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Local Symmetry Effects in Actinide 4f X-ray Absorption in Oxides

  • Author(s): Butorin, SM
  • Modin, A
  • Vegelius, JR
  • Suzuki, MT
  • Oppeneer, PM
  • Andersson, DA
  • Shuh, DK
  • et al.
Abstract

© 2016 American Chemical Society. 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 L2,3(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 N6,7spectral shape to changes in the compound crystal structure. For nonstoichiometric NpO2-x, the filling of the entire band gap with Np 6d states was observed thus supporting a phase coexistence of Np metal and stoichiometric NpO2which is in agreement with the tentative Np-O phase diagram.

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