Mechanisms of exchange and anion frenkel diffusion in uranium dioxide: Molecular dynamics study

Literature analysis reveals a significant discrepancy in the anion diffusion activation energies, calculated by the MD and the lattice statics methods using the point defects model, for a given interaction potentials set. To clarify this issue, the anion diffusion mechanisms and defect concentrations in a wide temperature range have been studied in detail. The stepped shape of the mean squared displacement (MSD) curves of anions, observed at low temperatures, is caused by the spontaneous formation and subsequent recombination of the long-living anion Frenkel defect. It is shown that correct MSD can be obtained, if at least one thermally activated defect exists (on average) in the simulation box. Otherwise the anion diffusion coefficients depend on the supercell size. Using the concentrations of defects and their mobility, calculated by the MD method, the diffusion coefficients of anions in the low-temperature region were calculated. They coincide well with the experimental data in absolute values. The mechanism of exchange diffusion has been quantitatively studied for the first time. Methods for detecting single exchange acts and calculating MSD for each type of exchange chains were developed. It is shown that exchange diffusion gives the greatest contribution at medium temperatures, before the superionic transition. (C) 2019 Elsevier B.V. All rights reserved.