Thermodynamic reassessment of U–Gd–O system

•CEF model of the Gibbs energy of the U1−yGdyO2±x.•Two sublattice ionic liquid model of the U−Gd−O melt.•Tentative Gibbs function for the rhombohedral UGd6O12 phase.•Computed phase diagrams for the U−Gd−O, UO2−GdO1.5, U−O, and Gd−O systems. The CALPHAD approach is used to describe the thermodynamic properties and phase relations in the U−Gd−O system. A compound energy formalism (CEF) model is developed for the U1−yGdyO2±x phase that includes the introduction of a U6+ cation to better reproduce the phase equilibria. The lattice stability for a fictive gadolinium oxide fluorite structure compound is calculated from density functional theory (DFT) for use in the model for U1−yGdyO2±x. A Gibbs function is proposed for the stoichiometric UGd6O12 compound and the partially ionic liquid sublattice model is used to represent the liquid phase. Reported experimental thermodynamic and phase equilibria data were then used in optimizations to develop representations of the phases in the system. These will be able to be extended to include other actinide and fission products to develop multi-component models within the CALPHAD framework. The models developed in this assessment well reproduce the reported phase relations for the U−Gd−O system. The computed oxygen potentials from the CEF for U1−yGdyO2±x are in good agreement with experimentally determined values.