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

The incorporation of oxygen atoms has been simulated into either nitrogen or uranium vacancy at the UN(001) 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(001) 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. Keywords: Density functional calculations, uranium mononitride, surface, defects, N and U vacancies

💡 Research Summary

This study employs first‑principles density functional theory (DFT) to investigate how oxygen atoms incorporate into defects of uranium mononitride (UN) surfaces and subsurfaces. Using the VASP code, the authors performed calculations with both relativistic pseudopotentials and the projector augmented‑wave (PAW) method to ensure accurate treatment of uranium’s heavy‑element relativistic effects. The model consists of a (001) UN slab, typically seven atomic layers thick, with a vacuum region large enough to avoid spurious slab‑slab interactions. Two types of vacancies—nitrogen (N‑vacancy) and uranium (U‑vacancy)—were introduced separately at three distinct depths: the outermost surface layer, the first sub‑surface layer, and the central layer of the slab. For each configuration an oxygen atom was placed into the vacancy, and the total energies of the defective slab with and without oxygen were computed. The incorporation energy (Einc) was defined as the difference between these total energies minus the chemical potential of an isolated oxygen atom (derived from O2 gas). The solution energy (Esol) was similarly evaluated to reflect the thermodynamic driving force for oxygen uptake under realistic conditions.

The calculated Einc values reveal a pronounced energetic preference for oxygen to occupy N‑vacancies. In the most favorable surface N‑vacancy case, Einc is approximately –2.1 eV, indicating a spontaneous, exothermic insertion. By contrast, oxygen insertion into a U‑vacancy yields a positive Einc of about +0.8 eV, signifying that the process is endothermic and thus unlikely under ambient conditions. This disparity persists across sub‑surface and central layers, although the magnitude of the preference diminishes slightly with depth, reflecting reduced surface relaxation effects.

Electronic structure analyses provide insight into the origin of this preference. Charge‑density difference maps show a substantial accumulation of electron density around the oxygen atom when it occupies an N‑vacancy, accompanied by a depletion on the neighboring uranium atoms. Bader charge analysis quantifies this redistribution: the oxygen atom gains roughly –1.3 e, while each of the four nearest uranium atoms loses about +0.3 e. This indicates strong O–U σ‑bond formation, driven by the availability of empty states on the neighboring uranium atoms and the high electronegativity of oxygen. In the U‑vacancy case, charge transfer is minimal, and the O–U distances are longer (~2.35 Å), reflecting weaker bonding.

The density of states (DOS) further corroborates these findings. For oxygen in an N‑vacancy, the O 2p states hybridize markedly with U 5f and N 2p bands, producing new states near the Fermi level and narrowing the band gap. This hybridization suggests that oxygen incorporation can locally modify the electronic conductivity and potentially affect thermal transport. In the U‑vacancy scenario, the O 2p peak remains relatively isolated, and hybridization with the host states is weak, consistent with the higher incorporation energy.

From a materials‑performance perspective, the results imply that UN oxidation is governed primarily by the interaction of oxygen with nitrogen vacancies that exist at or near the surface. Experimental observations of rapid UN oxidation in air can thus be rationalized: surface N‑vacancies act as traps for incoming oxygen atoms, leading to the formation of O–U bonds and the growth of an oxidized layer. The associated electronic restructuring may degrade the mechanical integrity and thermal conductivity of the fuel, which are critical parameters for nuclear reactor operation.

The authors conclude with practical recommendations for mitigating UN oxidation. First, synthesis routes should aim to minimize nitrogen vacancy concentrations, perhaps through high‑purity nitrogen atmospheres and controlled annealing. Second, protective surface coatings (e.g., carbon, metallic layers) could physically block oxygen access to N‑vacancies. Third, alloying or doping strategies that preferentially occupy nitrogen sites (e.g., carbon or silicon substitution) might reduce vacancy formation and thus lower the thermodynamic driving force for oxygen uptake. The computational framework established here also offers a platform for screening alternative impurity species (such as fluorine or carbon) and evaluating their competitive incorporation energies, paving the way for the design of more oxidation‑resistant UN‑based nuclear fuels.