Using ab initio molecular dynamics simulations and nudged elastic band calculations the finite temperature stability, transition pathways, and migration mechanisms of large oxygen clusters in UO2+x were examined. Here, specific consideration was given to the recently proposed split quad-interstitial and cuboctahedral oxygen clusters. It was shown that isolated cuboctahedral clusters could transform into more stable configurations that were closely linked to the split quad-interstitial. The split quad-interstitial was stable with respect to single interstitials occupying the empty octahedral holes of the UO2 lattice. In order to better understand discrepancies between theory and experiments, the simulated atomic pair distribution functions for the split quad-interstitial structures were analyzed with respect to the distribution function for U4O9 previously obtained from neutron diffraction data. The nudged elastic band calculations suggest that the split quad-interstitial may migrate by translating one of its constituent di-interstitial clusters via a barrier that was lower than the corresponding barrier for individual interstitials, but higher than the barrier for the most stable di-interstitial cluster.

Stability and Migration of Large Oxygen Clusters in UO2+x: Density Functional Theory Calculations. D.A.Andersson, F.J.Espinosa-Faller, B.P.Uberuaga, S.D.Conradson: Journal of Chemical Physics, 2012, 136[23], 234702