Modeling of punctual defects in UAL₄ from the U-Al system: Combination of CALPHAD method with first principles calculation

We investigated the point defect structure of oI20 UAl₄ in order to study aluminum diffusion. We performed ab initio calculations within a pseudopotentials method implemented in the Vienna Ab initio Simulation Package (VASP) to obtain point defect formation energies: vacancies (V^U and V^Al ) and antisites (Al^U and U^Al). Using a statistical-thermodynamic model we calculated defects concentrations as function of temperature and deviation from stoichiometry. For stoichiometric UAl₄ antisites are the dominant thermal defects. In off-stoichiometric UAl₄, antisites are the constitutional defects. For U-rich UAl₄, the thermal defect is called Entre Ramas, where one antisite U atom is replaced by five Al vacancies. For Al-rich UAl₄, the thermal defect is also an Entre Ramas, where four antisite Al atoms are replaced by five U vacancies. Our first principles results were used to model UAl₄ intermediate phase with a two sublattices Wagner-Schottky model. The Thermocalc data bases previously used were modified, including antisites and vacancies in both sublattices of the UAl₄ intermediate phase: (U,Al,VA)_0.2: (Al,U,VA)_0.8. We obtained a consistent thermodynamic database able to reproduce the entire U-Al equilibrium phase diagram (author)