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.