  Chloride molten salts are increasingly recognized for their applications in
pyroprocessing techniques for the separation of lanthanides. Understanding the
thermodynamic properties of these molten salts is essential to optimize the
separation process. Several thermodynamic models, including the associate
model, the two-sublattice ionic model, and the modified quasichemical model
with quadruplet approximation (MQMQA), are utilized to capture the complexity
of molten salts. In the present work, the Bayes factor was used to guide model
selection process for the thermodynamic modeling of the KCl-LaCl$_3$ system and
provide statistical comparisons of liquid models. The results indicate that the
MQMQA model is the most favorable model based on the available thermochemical
data. The LiCl-KCl-LaCl$_3$ system was further optimized with uncertainty
quantification using MQMQA. The thermodynamic properties of compounds in the
KCl-LaCl$_3$ system were obtained from DFT-based phonon calculations. The
calculated phase stability shows excellent agreement with experimental data,
indicating that an appropriate model is important for accurately predicting the
behavior of complex molten salts.
