Thermochemical modeling of the U_1-yGd _yO_2±x phase

A thermodynamic model for the U_1-yGd _yO_2±x phase was developed using the compound energy formalism (CEF) with a three sublattice approach and is an extension of the already successful CEF representation of the fluorite UO _2±x phase. The Gibbs energies for the end-members created by the addition of Gd to the cation sublattice are estimated using the lattice stability of a fictive gadolinium oxide fluorite structure compound from density functional theory. The model interaction parameters are determined from reported oxygen potential-temperature-composition measurements. The calculated results are in good agreement with the experimental data and the trends are consistent. The CEF for the U_1-yGd_yO _2±x solid solution can be combined with other representations of actinide and fission product containing fluorite UO ₂ phases to develop multi-component models within the CEF framework.