Phase equilibrium of PuO2−x − Pu2O3 based on first-principles calculations and configurational entropy change

Combination of an oxygen vacancy formation energy calculated using first-principles approach and the configurational entropy change treated within the framework of statistical mechanics gives an expression of the Gibbs free energy at large deviation from stoichiometry of plutonium oxide PuO2. An oxygen vacancy formation energy 4.20 eV derived from our previously first-principles calculation was used to evaluate the Gibbs free energy change due to oxygen vacancies in the crystal. The oxygen partial pressures then can be evaluated from the change of the free energy with two fitting parameters (a vacancy–vacancy interaction energy and vibration entropy change due to induced vacancies). Derived thermodynamic expression for the free energy based on the SGTE thermodynamic data for the stoichiometric PuO2 and the Pu2O3 compounds was further incorporated into the CALPHAD modeling, then phase equilibrium between the stoichiometric Pu2O3 and non-stoichiometric PuO2−x were reproduced.