International audienceThis work provides an illustration that density functional theory (DFT) + U calculations may quantitatively describe transport phenomena in uranium dioxide. Oxygen diffusion mechanisms are investigated using both ab initio calculations and experimental approaches mainly involving self-diffusion coefficient measurements. The dependences of the experimental data upon oxygen potential and sample impurity content demonstrate, by comparison with basic point defect and diffusion theory, that oxygen migration occurs via an interstitial mechanism. The temperature study provides an estimate of interstitial formation and migration energies which compare very favorably to energies calculated using the DFT+U approximation relating to the interstitialcy mechanism. Also, vacancy migration and Frenkel pair formation energies are shown to agree well with existing data

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The structure of ferroelectric domain walls in BaTiO3 has been investigated through two complementary approaches, a global one by the fine analysis of X-ray diffraction patterns, the other essentially local via a quantitative image analysis method developed and applied to High Resolution Transmission Electron Microscopy images. These two original approaches converge towards a clear description of 90○ walls which are shown to be a 4–6 nm wide region where the crystallographic discontinuity is accommodated by irregular atomic displacements. The results given here demonstrate that the usual structural theoretical description of walls commonly accepted for energy calculations are far too simplistic. The two underlying methodologies which have been developed to carry out these approaches can possibly be applied to other ferroelectrics, but without any doubt to other systems where twins or coherent interfaces are expected.