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Ab initio calculations of the atomic, electronic and vibrational structure of a pure and Co +2 doped MgF 2 crystals were performed and discussed. We demonstrate that Co +2 (3d 7 ) ions substituting for Mg is in the high spin state. In particular, the role of exact non-local exchange is emphasized for a proper reproduction of not only the band gap but also other MgF 2 bulk properties. It allows us for reliable estimate of the dopant energy levels position in the band gap, and its comparison with the experimental data. Thus, the present ab initio calculations and experiment data demonstrate that the Co +2 ground state level lies at ≈2 eV above the valence band top.

A molecular dynamic simulation of diffusion of intrinsic oxygen anions in the bulk of hypostoichiometric UO2-x nanocrystals with a free surface was carried out. The main diffusion mechanism turned out to be the migration of oxygen by the anionic vacancies. It is shown that in the range of values of the non-stoichiometry parameter 0.05 £x £ 0.275 the oxygen diffusion coefficient D is weakly dependent on temperature, despite the uniform distribution of the vacancies over the model crystallite. The reliable D values calculated for the temperature T = 923 K are in the range from 3×10-9 to 7×10-8 cm2/s, in quantitative agreement with the experimental data. The corresponding diffusion activation energy is in the range from 0.57 eV to 0.65 eV, depending on the interaction potentials used for the calculations.

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