Thermodynamics of mixed oxide compounds, Li2O · Ln2O3 (Ln=Nd or Ce)

The thermodynamics of mixed oxide compounds of the type Li2O · Ln2O3 (Ln=Nd or Ce) has been studied by measuring the lanthanide content of molten LiCl-Li2O-Li solutions in equilibrium with the solid mixed oxide compound at 923 K. The observed total lanthanide mole fraction in the molten salt when in equilibrium with both solid Ln2O3 and LiLnO2 was found to be 1.0×10−4 and 5.9×10−4 for Ln=Nd and Ce, respectively. The calculated mole fractions of LiLnO2 in the molten salt when the solid mixed oxide compound was present are 2.6×10−5 and 2.1×10−4 for LiNdO2 and LiCeO2, respectively. The unexpectedly large “solubilities” of the sesquioxides in the presence of Li2O can be explained by the formation of complex oxide species, LiLnO2 and Li3LnO3, in the molten salt solutions. The equilibrium constants and the free energies of formation for these species were derived from the measured data. The free energies of formation of the solid mixed oxide compounds at 923 K were found to be −1014±1.5 kJ/mol for LiNdO2 and −1006±11.0 kJ/mol for LiCeO2.