Narrow Results

The possible transition states, minimum energy paths (MEPs) and migration mechanisms of defect clusters and xenon-vacancy defect clusters in uranium dioxide (UO₂) have been investigated using the dimer and the nudged elastic-band (NEB) methods. The nearby O atom can easily hop into the oxygen vacancy position by overcoming a small energy barrier, which is much lower than that for the migration of a uranium vacancy. A simulation for a vacancy cluster consisting of two oxygen vacancies reveals that the energy barrier of the divacancy migration tends to decrease with increasing the separation distance of divacancy. For an oxygen interstitial, the migration barrier for the hopping mechanism is almost three times larger than that for the exchange mechanism. Xe moving between two interstitial sites is unlikely a dominant migration mechanism considering the higher energy barrier. A net migration process of a Xe-vacancy pair containing an oxygen vacancy and a xenon interstitial is identified by the NEB method. We expect the oxygen vacancy-assisted migration mechanism to possibly lead to a long distance migration of the Xe interstitials in UO₂. The migration of defect clusters involving Xe substitution indicates that Xe atom migrating away from the uranium vacancy site is difficult.

Nitrogen is an important impurity in Czochralski grown silicon (Cz–Si) as it enhances oxygen precipitation through the formation of vacancy–nitrogen–oxygen clusters and in particular the ${\mathrm{\text{V}}}_m{\mathrm{\text{N}}}_2{\mathrm{\text{O}}}_n$ complexes. Here, we employ density functional theory (DFT) to predict the structure of ${\mathrm{\text{V}}}_m{\mathrm{\text{N}}}_2{\mathrm{\text{O}}}_n$ ($m,n=1,2$). We report that the lowest energy ${\mathrm{\text{V}}}_m{\mathrm{\text{N}}}_2{\mathrm{\text{O}}}_n$ ($m,n=1,2$) defects are very strongly bound. These results are consistent, and support the previously reported theoretical and experimental conclusions that ${\mathrm{\text{V}}}_m{\mathrm{\text{N}}}_2{\mathrm{\text{O}}}_n$ structures could form.