Ab initio modeling of oxygen impurity atom incorporation into uranium mononitride surface and sub-surface vacancies

The incorporation of oxygen atoms has been simulated into either nitrogen or uranium vacancy at the UN(0 0 1) surface, sub-surface or central layers. For calculations on the corresponding slab models both the relativistic pseudopotentials and the method of projector augmented-waves (PAW) as implemented in the VASP computer code have been used. The energies of O atom incorporation and solution within the defective UN surface have been calculated and discussed. For different configurations of oxygen ions at vacancies within the UN(0 0 1) slab, the calculated density of states and electronic charge re-distribution was analyzed. Considerable energetic preference of O atom incorporation into the N-vacancy as compared to U-vacancy indicates that the observed oxidation of UN is determined mainly by the interaction of oxygen atoms with the surface and sub-surface N-vacancies resulting in their capture by the vacancies and formation of O-U bonds with the nearest uranium atoms.