Thermochemical modeling of the U1−yGdyO2±x phase

Highlights: •CEF model of the Gibbs energy of the U1−yGdyO2±x. •Phase diagram construction can be made using CEF representation for U1−yGdyO2±x. •CEF model can be used to compute defect fractions for U1−yGdyO2±x. -- Abstract: A thermodynamic model for the U1−yGdyO2±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 UO2±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 U1−yGdyO2±x solid solution can be combined with other representations of actinide and fission product containing fluorite UO2 phases to develop multi-component models within the CEF framework