Thermodynamic Assessment of the NaCl-DyCl₃ Molten Salt System

Phase equilibrium in the NaCl-DyCl₃ system were evaluated by numerical calculation with the Thermo-Calc software. Recent experimental thermodynamic data relative to pure DyCl₃ (entropy of phase transitions) and to the mixture (enthalpy of mixing) complemented the phase diagram reported in literature. All compounds (Na₃DyCl₆, NaDyCl₄, and NaDy₂Cl₇) were treated as stoichiometric compounds. A simple substitution model revealed unsuitable for this system which includes ions of different valences and also non-purely ionic interactions. A more realistic description is proposed in the present work. It takes into account recent structural features obtained from neutron diffraction experiments carried out both on DyCl₃ and NaCl-DyCl₃ melts and also the results of Molecular Dynamics (MD) simulation. All the above results suggest that stable DyCl/ species are likely to exist in the melt. Thermodynamic modeling was made accordingly. A better optimization of the system was obtained and yielded a consistent set of thermodynamic data. The experimental enthalpy of mixing, in particular, was represented quite satisfactorily while a poor agreement has been observed with a simple substitution model.