A comprehensive investigation on point defects and their clustering behavior in non-stoichiometric uranium dioxide UO2±x was carried out using the LSDA+U method based on density functional theory. Accurate energetic information and charge transfers available so far were obtained. With these energies that have improved more than 50% over that of pure generalized gradient approximation and local density approximation, it was shown that density functional theory predicted the predominance of O defects over U ones at any composition, which was possible only after properly treating the localized 5f electrons. Calculations also suggested an upper bound of x~0.03 for O clusters to start off. The volume change induced by point U defects was monotonic but non-linear whereas, for O defects, increasing x always reduced the system volume linearly, except for a dimer that required extra space for accommodation, which was identified as being a metastable ionic molecule. Though O dimers usually occupied Willis O” sites and mimicked a single O in energetics and charge state, they were rare under ambient conditions. The decomposition process and vibrational properties were studied carefully. In addition to a systematically obtained general clustering mechanism in anion-excess fluorites, also analyze were the local stabilities of possible basic clustering modes of O defects. The results offered a unified way of understanding the structure of Willis-type and cuboctahedral clusters in UO2+x and β-U4O9. Finally, the point defect model was generalized to the independent cluster approximation so as to include clustering effects.

Point Defects and Clustering in Uranium Dioxide by LSDA+U Calculations. H.Y.Geng, Y.Chen, Y.Kaneta, M.Iwasawa, T.Ohnuma, M.Kinoshita: Physical Review B, 2008, 77[10], 104120 (16pp)