–solid and solid–liquid phase transitions were determined by differential scanning calorimetry for pure TbCl3 and KTb2Cl7, RbTb2Cl7, CsTb2Cl7, K3TbCl6, Rb3TbCl6 and Cs3TbCl6 compounds. Both types of compounds, i.e. M3TbCl6 and MTb2Cl7 (M=K, Rb, Cs) melt congruently and show additionally a solid–solid phase transition with a corresponding enthalpy Δtrs H 0 of 6.1, 7.6 and 7.0 kJ mol–1 for potassium, rubidium and caesium M3TbCl6 compounds andΔtrs H 0 of 17.1 (rubidium) and of 12.1 and 10.9 kJ mol–1 (caesium) for MTb2Cl7 compounds, respectively. The enthalpies of fusion were measured for all the above compounds with the exception of Rb3TbCl6 and Cs3TbCl6. The heat capacities of the solid and liquid compounds have been determined by differential scanning calorimetry (DSC) in the temperature range 300–1100 K. The experimental heat capacity strongly increases in the vicinity of a phase transition, but varies smoothly in the temperature ranges excluding these transformations. C p data were fitted by an equation, which provided a satisfactory representation up to the temperatures of C p discontinuity. The measured heat capacities were checked for consistency by calculating the enthalpy of formation of the liquid phase, which had been previously measured. The results obtained agreed satisfactorily with these experimental data.