Clustering of excess oxygen in uranium dioxide: A first-principles study

To explore the behavior of excess oxygen in uranium dioxide (UO_2+x), we have investigated the energetics of oxygen interstitial clusters using density function theory (DFT+U). The calculations are based on transverse 3k antiferromagnetic UO_2+x with a $Pa\overline{3}$ configuration. The spin-orbit coupling increases the possibility of capturing the ground state of UO₂ without special control. The size of the simulation boxes significantly impacts the formation energies and thus the clustering energies of defects, especially for large oxygen interstitial clusters with charge states. The interactions between two negatively charged oxygen interstitials as a function of distance in UO_2+x demonstrate that the oxygen interstitials prefer a split di-interstitial configuration for distances less than 0.5 nm, but the mono-interstitial is the preferred configuration for distances greater than 0.6 nm. Cube-octahedral clusters are metastable and may relax to split-type clusters. The clustering energies of oxygen interstitial clusters imply that the interstitials are more prone to cluster for Fermi levels close to the valence band, while the clustering becomes energetically unfavorable at Fermi levels beyond the mid band gap. The Bader charge analysis indicates that neighboring uranium ions to the oxygen interstitials can oxidize to a +5 valence state.