Probing Chemical Bonding in Uranium Dioxide by Means of High-Resolution X-ray Absorption Spectroscopy

Abstract

A systematic x-ray absorption study at the U 3d, 4d and 4f edges of UO 2 was performed and the data were analyzed within framework of the Anderson impurity model. By applying the high-energy-resolution fluorescencedetection (HERFD) mode of x-ray absorption spectroscopy (XAS) at the U 3d 3/2 edge and conducting the XAS measurements at the shallower U 4f levels, fine details of the XAS spectra were resolved resulting from reduced corehole lifetime broadening. This multi-edge study enabled a far more effective analysis of the electronic structure at the U sites and characterization of the chemical bonding and degree of the 5f localization in UO 2. The results support the covalent character of UO 2 and do not agree with the suggestions of rather ionic bonding in this compound as expressed in some publications.

Resonant x-ray spectroscopy of uranium intermetallics at the M4,5 edges of uranium

“…However, the shift from 5f 2 to 5f 3 appears considerably less (0.3 eV) than found in the oxide systems (assuming some linearity in the oxide systems since for uranium no U 3+ state exists). Signals from all higher oxidation states (U 4+ and above) would fall at higher incident energies [28][29][30][31][32] .…”

“…To our knowledge, such measurements have only been reported on UO 2 28 and other uranium complex systems 24,[29][30][31][32] , so these efforts on U-intermetallics, particularly URu 2 Si 2 , should be of interest to those working in this field.…”

A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles

“…The HERFD spectrum of PuO 2 clearly shows two intense peaks, at ≈3970.2 eV and ≈3971.8 eV. According to the results of calculations carried out in the framework of the Anderson impurity model (AIM; Figure b), the intensity and energy of these two peaks are a result of multiple factors, such as the strength of the intra‐atomic and crystal‐field interactions, and the degree of the Pu 5f/ligand 2p hybridization in the ground and final states of the spectroscopic process. In comparison with PuO 2 , the HERFD spectrum of KPuO 2 CO 3(s) shifts towards higher incident energies and shows a narrow profile with an asymmetric shape and a shoulder at the higher incident energy side.…”

A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles

“…Figure 1a shows the first experimental HERFD data at the Pu M 4 edge for the Pu IV O 2 and KPu V O 2 CO 3(s) (solid) systems with Pu IV and Pu V oxidation states,respectively.Data were collected with an X-ray emission spectrometer [21] set to the maximum of the Mb emission line at 3534 eV.S ynthesis procedures and the characterization of both materials are reported in the Supporting Information. TheH ERFD spectrum of PuO 2 clearly shows two intense peaks,at% 3970.2 eV and % 3971.8 eV.A ccording to the results of calculations carried out in the framework of the Anderson impurity model (AIM;F igure 1b), [22][23][24] the intensity and energy of these two peaks are ar esult of multiple factors,s uch as the strength of the intra-atomic and crystal-field interactions,and the degree of the Pu 5f/ligand 2p hybridization in the ground and final states of the spectroscopic process.Incomparison with PuO 2 , the HERFD spectrum of KPuO 2 CO 3(s) shifts towards higher incident energies and shows an arrow profile with an asymmetric shape and as houlder at the higher incident energy side.T he results of the AIM calculations reported in Figure 1b show ag ood agreement with the experimental KPuO 2 CO 3(s) HERFD spectrum, confirming the presence of the pentavalent Pu oxidation state in KPuO 2 CO 3(s) .…”