Phase diagram of the TbBr3–CsBr binary system. Thermodynamic and transport properties of the Cs3TbBr6 compound

Abstract

Phase equilibria in the TbBr3–CsBr binary system were established from Differential Scanning Calorimetry (DSC) measurements. This binary system is characterized by three compounds, namely Cs3TbBr6, Cs3Tb2Br9 and CsTb2Br7, and two eutectics located at the TbBr3 mole fraction, x=0.095 (865 K) and x=0.552 (808 K), respectively. Cs3TbBr6 undergoes a solid–solid phase transition at 728 K and melts congruently at 1083 K with the related enthalpies 8.4 and 60.6 kJ mol−1, respectively. Cs3Tb2Br9, decomposes peritectically at 879 K, whereas CsTb2Br9 forms from Cs3Tb2Br9 and TbBr3 at 776 K and melts incongruently at 846 K. It undergoes also a solid–solid phase transition at 805 K, temperature very close to that (808 K) of the Cs3Tb2Br9–CsTb2Br7 eutectic. Separate investigations of the thermodynamic and transport properties were performed on the Cs3TbBr6 compound. These heat capacity and electrical conductivity experimental results suggest an order–disorder mechanism in the alkali metal cation sublattice whereas the TbBr6 octahedra, forming the anionic sublattice, retain their normal lattice positions. Compatibility of the experimental data was tested by the CALPHAD method. The entropy of mixing and Gibbs energies of formation of solid compounds were calculated. The temperature range of Cs3TbBr6 existence was discussed.