Investigation of cation self-diffusion mechanisms in UO2 plus or minus x using molecular dynamics

This article is devoted to investigation of cation self-diffusion mechanisms, taking place in UO2, UO2+x, and UO2-x crystals simulated under periodic (PBC) and isolated (IBC) boundary conditions using the method of molecular dynamics in the approximation of rigid ions and pair interactions. It is shown that under PBC the cations diffuse via an exchange mechanism (with the formation of Frenkel defects) with activation energy of 15-22 eV, while under IBC there is competition between the exchange and vacancy (via Schottky defects) diffusion mechanisms, which give the effective activation energy of 11-13 eV near the melting temperature of the simulated UO2.00 nanocrystals. Vacancy diffusion with lower activation energy of 6-7 eV was dominant in the non-stoichiometric crystals UO2.10, UO2.15 and UO1.85. Observations showed that a cation vacancy is accompanied by different number of anion vacancies depending on the deviation from stoichiometry: no vacancies in UO2.15, single vacancy in UO2.00 and four vacancies in UO1.85. The corresponding law of mass action formulas derived within the Lidiard-Matzke model allowed explaining the obtained activation energies and predicting a change in the activation energy within the temperature range of the superionic phase transition. The diffusion of cations on the surface of nanocrystals had activation energy of 3.1-3.6 eV.