Ab initio simulations of oxygen interaction with surfaces and interfaces in uranium mononitride

TL;DR

This study uses DFT simulations to analyze how oxygen interacts with uranium mononitride surfaces and grain boundaries, revealing easy oxygen penetration and mobility that lead to surface oxidation.

Contribution

It provides detailed atomistic insights into oxygen adsorption, migration, and incorporation into UN surfaces and grain boundaries, highlighting mechanisms of oxidation.

Findings

Oxygen easily penetrates UN surfaces and grain boundaries with N vacancies.

Adsorbed oxygen atoms are highly mobile on UN surfaces.

Oxygen incorporation into N vacancies facilitates surface oxidation.

Abstract

The results of DFT supercell calculations of oxygen behavior upon the UN (001) and (110) surfaces as well as at the tilt grain boundary are presented. Oxygen adsorption, migration, incorporation into the surface N vacancies on (001) and (110) surfaces have been modeled using 2D slabs of different thicknesses and supercell sizes. The temperature dependences of the N vacancy formation energies and oxygen incorporation energies are calculated. We demonstrate that O atoms easily penetrate into UN surfaces and grain boundaries containing N vacancies, due to negative incorporation energies and a small energy barrier. The Gibbs free energies of N vacancy formation and O atom incorporation therein at the two densely-packed surfaces and tilt grain boundaries are compared. It has been also shown that the adsorbed oxygen atoms are highly mobile which, combined with easy incorporation into surface N vacancies, explains efficient (but unwanted) oxidation of UN surfaces. The atomistic mechanism of UN oxidation via possible formation of oxynitrides is discussed.