Influence of charge states on energies of point defects and clusters in uranium dioxide

The variation of formation energies of point defects and clusters in uranium dioxide (UO2) as a function of their charge states is studied by density functional theory (DFT). Di- and trivacancies are considered as well as various assemblies of oxygen interstitials, namely the cuboctahedral defect (either empty or filled) and the split-di-interstitial. The energies of formation of these defects for various possible charge states are calculated using the DFT + U approach. The occurrence of multiple minima is circumvented by the use of the U-ramping technique [Meredig et al., Phys. Rev. B 82, 195128 (2010)]. One finds that point defects and vacancy clusters bear their formal charges, deduced from the ionic picture of bonding in UO2. Conversely, clusters of oxygen interstitials are much less charged than this fully ionic limit. The energy gain upon clustering is vastly modified when the possible charge of defects is taken into account. Vacancy clusters prove only marginally stable compared to their isolated counterparts. Clusters of oxygen interstitials are found energetically unstable with respect to isolated interstitials in the stoichiometric compound.