Self-diffusion of oxygen in superstoichiomertric uranium dioxide in the range of the superion phase transition

The self-diffusion of oxygen in the superion transition range (1300–3000 K) of superstoichiometric uranium dioxide UO$_{2+x}$ is studied by the method of molecular dynamics using the pair interaction potential recovered from data for the thermal expansion of the UO$_2$ lattice. It is shown that three portions can be distinguished in the temperature dependence of the coefficient of oxygen self-diffusion in UO$_{2+x}$, $(\ln(D)=f(1/T))$, for all the compositions studied ($x$ = 0, 0.008, and 0.030). These portions, each being described by the Arrhenius relationship, correspond to the crystalline, transition, and superion states of UO$_{2+x}$. At low temperatures (1300–1820 K), the activation energies of oxygen diffusion for the above compositions are, respectively, 2.66 $\pm$ 0.44, 1.33 $\pm$ 0.10, and 1.00 $\pm$ 0.09 eV. At the beginning of the transition region, these activation energies rise to 3.40 $\pm$ 0.11, 2.24 $\pm$ 0.10, and 1.66 $\pm$ 0.60 eV. In the superion state, the activation energy of oxygen diffusion for all the compositions is the same, 1.25 $\pm$ 0.15 eV, within the error limit. As the oxygen content in UO$_{2+x}$ grows, the phase transition temperature decreases considerably and may reach 1600 K at $x$ = 0.2. Comparison with experimental data for the low-temperature oxygen diffusion coefficient and with the data of UO$_2$ simulation using graphic processors shows good agreement of the results. By comparing the concentration dependences of the oxygen diffusion coefficient that are obtained by magnetic dynamics simulation with experimental data, it is shown that quantitative calculation of these dependences in the case of UO$_{2+x}$ can be carried out only for compositions with $x <$ 0.03 if the given type of potential is used.