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Formation Gibbs free energy of point defects and oxygen clusters in uraniumdioxide at high-pressure high-temperature conditions are calculated from firstprinciples, using the LSDA+U approach for the electronic structure and theDebye model for the lattice vibrations. The phonon contribution on Frenkelpairs is found to be notable, whereas it is negligible for the Schottky defect.Hydrostatic compression changes the formation energies drastically, makingdefect concentrations depend more sensitively on pressure. Calculations showthat, if no oxygen clusters are considered, uranium vacancy becomes predominantin overstoichiometric UO2 with the aid of the contribution from latticevibrations, while compression favors oxygen defects and suppresses uraniumvacancy greatly. At ambient pressure, however, the experimental observation ofpredominant oxygen defects in this regime can be reproduced only in a form ofcuboctahedral clusters, underlining the importance of defect clustering inUO2+x. Making use of the point defect model, an equation of state fornon-stoichiometric oxides is established, which is then applied to describe theshock Hugoniot of UO2+x. Furthermore, the oxidization and compression behaviorof uranium monoxide, triuranium octoxide, uranium trioxide, and a series ofdefective UO2 at zero Kelvin are investigated. The evolution of mechanicalproperties and electronic structures with an increase of the oxidation degreeare analyzed, revealing the transition of the groundstate of uranium oxidesfrom metallic to Mott insulator and then to charge-transfer insulator due tothe interplay of strongly correlated effects of 5f orbitals and the shift ofelectrons from uranium to oxygen atoms.

First-principles study on oxidation effects in uranium oxides and high-pressure high-temperature behavior of point defects in uranium dioxide