First-principles study of defects and phase transition inUO2

Abstract

Defect properties and phase transition inUO2 have been studied from first principles by the all-electron projector-augmented-wave (PAW)method. The generalized gradient approximation with empirical self-interaction correction,(GGA)+U, formalism has been used to account for the strong on-site Coulombrepulsion among the localized U 5f electrons. The Hubbard parameterUeff, magnetic ordering, chemical potential and heat of formationhave been systematically examined. By choosing an appropriateUeff = 3.0 eV it is possible to consistently describe structural properties ofUO2 and model the phase transition processes. The phase transition pressure forUO2 is about 20 GPa, which is less than the experimental value of 42 GPa but better than theLDA+U value of 7.8 GPa. Meanwhile our results for the formation energies of intrinsic defects partlyconfirm earlier calculations for the intrinsic charge neutral defects but reveallarge variations depending on the determination of the chemical potential andwhether the environment is O-rich or U-rich. Moreover, the results for extrinsicdefects of Xe, which are representative of mobile insoluble fission product inUO2, are consistent with experimental data in which Xe prefers to be trapped by Schottkydefects.