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

Abstract

Formation Gibbs free energy of point defects and oxygen clusters in uranium dioxide at high-pressure high-temperature conditions are calculated from first principles, using the LSDA+U approach for the electronic structure and the Debye model for the lattice vibrations. The phonon contribution on Frenkel pairs is found to be notable, whereas it is negligible for the Schottky defect. Hydrostatic compression changes the formation energies drastically, making defect concentrations depend more sensitively on pressure. Calculations show that, if no oxygen clusters are considered, uranium vacancy becomes predominant in overstoichiometric UO2 with the aid of the contribution from lattice vibrations, while compression favors oxygen defects and suppresses uranium vacancy greatly. At ambient pressure, however, the experimental observation of predominant oxygen defects in this regime can be reproduced only in a form of cuboctahedral clusters, underlining the importance of defect clustering in UO2+x. Making use of the point defect model, an equation of state for non-stoichiometric oxides is established, which is then applied to describe the shock Hugoniot of UO2+x. Furthermore, the oxidization and compression behavior of uranium monoxide, triuranium octoxide, uranium trioxide, and a series of defective UO2 at zero Kelvin are investigated. The evolution of mechanical properties and electronic structures with an increase of the oxidation degree are analyzed, revealing the transition of the groundstate of uranium oxides from metallic to Mott insulator and then to charge-transfer insulator due to the interplay of strongly correlated effects of 5f orbitals and the shift of electrons from uranium to oxygen atoms.

16 pages, 10 figures

References in corpus (7)

• Point defects and clustering in uranium dioxide by LSDA+U calculations
• Structural behavior of uranium dioxide under pressure by LSDA+U calculations
• Interplay of defect cluster and the stability of xenon in uranium dioxide by density functional calculations
• Stability mechanism of cuboctahedral clusters in UO2: First-principles calculations
• Ab initio investigation on oxygen defect clusters in UO2+x
• Shock Induced Order-disorder Transformation in Ni3Al
• Order-disorder Effects on Equation of State in FCC Ni-Al Alloys

Cited by in corpus (5)

• Phonon spectrum, thermodynamic properties, and pressure-temperature phase diagram of uranium dioxide
• Towards a first-principles determination of effective Coulomb interactions in correlated electron materials: Role of intershell interactions
• Prediction of novel ordered phases in U-X (X= Zr, Sc, Ti, V, Cr, Y, Nb, Mo, Hf, Ta, W) binary alloys under high pressure
• Anomalies in non-stoichiometric uranium dioxide induced by pseudo-phase transition of point defects
• Theoretical assessment on the possibility of constraining point defect energetics by pseudo-phase transition pressures