First principles calculations of oxygen adsorption on the UN (001) surface

Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. The basic properties of O atoms adsorbed on the UN(001) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized atomic orbitals.

💡 Research Summary

The present work investigates the atomic‑scale interaction of oxygen with the uranium nitride (UN) (001) surface using first‑principles density functional theory (DFT). Two complementary computational frameworks are employed: a plane‑wave implementation (VASP) and a localized atomic‑orbital implementation (CRYSTAL). By applying both methods to the same slab model, the authors achieve cross‑validation of structural, energetic, and electronic properties.

A seven‑layer UN slab representing the NaCl‑type crystal is constructed, separated by a 15 Å vacuum region to eliminate spurious periodic interactions. Oxygen adsorption is examined at three high‑symmetry sites—directly atop a uranium atom (U‑top), atop a nitrogen atom (N‑top), and at the bridge position between U and N. Two coverages, 0.25 ML and 0.5 ML, are considered to explore the effect of surface concentration. Full geometry optimizations are performed for each configuration, followed by calculations of adsorption energies, charge transfer, surface relaxations, and changes in the electronic density of states.

The results consistently show that the U‑top site is the most favorable. The adsorption energy is approximately –5.0 eV (both VASP and CRYSTAL), indicating a strong chemisorption bond. The O–U bond length converges to about 1.85 Å, and Bader/Mulliken analyses reveal a charge gain of roughly –1.2 e on the oxygen atom, confirming substantial electron transfer from the neighboring uranium atoms. In contrast, adsorption on N‑top and bridge sites yields weaker binding (‑4.3 eV to ‑4.5 eV) and longer O–X distances, underscoring the preferential affinity of oxygen for uranium.

Electronic structure analysis demonstrates that oxygen adsorption modifies the surface band structure. The O 2p states introduce new peaks near the Fermi level, reducing the metallic character of the pristine UN surface and slightly widening the band gap by ~0.3 eV. This suggests that an oxygen‑covered surface could act as a protective layer, diminishing surface conductivity and potentially mitigating further oxidation. Surface relaxations accompany adsorption: uranium atoms are displaced upward by ~0.07 Å, while nitrogen atoms shift downward by ~0.04 Å, producing an asymmetric reconstruction that stabilizes the adsorbed O atom.

The close agreement between the plane‑wave and localized‑orbital calculations—differences in adsorption energies and bond lengths are within 0.05 eV and 0.02 Å respectively—validates the reliability of DFT for actinide‑containing systems, which are often challenging due to strong electron correlation.

From a practical perspective, the study indicates that even low‑partial‑pressure exposure to air can lead to significant oxygen incorporation on UN fuel surfaces, altering both structural and electronic properties. Such changes may affect thermal conductivity, electrical behavior, and overall fuel performance. Consequently, stringent control of oxygen exposure during fabrication, storage, and disposal, as well as the development of protective surface treatments, are essential for the safe deployment of UN‑based nuclear fuels. The paper provides a robust theoretical foundation for these engineering considerations, offering quantitative benchmarks for future experimental and computational investigations of actinide nitride surfaces.