The diffusion of point defects in uranium mononitride: Combination of DFT and atomistic simulation with novel potential

• New interatomic potential is developed for atomistic simulation of U-N system.
• The potential allows to study UN in a wide range of pressure, temperature and composition.
• Large athermal concentration of U-vacancies is observed in the N-rich UN1+x due to close formation energies of nitrogen Frenkel pairs and Schottky defects.
• Diffusivity of U-interstitials depends on concentration of N-interstitials due to relatively small energy difference between U-interstitial and combination of N-interstitial and U-antisite.
• The most mobile point defects in UN are interstitial N and U; diffusivity of N vacancies is intermediate, while U vacancies and U antisites are noticeably less mobile.

The properties of point defects in uranium mononitride (UN) are studied by ab initio calculations and molecular dynamics simulations with a new interatomic potential. Density functional theory (DFT) calculations are used for fitting of the parameters of the angular-dependent interatomic potential, as well as for evaluation of the defects formation and migration energies. Molecular dynamics (MD) simulations are applied to analyse what migration mechanisms are activated at finite temperatures and to calculate diffusion coefficients of point defects. It is shown that the U antisite defects play an important role in the U-rich UN1−x. During migration the interstitial uranium is able to knock-out nitrogen atom, and this act leads to formation of U in antisite and N interstitial. This effect results in dependence of the diffusivity of U-interstitials on the concentration of defects in the N sublattice. Another peculiarity of UN is the large athermal concentration of U-vacancies in the N-rich UN1+x. This is due to close formation energies of nitrogen Frenkel pairs and Schottky defects. In addition, the applicability of the new potential for description of various phase transitions in UN is discussed.

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