This study offers an in-depth thermodynamic analysis of two ternary compounds, Na3CrF6(s) and α-Na3FeF6(s) which could form during reactor operation as a result of interactions between molten fluoride salts and structural materials. In the present study, a room temperature solution calorimeter was employed to determine the standard molar enthalpy of formation \(\left( {\Delta_{{\text{f}}} H_{{298.15{\text{K}}}}^{\rm o} } \right)\) of these compounds at 298.15 K. The \(\Delta_{{\text{f}}} H_{{298.15{\text{K}}}}^{\rm o}\) values for Na3CrF6(s) and α-Na3FeF6(s) were found to be –2976.0 ± 6.2 and – 2779.9 ± 6.7 kJ mol−1, respectively. Relaxation calorimetry in conjunction with differential scanning calorimetry was used to determine the molar heat capacity \(\left( {C_{{\text{p}}} } \right)\) of the compounds in the temperature range 2–650 K. Using the values of heat capacity in the temperature range (2–302 K), the value of standard molar entropy at 298.15 K \(\left( {S_{{298.15{\text{K}}}}^{\rm o} } \right)\) for Na3CrF6(s) and α-Na3FeF6(s) compounds were determined, which were found to be 254.9 and 264.6 J K−1 mol−1, respectively. The values of the enthalpy of formation at 298.15 K together with the molar heat capacity of these compounds were used to derive the temperature dependence of other thermodynamic functions, e.g. entropy, free energy function, and Gibbs energy of formation in the present report. These results provide important insights for thermodynamic modelling and for assessing the chemical compatibility of structural materials in molten fluoride salt-based reactor systems.
