Computer simulation of defect clusters in UO₂ and their dependence on composition

It is recognised that point defects play a key role in the behaviour and properties of many technologically significant oxides. What is less well understood is how these defects cluster together and, crucially, the extent to which the clusters change with composition. We chose to investigate this phenomenon by considering UO₂, a nuclear fuel material for which there is contradictory data in the literature concerning defect clustering as a function of oxygen content. Early studies of fluorite UO_2+x proposed a model based on 2:2:2 Willis clusters whilst more recent research suggests cuboctahedral or split quad-interstitial defect clustering. Here we use the PBE + U functional to simulate defective UO_2+x and find for 0.125 < x < 0.25, chains of edge-sharing 2:2:2 Willis clusters to be most stable. Below x = 0.125 these chains destabilise, transforming in to split di-interstitial clusters, demonstrating that the type of oxygen cluster present is dependent on local environment and stoichiometry.