Phase equilibria in LaBr3-TlBr pseudobinary system. Thermodynamic assessment of LaBr3-MBr (M = Li, Na, K, Rb, Cs, Tl) pseudobinary systems

The phase equilibria in the lanthanum(III) bromide-thallium bromide pseudobinary system was established by means of differential scanning calorimetry (DSC). The DSC investigations were performed on samples with different compositions in the whole mole fraction x(LaBr3) range. This system includes one intermediate compound Tl2LaBr5. It melts congruently at 829 K. The compositions of TlBr-Tl2LaBr5 and Tl2LaBr5-LaBr3 eutectics, corresponding to LaBr3 mole fraction x = 0.088 (T = 687 K) and x = 0.491 (T = 788 K) respectively, was found from Tammann's plot. The LaBr3-TlBr pseudobinary system was also optimized by CALPHAD method using the available experimental data. Experimental and calculated data concerning LaBr3-TlBr system were compared with data for LaBr3-MBr (M = Li-Cs) systems, which have also been optimized by CALPHAD method. The thermodynamic properties of the liquid phase in the systems were modelled using the Associated Solution Model with {3M(+) + LaBr63-} (M = Li-Cs, Tl) associate. The dependencies of mixing enthalpy and mixing entropy on mole fraction x(LaBr3) were estimated for all investigated systems. The dependencies of formation Gibbs energy on temperature and temperature range of existence of intermediate compounds have been calculated and discussed. The CALPHAD optimization indicates that the participation of the associate in the liquid phase in the LaBr3-MBr (M = K-Cs, Tl) systems increases with the increase of the ionic radius of the monovalent metal.