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Comment on "Modeling oxygen self-diffusion in UO2 under pressure by M.W.D Cooper et al., Solid State Ionics 282 (2015) 26-30"

The oxygen self-diffusion coefficient in UO${_2}$ has been recently studied [Cooper et al. Solid State Ionics 282 (2015) 26-30] over a range of pressures (0-10GPa) and temperatures (300-1900K) by combining molecular dynamics calculations with a thermodynamical model, the cBΩ model. A significant reduction in oxygen self-diffusion as a function of increasing hydrostatic pressure, and the associated increase in activation energy was identified. Here, we extend this study and find that the compressibility of the corresponding activation volume exceeds significantly the compressibility of the bulk material by almost one order of magnitude. This results is important since in the literature it is usually assumed that these two compressibilities are equal. The same holds when comparing the thermal expansion coefficient of this volume with that of the bulk solid.