JG Tobin1 S.-W. Yu2 C.H. Booth3 Tolek Tyliszczak3 David Shuh3 G. van der Laan4 Dimosthenis Sokaras5 Dennis Nordlund5 T.C. Weng5

1, University of Wisconsin-Oshkosh, Oshkosh, Wisconsin, United States
2, Lawrence Livermore National Laboratory, Livermore, California, United States
3, Lawrence Berkeley National Laboratory, Berkeley, California, United States
4, Diamond Light Source, Diamond, , United Kingdom
5, SSRL, Palo Alto, California, United States

An extensive investigation of oxidation in uranium has been pursued. [1] This includes the utilization of soft x-ray absorption spectroscopy, hard x-ray absorption near-edge structure, resonant (hard) x-ray emission spectroscopy, cluster calculations, and a branching ratio analysis founded on atomic theory. The samples utilized were uranium dioxide (UO2), uranium trioxide (UO3), and uranium tetrafluoride (UF4). A discussion of the role of nonspherical perturbations, i.e., crystal or ligand field effects, will be presented. The conclusions are as follows. (1) The hypothesis of the potential importance of CF effects in the XAS branching ratio (BR) analysis of 5f states was incorrect. (2) Both UO2 and UF4 are n5f = 2 materials. The combination of the 4d XAS BR and RXES analyses is particularly powerful. (3) CF broadening in the L3 RXES spectroscopy does not preclude a successful analysis. (4) The prior experimental result that n5f (UO2) = 3 and the proposed causation by covalent bonding was incorrect. UO2 is an n5f = 2 material and analysis within a simple, ionically localized picture provides the correct result. (5) UO3 appears to be an n5f = 1 material. (6) While the 4d XAS BR analysis is blind to CF effects, crystal field and covalence remain important. (7) For localized actinide systems, the 4d XAS BR analysis founded upon the utilization of the intermediate coupling scheme remains a powerful tool. (8) For delocalized actinide systems, the BR analysis is problematic.