LAUSR

Abstract The standard molar Gibbs energy of formation of Na2ThF6(s) has been determined using an e.m.f. technique. The fluoride cell: (-) Pt, NiO(s) + NiF2(s)│CaF2(s)│ThOF2(s)+ Na2ThF6(s) + NaF(s), Pt (+) has been constructed to measure Gibbs energy of formation of Na2ThF6(s) using CaF2(s) as solid electrolyte. From the measured e.m.f. values of the cell and required Gibbs energy values from the literature, ΔrG°m (Na2ThF6, s, T) has been calculated as: ΔfG°m (Na2ThF6, s, T) (kJ mol−1) ± 10 = − 3378 + 0.5296·(T/K) (700 K ≤ T ≤ 945 K). The heat capacity of Na2ThF6(s) was measured with Differential Scanning Calorimeter in the temperature range 300–800 K. The enthalpy of formation of Na2ThF6(s) at 298.15 K has been calculated by the second method from the measured Gibbs energy and heat capacity data. The chemical potential diagram of Na-Th-F-O system has been computed based on measured data in this study and required data from the literature. Based on the thermodynamic data, melting points and compositions of several potential salt mixtures have been suggested for Molten Salt Reactors.