Thermodynamics and Phase Stability of SmF₃ with LiF, NaF, and KF for Molten Salt Reactor Applications

Understanding and controlling the thermodynamic behavior of molten fluoride salts is critical for their deployment in advanced energy systems, including molten salt reactors. Self-consistent Gibbs energy functions were developed to describe the thermochemical behavior of the fission product SmF₃ with LiF, NaF, and KF using the modified quasi-chemical model in the quadruplet approximation. Enthalpies of mixing for each pseudobinary system were estimated through a correlative method based on the ionic size difference parameter, δ₁₂. Targeted differential scanning calorimetry experiments, supported by X-ray diffraction, were performed to establish phase equilibria in LiF-SmF₃, NaF-SmF₃, and KF-SmF₃. These measurements confirm the thermodynamic instability of the reported LiSmF₄ phase and establish the melting and decomposition temperatures of K₃SmF₆. Low-temperature heat capacities of intermediate compounds NaSmF₄, KSmF₄, and K₂SmF₅ were measured for the first time and used to constrain the entropies of these compounds in thermochemical assessments. The resulting description provides a validated basis for predicting Sm-bearing phase stability and thermochemistry in the constituent salts of FLiNaK.