Phase equilibrium of PuO_2-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 Pu ₂O₃ compounds was further incorporated into the CALPHAD modeling, then phase equilibrium between the stoichiometric Pu₂O ₃ and non-stoichiometric PuO_2-x were reproduced.