Thermodynamic modeling of the (U,La)O_2±x solid solution phase

Lanthanide (Ln) fission products have high fission yields and are known to form solid solutions with UO₂ over a wide range of composition. As part of a larger effort to predict phase stability of the mixed metal oxide (U,Ln)O_2±x solid solution phase, a comprehensive and self-consistent thermodynamic model for (U,La)O_{2± x} has been developed through the use of the compound energy formalism (CEF) as implemented in the CALPHAD (CALculation of PHAse Diagram) computational thermodynamic approach. The reported experimental oxygen chemical potentials for both hyper- and hypo-stoichiometric (U,La)O _2±x have been assessed and used to evaluate interaction parameters for the phase representation. The lattice stability of hypothetical "LaO₂" in the CaF₂ structure necessary to describe the Gibbs energy of end-members for the La-doped UO ₂ solution phase has been obtained from first-principles calculations based on density functional theory. With respect to LaO_1.5 it is determined to equal +8739 J/mol. Good agreements between the calculated and experimental oxygen partial pressures have been obtained by introducing interaction parameters for the mixing between U and La in the metal cation sublattice. Calculated partial pressures of oxygen in equilibrium with the (U,La)O_2±x solution phase at various temperatures are presented.