States of the Schottky defect in uranium dioxide and other fluorite type crystals: Molecular dynamics study

• We derived the degree of vacancy association in Schottky defects from temperature.
• At high temperatures cation vacancy forms complexes with anion vacancies.
• Point defects model in the presence of complexes was considered.
• The presence of anion vacancies around cation vacancy lowers its migration energy.
• The formation of nanovoids at high cation vacancy concentrations was shown.

Mass transfer processes in fluorite-type systems are determined by the diffusion of cations via vacancies from Schottky defects. To predict diffusion coefficients of cations and other parameters based on it the Lidiard and Matzke approximation of the point defects model, namely the assumption of isolated vacancies, is widely used. States of the Schottky defect were studied with the high-speed molecular dynamics method in a wide temperature range, with six different interaction potentials. Schottky vacancies were dynamically detected during the simulation. It is shown that contrary to the Lidiard and Matzke model, the Schottky cation vacancy is always associated with anion vacancies.The degree of the Schottky defect association depends on the temperature, at high temperatures near the cation vacancy two or more anion vacancies are located. It is shown that the calculated formation energy of Schottky defects in the form of the trivacancy (5.8–7.4) eV for all potentials are close to the experimental value (6–7) eV, in contrast to the formation energy of Schottky defects in the form of isolated vacancies that exceeds 10 eV.Point defects model of the simulated system in the presence of an artificially created Schottky defect was constructed and compared with calculation results. According to our study the point defects model is applicable only at low temperatures less than half of the melting temperature.It is shown that the presence of anion vacancies near the cation vacancy reduces the migration energy of cations. However, for systems with several Schottky defects the vacancy clusterization and the formation of voids are observed. This leads to an increase of the diffusion activation energy due to the contribution of the energy that needs to separate the single cation vacancy from the void.