Chemical State of Complex Uranium Oxides

Kvashnina, Kristina O.; Butorin, Sergei M.; Martin, Philippe; Glatzel, Pieter

doi:10.1103/physrevlett.111.253002

Cited by 233 publications

(269 citation statements)

References 39 publications

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“…4), broadly consistent with the DFT results and previous spectroscopic measurements, e.g. [70,71]. Although U(VI) is the thermodynamically stable form of U under our measurement conditions, the oxidized U speciation is dominated by U(V) as a result of kinetic limitations to oxidation.…”

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confidence: 80%

UO2Oxidative Corrosion by Nonclassical Diffusion

et al. 2015

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Using x-ray scattering, spectroscopy, and density-functional theory we determine the structure of the oxidation front when a UO 2 (111) surface is exposed to oxygen at ambient conditions. In contrast to classical diffusion and previously reported bulk UO 2+x structures, we find oxygen interstitials order into a nanoscale superlattice with three-layer periodicity and uranium in three oxidation states: IV, V, and VI. This oscillatory diffusion profile is driven by the nature of the electron transfer process, and has implications for understanding the initial stages of oxidative corrosion in materials at the atomistic level. Main textOxidative corrosion is a key cause of material failure. This is especially true of uranium dioxide, which is the most economically important uranium mineral [1], the primary constituent of most nuclear fuels [2], and the desired product of many bioremediation strategies for uranium contamination [3]. UO 2 is an end-member in a complex metal-oxide system that is fundamentally important to experimental and computational actinide science [e.g., 4-7]. Despite more than 60 years of UO 2 oxidation research [8], moving beyond a macroscopic or empirical description to an understanding of the underlying atomistic processes has been difficult due to experimental challenges and the complex oxidation behavior of uranium oxides.Uranium dioxide exhibits a broad range of structural transformations due to oxidation. The UO 2 lattice readily incorporates interstitial oxygen atoms up to a stoichiometry near UO 2.25 (U 4 O 9 ) with minimal unit cell distortion [9]. Further oxidation to U 3 O 8 leads to structural rearrangement, volume expansion, and material failure [10,11]. When U(IV) in UO 2 is oxidized to U(VI) under water, dissolution occurs since U(VI) readily forms soluble uranyl (UO 2 2+) that can be released into the environment, although surfaces can be passivated [12]. Single crystal surface structures and oxidation have been studied under vacuum [10,11,[13][14][15][16][17][18][19][20][21], and by computational methods [22,23], but little is known about atomic level oxidation mechanisms under atmospheric conditions, especially in the earliest stages of oxidation.We have combined crystal truncation rod (CTR) x-ray diffraction -an in situ method that is sensitive to surface atomic structure [24,25] -with density functional theory (DFT) and x-ray photoelectron spectroscopy (XPS) to detail the initial stages of UO 2 oxidation via the (111) surface, which is the natural cleavage plane and predicted to be the most stable when dry [26][27][28]. We show that the oxidation front does not follow classical diffusion, but instead exhibits complex self-organization behavior, with interstitial oxygen atoms preferentially occupying every third layer below the surface.A freshly polished surface was measured with CTR (time 0), exposed to ~1 atm dry oxygen gas, and re-measured several times up to 21 days (504 hours) of exposure [29]. As the surface oxidizes, broad, bulk-forbidden peaks develop at L valu...

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confidence: 80%

UO2Oxidative Corrosion by Nonclassical Diffusion

et al. 2015

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“…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] .…”

Section: Resultsmentioning

confidence: 99%

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

et al. 2017

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We present resonant x-ray emission spectroscopic (RXES) data from the uranium intermetallics UPd3, USb, USn3 and URu2Si2 at the U M4,5 edges and compare the data to those from the well-localized 5f 2 semiconductor UO2. The technique is especially sensitive to any oxidation of the surface, and this was found on the USb sample, thus preventing a good comparison with a material known to be 5f 3 . We have found a small energy shift between UO2 and UPd3, both known to have localized 5f 2 configurations, which we ascribe to the effect of conduction electrons in UPd3. The spectra from UPd3 and URu2Si2 are similar, strongly suggesting a predominant 5f 2 configuration for URu2Si2. The valence-band resonant inelastic x-ray scattering (RIXS) provides information on the U P3 transitions (at about 18 eV) between the U 5f and U 6p states, as well as transitions of between 3 and 7 eV from the valence band into the unoccupied 5f states. These transitions are primarily involving mixed ligand states (O 2p or Pd, Ru 4d) and U 5f states. Calculations are able to reproduce both these low-energy transitions reasonably well.

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“…The HERFD-XAS spectrum is represented by a linear cut of such a RIXS map (14) along the diagonal of the plane defined by the incident energy axis and energy transfer axis or parallel to the incident energy axis at a constant emitted energy (the energy of the line maximum in this case) in the plane of the emitted vs. incident energies. The required Slater integrals F k , G k , and R k (25); spin-orbit coupling constants ζ; and matrix elements were obtained with the modified TT-MULTIPLETS package (26)(27)(28).…”

Section: Significancementioning

confidence: 99%

“…The effects of the hybridization of the valence states of actinides with ligand states in compounds and the charge transfer between ligand and actinide sites are expected to result in the appearance of chargetransfer satellites in the XAS spectra. The interpretation of such satellites can provide information on the degree of the 5f hybridization, covalency of the compound, and the nature of the ground state.An advanced technique, high-energy resolution fluorescencedetection X-ray absorption spectroscopy (HERFD-XAS), has been recently used for measurements at the 3d edge of U compounds (14,15). This has led to a tremendous improvement in the experimental resolution and revealed additional structures in the XAS spectra, not measurable before.…”

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confidence: 99%

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High-resolution X-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds

Butorin

Kvashnina

Vegelius

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Applying the high-energy resolution fluorescence-detection (HERFD) mode of X-ray absorption spectroscopy (XAS), we were able to probe, for the first time to our knowledge, the crystalline electric field (CEF) splittings of the 5f shell directly in the HERFD-XAS spectra of actinides. Using ThO 2 as an example, data measured at the Th 3d edge were interpreted within the framework of the Anderson impurity model. Because the charge-transfer satellites were also resolved in the HERFD-XAS spectra, the analysis of these satellites revealed that ThO 2 is not an ionic compound as previously believed. The Th 6d occupancy in the ground state was estimated to be twice that of the Th 5f states. We demonstrate that HERFD-XAS allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.actinide compounds | crystal field | covalency F or strongly electron-correlated systems, X-ray absorption spectroscopy (XAS) is viewed as a technique capable of providing information on the ground-state character despite the presence of the core hole in the final state of the spectroscopic process. In the 3d transition metal compounds, XAS at the metal 2p edge is routinely used to probe the magnitude of the crystalline electric field (CEF) splittings in the 3d shell and the degree of the 3d hybridization in the ground state (1).However, for the 5f shell of actinides, such a use of XAS is difficult. At the U 3d and 4d edges, where the transitions to the 5f states are probed, core-hole lifetime broadenings [full width at half maximum (FWHM)] are about 3.2 eV and 4.2 eV, respectively (2), whereas at the 5d edge, the broadening can be as large as 6.0 eV. Therefore, the conventional XAS spectra of actinide materials at the 3d and 4d edges often appear as single lines where splittings of the 5f states (e.g., due to the CEF interaction) are completely smeared out (3-8).The CEF effects for actinides are usually characterized with the help of optical absorption (reviewed in ref. 9) and electron paramagnetic resonance (EPR) (10, 11) spectroscopies but that is often done on doped systems when the actinide ions in question are introduced into the lattice of a compound without 5f electrons. Another high-resolution technique that can provide information about the CEF splittings is inelastic neutron scattering (INS) (12, 13) but the spectra usually cover a very limited energy range. In both optical absorption and INS spectroscopies, the CEF strength and parameters are evaluated by fitting the calculated energy levels of the CEF Hamiltonian to the energies of the measured lines of the 5f-5f transitions, whereas attempts to calculate spectral intensities are rare.On the other hand, for XAS at the 3d threshold of actinides it is relatively straightforward to calculate both energy levels and spectral intensities due to dipole selection rules. The effects of the hybridization of the valence states of actinides with ligand states in compounds and the ch...

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Chemical State of Complex Uranium Oxides

Cited by 233 publications

References 39 publications

UO2Oxidative Corrosion by Nonclassical Diffusion

UO2Oxidative Corrosion by Nonclassical Diffusion

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

High-resolution X-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds

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