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Abstract

X-ray absorption and resonant X-ray emission measurements at the O 1s edge of the uranium oxides UO2, U3O8 and UO3 are presented. The spectral shapes of the O Kα X-ray emission spectra of UO3 exhibit significant excitation energy dependence, from an asymmetric to a symmetric form, which differs from those of UO2 and U3O8. This energy dependence is attributed to a significant difference in the oxygen–uranium hybridization between two different sites in the crystal structure of UO3. The spectral shapes of UO2 and U3O8 are also found to be different but without significant energy dependence. The experimental spectra of the valence and conduction bands of the uranium oxides are compared to the results of electronic structure calculations available in the literature.

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... A component of these studies has been XAS that contains information on the unoccupied electronic states. In UO 2 these are the states of the upper Hubbard band (UHB) above the Mott gap. Figure 1(a) shows the O K XAS of: UO 2 that is cubic with U(IV); U 3 O 8 that is layered with two distinct U(V) and (VI) sites; α UO 3 that is layered and U(VI) [31][32][33][34]; and the molecular complex Cs 2 UO 2 Cl 4 that possesses the U(VI) trans dioxo moiety that is inherent to the higher U valences and defines the oblate local geometry that is the basis for layering [35]. Because of the symmetry relationship between the cubic U site of UO 2 and the quasioctahedral site of the layered materials and uranyl complexes, these spectra for both geometries consist of two peaks. ...
... This value for the leading edge of UO 2 is, however, somewhat arbitrary because it is not smooth but exhibits significant structure at or slightly above the noise level that makes it vary from 2.5 eV near its maximum amplitude to 1.5 eV near the baseline. The peak of the U 5f states is only ∼0.5 eV higher for U 3 O 8 relative to UO 2. The overall separation between these is therefore substantially reduced and [32] to this UO 2. The broadening of the overall U 5f-6d manifold of the mixed valence compounds occurs on the low energy edge of the upper Hubbard band. In addition to some variability in the relative amplitudes between different samples of the same compound ( Fig. S1), the NIXS that is not distorted by self-absorbance shows that the 531.5 eV peak in the U 3 O 7 spectrum is quite low relative to the others ( Fig. S3). ...
Article
Mixed valence O-doped UO2+x and photoexcited UO2 containing transitory U3+ and U5+ host a coherent polaronic quantum phase (CPQP) that exhibits the characteristics of a Fröhlich-type, nonequilibrium, phonon-coupled Bose-Einstein condensate whose stability and coherence are amplified by collective, anharmonic motions of atoms and charges. Complementary to the available, detailed, real space information from scattering and EXAFS, an outstanding question is the electronic structure. Mapping the Mott gap in UO2, U4O9, and U3O7 with O XAS and NIXS and UM5 RIXS shows that O doping raises the peak of the U5f states of the valence band by ∼0.4 eV relative to a calculated value of 0.25 eV. However, it lowers the edge of the conduction band by 1.5 eV vs the calculated 0.6 eV, a difference much larger than the experimental error. This 1.9 eV reduction in the gap width constitutes most of the 2–2.2 eV gap measured by optical absorption. In addition, the XAS spectra show a tail that will intersect the occupied U5f states and give a continuous density-of-states that increases rapidly above its constricted intersection. Femtosecond-resolved photoemission measurements of UO2, coincident with the excitation pulse with 4.7 eV excitation, show the unoccupied U5f states of UO2 and no hot electrons. 3.1 eV excitation, however, complements the O-doping results by giving a continuous population of electrons for several eV above the Fermi level. The CPQP in photoexcited UO2 therefore fulfills the criteria for a nonequilibrium condensate. The electron distributions resulting from both excitations persist for 5–10 ps, indicating that they are the final state that therefore forms without passing through the initial continuous distribution of nonthermal electrons observed for other materials. Three exceptional findings are: (1) the direct formation of both of these long lived (>3–10 ps) excited states without the short lived nonthermal intermediate; (2) the superthermal metallic state is as or more stable than typical photoinduced metallic phases; and (3) the absence of hot electrons accompanying the insulating UO2 excited state. This heterogeneous, nonequilibrium, Fröhlich BEC stabilized by a Fano-Feshbach resonance therefore continues to exhibit unique properties.
... The formal valency of uranium in UO 2 is tetravalent (U 4+ ) with O h 5 symmetry around the uranium ion. The lattice structure has identical O sites and has a O-U bond length of~2.37 Å [28]. In contrast to UO 2 , the crystal structure is more complicated in the mixed-valency oxide U 3 O 8 that contains both U 4+ and U 6+ in a 1:2 ratio. ...
... Additionally, two types of U-O lengths are known: short axial oxygen uranyl bonds (~1.79 Å) and longer, planar, equatorial bonds (~2.30 Å) [28]. ...
... O 1s x-ray absorption (XA) [29][30][31][32]] and x-ray emission (XE) [30] measurements of UO 2 have been reported earlier. ...
... O 1s x-ray absorption (XA) [29][30][31][32]] and x-ray emission (XE) [30] measurements of UO 2 have been reported earlier. ...
Article
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Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5 f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5 f -shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4-5 eV.
... Sinkov et al studied the effect of oxidation on both uraninite and metaschoepite [9]. Magnuson et al studied the X-ray absorption and resonant X-ray emission measurements at the O 1s edge of the uranium oxides UO 2 , UO 3 and U 3 O 8 [10]. Taylor et al developed an X-ray diffraction technique to measure specifically U 3 O 8 formation on the surface of UO 2 pellets, and an appropriate mathematical model to analyse the kinetic data. ...
Conference Paper
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Nuclear power has rapidly come to the forefront as a cheaper, more reliable fuel source to solve the world's energy crisis. In order to meet future demands for nuclear fuel there has been increased interest in optimizing the extraction of uranium from its various minerals. One of the major uranium minerals used to produce nuclear fuel is tetravalent uraninite (UO2). Various aspects of the extraction of uranium from this mineral are however poorly understood such as the influence of dopants (impurities) and gangue minerals found in the same ore deposits. In this study three natural uraninite samples were characterized using in-situ High temperature X-ray diffraction (HT-XRD) a General Area Detector Diffraction System (GADDS) (using whole rock samples). The results were then compared to synthetic UO2 to determine structural and compositional differences between natural and synthetic UO2. XRD results showed that the samples were all crystalline at room temperature and contained high amounts of uranium dioxide. The common gangue minerals associated with the minerals included silicon dioxide and titanium dioxide.
... Energy calibration was performed with respect to the K edge of yttrium foil (17.038 keV). m-XANES spectra of uranium reference compounds of different oxidation state were recorded to aid interpretation, including UO 2 (U 4+ ), U 0.5 Y 0.5 Ti 2 O 6 (U 5+ ), 19,21 U 3 O 8 (U 5.33+ ) 26 and UO 3 (U 6+ ). Reference compounds were prepared as 3 mm diameter pellets of ceramic powders distributed in polyethylene glycol (PEG). ...
Article
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An environmentally aged radioactive particle of UFeO4 recovered from soil contaminated with munitions depleted uranium (DU) was characterised by microbeam synchrotron X-ray analysis. Imaging of uranium speciation by spatially resolved X-ray diffraction (µ-XRD) and X-ray absorption spectroscopy (µ-XAS) was used to localise UFeO4 in the particle, which was coincident with a distribution of U(V). The U oxidation state was confirmed using X-ray Absorption Near Edge Structure (µ-XANES) spectroscopy as +4.9 ± 0.15. Le-Bail fitting of the particle powder XRD pattern confirmed the presence of UFeO4 and a minor alteration product which was modelled as meta-ankoleite (K(UO2)(PO4)·3H2O). Refined unit cell parameters for UFeO4 were in good agreement with previously published values. Uranium-oxygen interatomic distances in the first co-ordination sphere were determined by fitting of Extended X-ray Absorption Fine Structure (µ-EXAFS) spectroscopy. The average first shell U-O distance was 2.148 ± 0.014 Å, corresponding to a U valence of +4.97 ± 0.13 using bond valence sum analysis. Using bond distances from the published structure of UFeO4, U and Fe bond valence sums were calculated as +5.00 and +2.78 respectively, supporting the spectroscopic analysis and confirming the presence of a U(V)/Fe(III) pair. Overall this investigation provides important evidence for the stability of U(V) ternary oxides, in oxic, variably moist surface environment conditions for at least 25 years.
... For example, the theoretical and experimental results on bandgap-E g varied between 0 and 2.34 eV [11,18,24,28]. Moreover, some studies suggest U 4+ /U 6+ mix valency state [18,24,29] for uranium while other investigations propose U 5+ /U 6+ [23]. Also, some studies predict the magnetic state of U 3 O 8 to be ferromagnetic (FM) and others predict antiferromagnetic (AFM) configurations [18,23,24]. ...
Article
Triuranium octoxide (U3O8), the most stable form of uranium oxide, is an important material that finds application in nuclear and semiconductor industry. The understanding of electronic, magnetic and optical properties of U3O8 is essential for these advanced applications. Therefore, in this work, we have performed the density functional theory (DFT) study to investigate the structural, electronic, magnetic and optical properties of the low-temperature orthorhombic phase of U3O8 (𝛼-U3O8) within the generalized gradient approximation (GGA) by using WIEN2k software package. To capture the highly correlated nature of 5f electrons in uranium atoms, an on-site Coulomb repulsion term (Hubbard-U) of 4.5 eV, was considered. Further, the effect of spin orbital coupling (SOC) on the electronic structure and band gap of 𝛼-U3O8 is demonstrated. Work functions (𝜙) were evaluated for the planes [001], [100], [010] and [111] using the first principles code QUANTUM ESPRESSO (QE). This study verifies the importance of SOC on structural, electronic and optical properties of 𝛼-U3O8 and claim for indirect theoretical band gap-𝐸 of 2.03 eV verifying the semiconductor behaviour. The 𝑔 optical anisotropy is analyzed through the frequency-dependent optical properties, namely, the real and imaginary parts of the dielectric tensor (𝜀1(ω) and 𝜀2(ω)), absorption coefficient (𝛼(ω)), optical conductivity (𝜎(ω)), refractive index (𝑛(ω)) and Loss-function (𝐿(ω)). By comparing the Fermi energy and the vacuum level energy, the work functions for the planes (100), (001), (010) and (111) are predicted as 6.31, 6.73, 7.01 and 7.03 eV respectively. Furthermore, in this article, we present our experimental investigation of 𝐸 based on diffuse reflectance spectra (DRS) method. Measured DRS 𝑔 of three powder samples are analyzed using the Kubelka-Munk model. Three powder samples, namely 1wt%, 2wt% and 4wt% of U3O8 diluted with KCl show that U3O8 exhibits semiconducting behaviour with indirect band gaps of 1.86, 1.81 and 1.72 eV respectively.
... Furthermore, the colors of the oxides can qualitatively help differentiate their compositions. U 3 O 8 and UO 3 colors are reported as dark green-black and orange-yellow, respectively [39][40][41] . We observed accumulation of yellow material on the walls of the quartz tube. ...
Article
Full-text available
We use a recently developed plasma-flow reactor to experimentally investigate the formation of oxide nanoparticles from gas phase metal atoms during oxidation, homogeneous nucleation, condensation, and agglomeration processes. Gas phase uranium, aluminum, and iron atoms were cooled from 5000 K to 1000 K over short-time scales (∆t < 30 ms) at atmospheric pressures in the presence of excess oxygen. In-situ emission spectroscopy is used to measure the variation in monoxide/atomic emission intensity ratios as a function of temperature and oxygen fugacity. Condensed oxide nanoparticles are collected inside the reactor for ex-situ analyses using scanning and transmission electron microscopy (SEM, TEM) to determine their structural compositions and sizes. A chemical kinetics model is also developed to describe the gas phase reactions of iron and aluminum metals. The resulting sizes and forms of the crystalline nanoparticles (FeO-wustite, eta-Al2O3, UO2, and alpha-UO3) depend on the thermodynamic properties, kinetically-limited gas phase chemical reactions, and local redox conditions. This work shows the nucleation and growth of metal oxide particles in rapidly-cooling gas is closely coupled to the kinetically-controlled chemical pathways for vapor-phase oxide formation.
... UO 2 can easily be distinguished from U 3 O 8 and UO 3 because of their different crystal structures. [15] Because of the ability of uranium to stabilise multioxidation states (III, IV, V and VI) and because the intercalation of interstitial oxygen into the fluorite lattice of UO 2 is a spontaneous process, [16] hyperstoichiometric UO 2 + x (0 < ...
Article
Very little is known about the size and shape effects on the properties of actinide compounds. As a consequence, the controlled synthesis of well-defined actinide-based nanocrystals constitutes a fundamental step before studying their corresponding properties. In this paper, we report on the non-aqueous surfactant-assisted synthesis of thorium and uranium oxide nanocrystals. The final characteristics of thorium and uranium oxide nanocrystals can be easily tuned by controlling a few experimental parameters such as the nature of the actinide precursor and the composition of the organic system (e.g., the chemical nature of the surfactants and their relative concentrations). Additionally, the influence of these parameters on the outcome of the synthesis is highly dependent on the nature of the actinide element (thorium versus uranium). By using optimised experimental conditions, monodisperse isotropic uranium oxide nanocrystals with different sizes (4.5 and 10.7 nm) as well as branched nanocrystals (overall size ca. 5 nm), nanodots (ca. 4 nm) and nanorods (with ultra-small diameters of 1 nm) of thorium oxide were synthesised.
... Standards of UO 2 (average U oxidation state (OS) 4), U 0.5 Y 0.5 Ti 2 O 6 (average OS 5), 24 U 3 O 8 (average OS 5.33), 25 and UO 3 (average OS 6) were recorded. Analytical-grade UO 2 and UO 3 were used (British Drug Houses Ltd.), and U 3 O 8 and U 0.5 Y 0.5 Ti 2 O 6 were synthesized using previously described methodologies in Grenthe et al. 15 and James et al,. ...
Article
Use of depleted uranium (DU) munitions has resulted in contamination of the near-surface environment with penetrator residues. Uncertainty in the long-term environmental fate of particles produced by impact of DU penetrators with hard targets is a specific concern. In this study DU particles produced in this way and exposed to the surface terrestrial environment for longer than 30 years at a U.K. firing range were characterized using synchrotron X-ray chemical imaging. Two sites were sampled: a surface soil and a disposal area for DU-contaminated wood, and the U speciation was different between the two areas. Surface soil particles showed little extent of alteration, with U speciated as oxides U3O7 and U3O8. Uranium oxidation state and crystalline phase mapping revealed these oxides occur as separate particles, reflecting heterogeneous formation conditions. Particles recovered from the disposal area were substantially weathered, and U(VI) phosphate phases such as meta-ankoleite (K(UO2)(PO4)·3H2O) were dominant. Chemical imaging revealed domains of contrasting U oxidation state linked to the presence of both U3O7 and meta-ankoleite, indicating growth of a particle alteration layer. This study demonstrates that substantial alteration of DU residues can occur, which directly influences the health and environmental hazards posed by this contamination.
Article
Proper and accurate placement of the components of the unoccupied density of states in uranium dioxide requires a careful calibration of the energy scales in the spectroscopic investigations. Here, the energy scale calibrations and corresponding spectroscopic measurements are described in detail, including photoelectron spectroscopy, inverse photoelectron spectroscopy and, most important, x-ray absorption spectroscopy.
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Soft X-ray spectromicroscopy at the O K-edge, U N 4,5-edges and Ce M 4,5-edges has been performed on focused ion beam sections of spent nuclear fuel for the first time, yielding chemical information on the sub-micrometer scale. To analyze these data, a modification to non-negative matrix factorization (NMF) was developed, in which the data are no longer required to be non-negative, but the non-negativity of the spectral components and fit coefficients is largely preserved. The modified NMF method was utilized at the O K-edge to distinguish between two components, one present in the bulk of the sample similar to UO 2 and one present at the interface of the sample which is a hyperstoichiometric UO 2+x species. The species maps are consistent with a model of a thin layer of UO 2+x over the entire sample, which is likely explained by oxidation after focused ion beam (FIB) sectioning. In addition to the uranium oxide bulk of the sample, Ce measurements were also performed to investigate the oxidation state of that fission product, which is the subject of considerable interest. Analysis of the Ce spectra shows that Ce is in a predominantly trivalent state, with a possible contribution from tetravalent Ce. Atom probe analysis was performed to provide confirmation of the presence and localization of Ce in the spent fuel.
Article
X-ray Emission Spectroscopy (XES) and Bremstrahlung Isochromat Spectroscopy (BIS), two electron-in/photon-out measurements, have been used to probe multi-electronic effects in the important actinide compound, uranium dioxide (UO2). The new observables in the XES are a distinct satellite, a core-to-core-transition, and the lack of a spin-orbit-split satellite peak. New XES data for 4f and 4p derived transitions are presented and discussed. Electron shielding in the 4d5/2 main and satellite XES peaks is discussed in light of the experimental observations.
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X-ray pair distribution function (pdf) and U L 3 extended x-ray absorption fine structure (EXAFS) and neutron pdf measurements that give identical results for UO 2 show U(VI)-oxo moieties with x rays for mixed valence U 4 O 9 and U 3 O 7 , in contrast to the neutron data that indicate only U(V) sites with no short U-O bonds as well as other differences. In addition, although the EXAFS spectra of UO 2 are essentially identical at 30, 100, and 200 K, those of the UO 2+x compounds exhibit different nearest-neighbor U-O distributions at each temperature. Further tunneling polaron-type behavior is found in the broadening of the features of the O K-edge x-ray absorption spectra (XAS) of the UO 2+x compounds. Raman spectra of powders also show a large increase in scattering cross section with increasing O content that would originate in a change in the electronic structure that increases the overall polarizability. The XAS and Raman also show that U 4 O 9 does not behave as a linear combination of the UO 2 and U 3 O 7 fluorite endpoints. The properties induced by mobile rather than static charged quasiparticles were explored by optical pumping of the metal-to-metal charge-transfer transition. The temperature dependence of 4.71 eV pump–1.57 eV probe reflectivity on UO 2 that initially populates the U 6d-dominated portion of the upper Hubbard band (UHB) shows a sharp 28-μsec lifetime peak at 25 K that may be associated with the fluctuations of its antiferromagnetic transition. Pumping at 3.14 eV into the 5f -dominated portion of the UHB shows an analogous 2.8-μsec peak but also a plateau bracketing this peak that ends in a cusp at 50–60 K and an abrupt change in the hardening rate of a novel 12–15 GHz phonon that is the signature for the quasiparticle quantum phase. The different results for the different excitation channels indicate a highly specific nonthermal relaxation mechanism. These results constitute the first observation of a distinct phase of photoinduced quasiparticles that is sufficiently coupled to the lattice to undergo a gap-opening transition. When the intragap state is probed with a terahertz time domain spectroscopy (TTDS) measurement 33 psec after a 3.14 excitation pulse, it shows increased absorption in the 0.5–1.1 THz range with a decrease in temperature from ∼30 to 10 K instead of the expected decrease, a result consistent with the presence of a condensate. These results are too extreme to originate in the dynamical, nonadiabatic, coupled charge-transfer–phonon/tunneling polaron scenario previously used for doped Mott-Hubbard insulators with intermediate electron-phonon coupling and therefore indicate novel physics. One possibility that could cause all of these behaviors is that a collective, dynamical, charge transfer-coupled Peierls distortion involving the 2 U(V) ↔ U(IV) + U(VI)-oxo excitation occurs coherently over an entire domain to cause the atoms in this domain to condense into a system with Bose-Einstein or Bose-Einstein-Hubbard properties.
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A computational study is presented of the structural, electronic, and magnetic properties of U3O8 and Np2O5, which are actinide oxides in a higher oxidation state than the tetravalent state of the common dioxide phases, UO2 and NpO2. The calculations are based on the density functional theory+U approach, in which additional Coulomb correlations on the actinide atom are taken into account. The calculated properties of these two higher oxidized actinide oxides are analyzed and compared to those of their tetravalent analogs. The optimized structural parameters of these noncubic oxides are found to be in reasonable agreement with available experimental data. U3O8 is predicted to be a magnetic insulator, having one U atom in a hexavalent oxidation state and two U atoms in a pentavalent oxidation state. For Np2O5, which is also predicted to be an insulator, a complicated noncollinear magnetic structure is computed, leading to a nonzero overall magnetization with a slight antiferromagnetic canting. The calculated electronic structures are presented and the variation of the U 5f or Np 5f-O 2p hybridization with the oxidation state is analyzed. With increasing oxygen content, the nearly localized 5f electrons of the actinide elements are more positioned near the Fermi level and the hybridization between 5f and 2p states is markedly increased.
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One of the crucial questions of all actinide electronic structure determinations is the issue of 5f versus 6d character and the distribution of these components across the density of states. Here, a breakthough experiment is discussed, which has allowed the direct determination of the U5f and U6d contributions to the unoccupied density of states in uranium dioxide. A novel resonant inverse photoelectron and x-ray emission spectroscopy investigation of UO(2) is presented. It is shown that the U5f and U6d components are isolated and identified unambiguously.
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An extensive investigation of oxidation in uranium has been pursued. 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.
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Using x-ray emission spectroscopy and absorption spectroscopy, it has been possible to directly access the states in the unoccupied conduction bands that are involved with 5f and 6d covalency in oxidized uranium. By varying the oxidizing agent, the degree of 5f covalency can be manipulated and monitored, clearly and irrevocably establishing the importance of 5f covalency in the electronic structure of the key nuclear fuel, uranium dioxide. © 2015 American Physical Society.
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In recent years, scientists have progressively recognized the role of electronic structures in the characterization of chemical properties for actinide containing materials. High-energy resolution X-ray spectroscopy at the actinide M4,5 edges emerged as a promising direction because this method can probe actinide properties at the atomic level through the possibility of reducing the experimental spectral width below the natural core-hole lifetime broadening. Parallel to the technical developments of the X-ray method and experimental discoveries, theoretical models, describing the observed electronic structure phenomena, have also advanced. In this feature article, we describe the latest progress in the field of high-energy resolution X-ray spectroscopy at the actinide M4,5 and ligand K edges and we show that the methods are able to (a) provide fingerprint information on the actinide oxidation state and ground state characters (b) probe 5f occupancy, non-stoichiometry, defects, and ligand/metal ratio and (c) investigate the local symmetry and effects of the crystal field. We discuss the chemical aspects of the electronic structure in terms familiar to chemists and materials scientists and conclude with a brief description of new opportunities and approaches to improve the experimental methodology and theoretical analysis for f-electron systems.
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The electronic structure in the complete series of stable lanthanide sesquioxides, Ln2O3 (Ln = La to Lu, except radioactive Pm), has been evaluated using oxygen K-edge X-ray absorption spectroscopy (XAS) with a scanning transmission X-ray microscope (STXM). The experimental results agree with recent synthetic, spectroscopic and theoretical investigations that provided evidence for 5d orbital involvement in lanthanide bonding, while confirming the traditional viewpoint that there is little Ln 4f and O 2p orbital mixing. However, the results also showed that changes in the energy and occupancy of the 4f orbitals can impact Ln 5d and O 2p mixing, leading to several different bonding modes for seemingly identical Ln2O3 structures. On moving from left to right in the periodic table, abrupt changes were observed for the energy and intensity of transitions associated with Ln 5d and O 2p antibonding states. These changes in peak intensity, which were directly related to the amounts of O 2p and Ln 5d mixing, were closely correlated to the well-established trends in the chemical accessibility of the 4f orbitals towards oxidation or reduction. The unique insight provided by the O K-edge XAS is discussed in the context of several recent theoretical and physical studies on trivalent lanthanide compounds.
Chapter
Differences and similarities between 5f and 4f electrons are pointed out. Changes of the characteristics of the 5f electrons along the series are described. Spectroscopic analysis of actinide compounds throwing light on the localized or delocalized character of the 5f electrons are discussed by analogy with the observations made for the rare-earths.
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A tetravalent cerium macrocyclic complex (CeLK4) was prepared with an octadentate terephthalamide ligand comprised of hard catecholate donors and characterized in the solution state by spectrophotometric titrations and electrochemistry and in the crystal by X-ray diffraction. The solution-state studies showed that L exhibits a remarkably high affinity toward Ce(4+), with log β110 = 61(2) and ΔG = -348 kJ/mol, compared with log β110 = 32.02(2) for the analogous Pr(3+) complex. In addition, L exhibits an unusual preference for forming CeL(4-) relative to formation of the analogous actinide complex, ThL(4-), which has β110 = 53.7(5). The extreme stabilization of tetravalent cerium relative to its trivalent state is also evidenced by the shift of 1.91 V in the redox potential of the Ce(3+)/Ce(4+) couple of the complex (measured at -0.454 V vs SHE). The unprecedented behavior prompted an electronic structure analysis using L3- and M5,4-edge X-ray absorption near-edge structure (XANES) spectroscopies and configuration interaction calculations, which showed that 4f-orbital bonding in CeLK4 has partial covalent character due to ligand-to-metal charge transfer (LMCT) in the ground state. The experimental results are presented in the context of earlier measurements on tetravalent cerium compounds, indicating that the amount of LMCT for CeLK4 is similar to that observed for [Et4N]2[CeCl6] and CeO2 and significantly less than that for the organometallic sandwich compound cerocene, (C8H8)2Ce. A simple model to rationalize changes in 4f orbital bonding for tri- and tetravalent lanthanide and actinide compounds is also provided.
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Sinterable ThO2 and mixed 70%ThO2-30%UO2 nanopowders were synthesized and calcined at a low temperature by surfactant assisted sol-gel process using three different nonionic structure directing (NSD) agents: TritonX100, Polysorbate 80 and Polysorbate 20. The produced powder samples, with high surface area and micro and meso pores, were pressed and sintered to form high density ThO2 and (Th,U)O2 pellets. The calculated green and sintered densities of the fabricated pellets, and the results of their microstructural characteristics studies, utilizing SEM images, show that developed synthesizing method yields a satisfactory sinterability of the synthesized ThO2 and ThO2-UO2 nanopowders at low temperature. Also, the final grain size of the pellets reached less than a micrometer in size. The ThO2 and (Th,U)O2 pellets fabricated by synthesized powder using Triton X100 had a higher density and better grain growth. The uranium presence in the mixed oxide pellets led to better sinterability and more normal grain growth.
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A detailed analysis of a single crystal of uranium tetrafluoride has been carried out. The techniques include x-ray absorption spectroscopy, as well as x-ray photoelectron spectroscopy and x-ray emission spectroscopy. Evidence will be presented for the presence of a uranyl species, possibly UO2F2, as a product of, or participant in the surface degradation.
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Understanding the nature of covalent (band-like) vs ionic (atomic-like) electrons in metal oxides continues to be at the forefront of research in the physical sciences. In particular, the development of a coherent and quantitative model of bonding and electronic structure for the lanthanide dioxides, LnO2 (Ln = Ce, Pr, and Tb), has remained a considerable challenge for both experiment and theory. Herein, relative changes in mixing between the O 2p orbitals and the Ln 4f and 5d orbitals in LnO2 are evaluated quantitatively using O K-edge X-ray absorption spectroscopy (XAS) obtained with a scanning transmission X-ray microscope and density functional theory (DFT) calculations. For each LnO2, the results reveal significant amounts of Ln 5d and O 2p mixing in the orbitals of t2g (σ-bonding) and eg (π-bonding) symmetry. The remarkable agreement between experiment and theory also shows that significant mixing with the O 2p orbitals occurs in a band derived from the 4f orbitals of a2u symmetry (σ-bonding) for each compound. However, a large increase in orbital mixing is observed for PrO2 that is ascribed to a unique interaction derived from the 4f orbitals of t1u symmetry (σ- and π-bonding). O K-edge XAS and DFT results are compared with complementary L3-edge and M5,4-edge XAS measurements and configuration interaction calculations, which shows that each spectroscopic approach provides evidence for ground state O 2p and Ln 4f orbital mixing despite inducing very different core–hole potentials in the final state.
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Uranium-containing metallic systems such as U3Si2 are potential Accident Tolerant Fuel (ATF) for Light Water Reactors (LWRs) and the next generation of nuclear reactors. Its oxidation behaviour, especially in oxygen and water-enriched environments, plays a critical role in determining its applicability in commercial reactors. In this work, we have investigated the oxidative behaviour of U3Si2 experimentally and by theoretical computation. The appearance of oxide signatures has been established from X-ray diffraction (XRD) and Raman spectroscopic techniques after oxidation of the solid U3Si2 sample in synthetic air (oxygen and nitrogen). We have also studied the changes in electronic structure as well as the energetics of oxygen interactions on U3Si2 surfaces using first-principles calculations in the Density Functional Theory (DFT) formalism. Detailed charge transfer and bond length analyses revealed the preferential formation of mixed oxides of UO2 and SiO2 on the U3Si2 {001} surface as well as UO2 alone on the U3Si2 {110} and {111} surfaces. Formation of peroxo (O22-) state confirmed the dissociation of molecular oxygen before U3Si2 oxidation. Core experimental analyses of the oxidized U3Si2 samples have revealed the formation of higher oxides from Raman spectroscopy and XRD techniques. This work is introduced to further a better understanding of the oxidation of U-Si metallic fuel compounds.
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sı Supporting Information 5 ABSTRACT: Rare-earth based A 2 B 2 O 7 compounds have been considered as potential 6 host materials for nuclear waste due to their exceptional chemical, physical, capability of 7 accommodating high concentration of actinides at both A-and B-sites, negligible leaching, 8 tendency to form antisite defects, and radiation stabilities. In this work, La 2 Hf 2 O 7 (LHO) 9 and Gd 2 Hf 2 O 7 (GHO) nanoparticles (NPs) were chosen as the RE-based hafnates to 10 study the structural changes and the formation of different U molecular structures upon 11 doping (or alloying) at high concentration (up to 30 mol %) using a combined 12 coprecipitation and molten-salt synthesis. These compounds form similar crystal 13 structures, i.e., ordered pyrochlore (LHO) and disordered fluorite (GHO), but are 14 expected to show different phase transformations at high U doping concentration. X-ray 15 diffraction (XRD) and Rietveld refinement results show that the LHO:U NPs have high 16 structural stability, whereas the GHO:U NPs exhibit a highly disordered structure at high 17 U concentration. Alternatively, the vibrational spectra show an increasingly random 18 oxygen distribution with U doping, driving the LHO:U NPs to the disordered fluorite phase. X-ray spectroscopy indicates that U is 19 stabilized as different U 6+ species in both LHO and GHO hosts, resulting in the formation of oxygen vacancies stemming from the U 20 local coordination and different phase transformation. Interestingly, the disordered fluorite phase has been reported to have 21 increased radiation tolerance, suggesting multiple benefits associated with the LHO host. These results demonstrate the importance 22 of the structural and chemical effect of actinide dopants on similar host matrices which are important for the development of RE-23 based hafnates for nuclear waste hosts, sensors, thermal barrier coatings, and scintillator applications.
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The mechanism of uranium (VI) removal by two anaerobic bacterial consortia, recovered from an uncontaminated site (consortium A) and other from an uranium mine (consortium U), was investigated. The highest efficiency of U (VI) removal by both consortia (97%) occurred at room temperature and at pH 7.2. Furthermore, it was found that U (VI) removal by consortium A occurred by enzymatic reduction and bioaccumulation, while the enzymatic process was the only mechanism involved in metal removal by consortium U. FTIR analysis suggested that after U (VI) reduction, U (IV) could be bound to carboxyl, phosphate and amide groups of bacterial cells. Phylogenetic analysis of 16S rRNA showed that community A was mainly composed by bacteria closely related to Sporotalea genus and Rhodocyclaceae family, while community U was mainly composed by bacteria related to Clostridium genus and Rhodocyclaceae family.
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The influence of Mott-Hubbard electron-electron correlations on the electronic structure and structural stability of uranium dioxide (UO2) has been analysed using the local spin-density approximation (LSDA) + U approach. We have found that the inclusion of a term describing the Hubbard on-site repulsion between 5f electrons results in a dramatic improvement in the description of the equilibrium electronic and magnetic structure of UO2 for which conventional LSDA calculations incorrectly predict a non-magnetic metallic ground state. We have found that the presence of electron-electron correlations in the 5f band modifies the character of chemical bonding in the material, leading to a Heitler-London type of hybridization between the 5f orbitals and giving rise to a larger value of the equilibrium lattice constant in better agreement with experimental observations.
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X-ray-absorption near-edge structure (XANES) spectroscopy at the M3,4,5 and L3 edges of uranium and thorium using synchrotron radiation has been used to probe the unoccupied 5f electronic states and local structure of uranium sites in oxides and glasses. The uranium sites in hyperstoichiometric UO2+x (x∼0.25, x∼0.66) oxides have been studied. The multiple-scattering resonance in the direction of the linear uranyl group UO2 2+ is identified in the L3 XANES spectra of uranyl nitrate hexahydrate. The localization of unoccupied 5f states in uranium and thorium oxides and glasses has been probed by M4- and M5-edge spectra. The presence of narrow 5f localized unoccupied states in Th- and U-containing glasses is indicated by the symmetric sharp white line due to 3d5fn+1 final states. The variable width of the white line indicates that the bandwidth of unoccupied 5f states increases by 4 eV going from glasses to oxides. The presence of 5f components in the U 6d conduction band of oxides, due to hybridization between the U 6d,7s and U5f states is shown by a long asymmetric tail of the white line at its high-energy side extending up to 12 eV. No evidence of many-body effects in XANES spectra, due to the configuration interaction between localized 5f configurations which was observed in mixed-valence rare-earth-metal compounds, has been found in Th and U oxides.
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Recent spectroscopic results obtained on UO2 single crystals are reviewed. Special emphasis is put on the various optical spectroscopies such as reflectivity, absorption, Faraday rotation, magnetic circular dichroism and Raman scattering, but also X-ray photoemission, bremsstrahlung isochromat spectra and neutron scattering data are discussed to show the state of the art of our knowledge of the electronic structure of UO2.
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The study of high-Tc superconductors and related compounds by means of soft X-ray fluorescence spectroscopy with monochromatic photon excitation is reviewed. It is shown that this spectroscopy provides a powerful tool for probing the local electronic structure at inequivalent O sites. The states in the valence band belonging to different sites are selectively studied by virtue of large variations in the O 1s X-ray absorption cross-section for these sites at certain excitation energies as well as by taking advantage of 1s chemical shifts between the sites. The character of unoccupied states is analyzed by monitoring the changes in the shape of X-ray fluorescence spectra with varying energies of incident photons. Issues are addressed, such as symmetry of doping-induced states and the origin of charge-carriers with respect to inequivalent sites, as well as the influence of electron–electron interaction.
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Systematic x-ray photoelectron spectroscopy (XPS) measurements are reported for both core and valence electrons for oxides of the actinides thorium, uranium, neptunium, plutonium, americum, curium, berkelium, and californium. The XPS spectra of the localized 5f electron states were compared to the appropriate multiplet calculations for the neutral atom. Agreement between theory and experiment was generally good but substantial solid-state broadening of the oxide spectra limited the capability for comparing detailed spectral features. Binding energies of several prominent core lines are systematically presented as a function of atomic number for the series of actinide oxides. Comparisons are made between the measured energies and the corresponding calculated energies reported by other investigators. Anomalous core-level spectra (most notably the 5d's), which probably result from coupling of the hole-state magnetic moment with the moment of the unfilled 5f shell, are presented.
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The reactions of acetone were investigated on α  U 3 O 8 (stoichiometric and H 2 reduced) by temperature programmed desorption. The surface and bulk characteristics of U oxides were investigated by x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD). The comparison between the XPS U 4 f 7/2 binding energy of β  UO 3 , α  U 3 O 8 , and UO 2 indicated that the U cations in U 3 O 8 are composed of +5/+6 (2/1) or +4/+6 (1/2) oxidation states. XPS of H 2 reduced α  U 3 O 8 (at 800 K) indicated a shift of the U  4f 7/2 binding energy from 381.6 (unreduced) to 380.5 eV (reduced) as well as the appearance of two satellites at 387.0 and 397.4 eV. These new XPS lines’ positions are characteristic of U <sup>+4</sup> cations of UO 2 . Similar results were observed upon Ar-ion sputtering of α  U 3 O 8 . Ar-ion sputtering of β  UO 3 also results in the reduction of U</roma- - n><sup> +6 </sup> cations to U <sup> +4 </sup> cations. XRD of H 2 reduced α  U 3 O 8 indicated that all of the α  U 3 O 8 phase was transformed to the UO 2 phase. A considerable difference between the reactivity of the surfaces of U 3 O 8 and UO 2 towards acetone was observed. A carbon–carbon bond formation reaction giving isobutene was observed only on the α  U 3 O 8 surface indicating its structure-sensitive nature and/or its sensitivity towards changing the oxidation state of U cations. This reaction has not been observed on the surfaces of actinide oxides before. On the other hand, acetone reacted on UO 2 to give mainly propene. This latter reaction (which is a C–O bond dissociation) is most likely due to the capacity of UO 2 to accommodate large numbers of atomic oxygen in interstitial positions while maintaining its structure intact. © 1997 American Vacuum Society.
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A detailed study of angle resolved photoemission of UO2 〈100〉 in the normal emission configuration is presented. The results are compared with a semirelativistic linearized augmented plane wave (LAPW) band calculation with the actual calculated empty bands used in the data reduction. Excellent agreement is found with calculations both for filled states as well as critical points in the empty states with no adjustment of bands, f-p Hybridization is found at the leading edge of the valence band and not at the bottom of the valence band, p-d Hybridization is found throughout the valence bands.
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A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
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The f-electron elements in actinide dioxide crystals have cubic site symmetry. Nevertheless, our understanding of their electronic structure has advanced rather slowly. Now, in addition to the magnetic susceptibility, magnetic resonance, optical and X-ray absorption data, there is information from both elastic and inelastic neutron magnetic scattering experiments.This report compares experimental data with results from two types of calculations: discrete-variational Xα molecular-orbital studies and traditional fn crystal-field studies. In this way, we not only gain an insight into the single-site crystal-field parameters but we learn something about the significance and the approximate magnitude of the interactions between the f electrons on neighboring actinide sites.
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