Diffusion of oxygen in uranium dioxide: A first-principles investigation
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Results of ab initio density-functional theory calculations of the migration energies of oxygen vacancies and interstitials in stoichiometric ${\text{UO}}_{2}$ are reported. The diffusion of oxygen vacancies in ${\text{UO}}_{2}$ is found to be highly anisotropic, and the [1 0 0] direction is energetically favored. The atomic relaxations play an important role in reducing the migration barriers. Within the generalized gradient approximation (GGA), we find that the migration energies of the preferred vacancies and interstitials paths are, respectively, 1.18 and 1.09 eV. With the inclusion of the Hubbard $U$ parameter to account for the $5f$ electron correlations in $\text{GGA}+U$, the vacancy migration energy is lowered to 1.01 eV while the interstitial migration energy increases slightly to 1.13 eV. We find, however, that the correlation effects have a drastic influence on the mechanism of interstitial migration through the stabilization of Willis-type clusters. Indeed, in contrast to GGA, in $\text{GGA}+U$ there is an inversion of the migration path with the so-called ``saddle-point'' position being lower in energy than the usual starting position. Thus while the migration barriers are nearly the same in GGA and $\text{GGA}+U$, the mechanisms are completely different. Our results clearly indicate that both vacancies and interstitials contribute almost equally to the diffusion of oxygen in ${\text{UO}}_{2}$.