Charge states of point defects in uranium oxide calculated with a local hybrid functional for correlated electrons

The formation energies and charge states of point defects in uranium dioxide are calculated from first principles, using the local hybrid functional for correlated electrons, which offers a nice alternative to LDA+U calculations. The possible occurrence of multiple minima in such calculations is carefully taken into account. Point defects in UO2 are generally found to be charged with a -4 charge for uranium vacancies, -2 for oxygen interstitials, and a charge ranging from 0 to +2 for oxygen vacancies depending on the position of the Fermi level. The calculated formation energies of noninteracting oxygen Frenkel pairs and Schottky defects made of the association of charged defects are in very good agreement with experimental values. A Brouwer diagram based on a point-defect model for stoichiometry variations in UO2+x is built. It fails to predict the dominant concentration of oxygen interstitials in the overstoichiometric oxide but predicts that oxygen vacancies are in a + 1 charge state in the hypostoichiometric oxide. This charge state, which differs from the one assumed in the traditional ( fully ionic) picture of UO2, is confirmed by the obtained variation of the deviation from stoichiometry with oxygen pressure, which is consistent with experiments.