Study of phase equilibrium of Pu2O3-PuO2 system by the first-principles calculation and CALPHAD approach

A combination of a first-principles calculation, lattice dynamics and CALPHAD (CALculation of PHAse Diagrams) modeling is proven as a powerful tool so as to evaluate the Gibbs free energy and a phase equilibrium between compounds including large amount of vacancies. In this work, non-stoichiometric PuO2-x (dioxide) and Pu2O3 (sesquioxide) has been studied. An electron cohesive energy was evaluated from a first-principles calculations to estimate total energy of the compounds and a vacancy formation energy, and the theory of statistical mechanics was applied to evaluate enthalpy/entropy change due to oxygen vacancies for the non-stoichiometry of the PuO2 (i.e. PuO2-x). Then a vacancy-vacancy interaction energy was determined by fitting to the experimental data of a quantity of non-stoichiometry of the PuO2 compounds as a function of oxygen potentials at large deviation from stoichiometry. The resulting Gibbs free energy yields phase boundary between the phases with good agreement with to the experimental data.