DFT plus U Study of Chemical Impurities in PuO2

We employ density functional theory to explore the effects of impurities-in the fluorite crystal structure of PuO2. The impurities that were considered are known impurities that exist in metallic delta-phase Pu, including H, C, Fe, and Ga. These impurities were placed at various high-symmetry sites within the PuO2 structure including an octahedral interstitial site, an interstitial site with coordination to two neighboring O atoms, an O substitutional site, and a Pu substitutional site. Incorporation energies were calculated to be energetically unfavorable for all sites except the Pu substitutional site. When impurities were placed in a Pu substitutional site, complexes incorporating the impurities and O formed within the PuO2 structure. The observed defect-oxygen structures were OH, CO3, FeO5, and GaO3. The presence of these defects led to distortion of the surrounding O atoms within the structure, producing long-range disorder of O atoms. In contrast, perturbations of Pu atoms had a relatively short-range effect on the relaxed structures. These effects are demonstrated via radial distribution functions for O and Pu vacancies. Calculated electronic structure revealed hybridization of the impurity atom with the O valence states and a relative decrease in the Pu 5f states. Minor differences in band gaps were observed for the defected PuO2 structures containing H, C, and Ga. Fe-containing structures, however, were calculated to have a significantly decreased band gap, where the implementation of a Hubbard U parameter on the Fe 3d orbitals will maintain the calculated PuO2 band gap.