First-principle Study Of Uranium Dioxide

We report a study of the electronic structure and optical properties of uraniumdioxide based on ab-initio density-functional theroy using generalized gradientapproximation (GGA).In order to describe the strong correlation between5f electronic ofuranium correctly,we employ hubbard U correction. We first optimized the variation ofthe bulk uranium dioxide properties with the different (U-J) values. We found that thevalues of (U-J) and the different XC functional in the DFT+U calculations indicate thatwe get a better result in the LDA+U using a (U-J)4.5ev value which is in goodagreement with the experiment value. As well we found a relatively accurate value for thePW91functional in most calculation existing a (U-J)3.5ev. So it is difficult to select aabsolute accurate (U-J) value for the two different functionals. Then we determine thestructural and electronic properties of the ground state of uranium dioxide and comparethem with the results obtained in published literature and experimental data. According tothe optimization of the UO2cell structure, the lattice constant is a=b=5.558、c=5.496,Then we succeed in predicting UO2to be a semi-conductor,the band gap is2.03ev which is only0.07ev smaller than the experimental data. We can clear see that thevalence bands are mainly derived from U5f and O2p orbitals. the peak around the Fermilevel is mainly constitute by U5f and a little O2p orbital.Based on the accurateelectronic structure, we also calculate the complex dielectric function of UO2and therelated optical properties. Such as reflectivity, refractive index, exinction index,energy-loss spectra and absorption coefficient.The values in the optical spectrum are allconsistent well with the experimental results.Because of uranium dioxide can produce continuous fission products, we study thestability of oxygen or uranium defect in the uranium dioxide structure. The study ofradiation behavior in fission product solubility has great help to the safety of the nuclearreaction. We also report here DFT+U calculation of the point defects in the fluoritestructure of UO2. In non-stoichiometric uranium dioxideUO2x, we investigated thevariation of the formation energy and the volume for the oxygen interstitial or vacancyand uranium vacancy in UO2. Calculation results shown that for oxygen defects we found that except the oxygen dimmer,the system volume reduces linearly with the increase ofparameter x.