Thermodynamic investigations of oxyfluoride of thorium and uranium

Abstract

The standard molar Gibbs energy of formation of ThOF2(s) and UO2F2(s) has been determined using an e.m.f. technique. For this purpose, separate fluoride cell has been constructed using CaF2(s) as the solid electrolyte. From the measured e.m.f. values and required Gibbs energy data available in the literature, ΔfGom(T) for these oxyfluorides has been calculated. The enthalpy of formation of ThOF2(s) and UO2F2(s) at 298.15 K has been calculated from the experimentally measured Gibbs energy data using the second and the third law methods. To determine the stability domains of ThOF2(s) and UO2F2(s), the phase diagram and chemical potential diagrams of Th–F–O and U–F–O systems were calculated by the CALPHAD method and FactSage software. These calculations can be used to predict the oxygen partial pressures and the temperature domains in which thorium and uranium oxyfluorides might be formed in the molten salt medium.