GGA plus U study of the incorporation of iodine in uranium dioxide

Ab initio calculations based on the Density Functional Theory are carried out in order to investigate the incorporation of iodine in uranium dioxide. The GGA+U approximation is used to describe the strong correlations of uranium 5f electrons. We studied several defects that are likely to accommodate the incorporation of iodine in the material, such as uranium and oxygen vacancies, divacancy and Schottky defects. We find the iodine atoms to be stable in a neutral Schottky defects, with an incorporation energy of -1.3 eV. This result may account for the solubility of iodine in uranium dioxide observed experimentally. We also notice that the incorporation of iodine involves steric and electronic contributions. The larger the defect iodine is incorporated in, the lower is its incorporation energy. Besides, we find iodine to be charged -1, thus getting the stable electronic configuration of rare gases. We also highlight the fact that the use of GGA+U increases the number of metastable states (non global energy minima), compared to the LDA/GGA approximations. Consequently, special care has to be taken on the 5f electronic occupancies in order to ensure that the absolute energy minimum has been reached.