title: Local Symmetry Effects in Actinide 4f X-ray Absorption in Oxides.
abstract: A systematic X-ray absorption study at actinide N6,7 (4f → 6d transitions) edges was performed for light-actinide oxides including data obtained for the first time for NpO2, PuO2, and UO3. The measurements were supported by ab initio calculations based on local-density-approximation with added 5f-5f Coulomb interaction (LDA+U). Improved energy resolution compared to common experiments at actinide L(2,3) (2p → 6d transitions) edges allowed us to resolve the major structures of the unoccupied 6d density of states (DOS) and estimate the crystal-field splittings in the 6d shell directly from the spectra of light-actinide dioxides. The measurements demonstrated an enhanced sensitivity of the N(6,7) spectral shape to changes in the compound crystal structure. For nonstoichiometric NpO(2-x), the filling of the entire band gap with Np 6d states was observed thus supporting a phase coexistence of Np metal and stoichiometric NpO2 which is in agreement with the tentative Np-O phase diagram.
authors.affiliation: Department of Physics and Astronomy, Uppsala University , P.O. Box 516, SE-751 20 Uppsala, Sweden.
authors.forename: Sergei M
authors.initials: SM
authors.lastname: Butorin
authors.affiliation: Department of Physics and Astronomy, Uppsala University , P.O. Box 516, SE-751 20 Uppsala, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Modin
authors.affiliation: Department of Physics and Astronomy, Uppsala University , P.O. Box 516, SE-751 20 Uppsala, Sweden.
authors.forename: Johan R
authors.initials: JR
authors.lastname: Vegelius
authors.affiliation: Department of Physics and Astronomy, Uppsala University , P.O. Box 516, SE-751 20 Uppsala, Sweden.|RIKEN Center for Emergent Matter Science , 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
authors.forename: Michi-To
authors.initials: MT
authors.lastname: Suzuki
authors.affiliation: Department of Physics and Astronomy, Uppsala University , P.O. Box 516, SE-751 20 Uppsala, Sweden.
authors.forename: Peter M
authors.initials: PM
authors.lastname: Oppeneer
authors.affiliation: Materials Science in Radiation and Dynamical Extremes, Materials Science and Technology Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87545, United States.
authors.forename: David A
authors.initials: DA
authors.lastname: Andersson
authors.affiliation: Chemical Sciences Division, Lawrence Berkeley National Laboratory , MS 70A1150, One Cyclotron Road, Berkeley, California 94720, United States.
authors.forename: David K
authors.initials: DK
authors.lastname: Shuh