Bonding in uranium oxides: The role of 5f electrons

X-ray photoemission spectra of valence and bonding bands of UO₂, U₃O₈ and UO₃ are presented. The gradual transfer of localized 5f electrons into the bonding band as a function of oxygen content has been studied in these oxides.     

X-ray photoelectron spectra of uranium and uranium oxides. Correlation with the half-life of 235Um.

Uranium metal and the uranium oxides UO₂, U₃O₈, UO₃ are studied by photoelectron spectroscopy. Chemical shifts of the 4f electronic cor     

The characterization of Cr secondary oxide phases in ZnO films studied by X-ray spectroscopy and photoemission spectroscopy

X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS) were used to characterize the Cr secondary oxide phases in ZnO films that had been prepared using a co-sputtering method. Analysis of the Cr L_3,2-edge XANES spectra reveals that the intensity of white-line features decreases subtly as the sputtering power increases, indicating that the occupation of Cr 3d orbitals increases with Cr concentration in (Zn, Cr)O films. The O K-edge spectra show that the intensity of XANES features of (Zn, Cr)O films is lower than those of ZnO film, suggesting enhanced occupation of O 2p-derived states through O 2p-Cr 3d hybridization. The XES and XPS spectra indicate that the line shapes in the valence band of (Zn, Cr)O films are quite different from those of ZnO and that the Cr₂O₃ phase dominates the spinel structure of (Zn, Cr)O films increasingly as the Cr sputtering power is increased. Over all results suggest that the non-ferromagnetic behavior of (Zn, Cr)O films can be attributed to the dominant presence of Cr₂O₃, whereas the bulk comprise phase segregations of Cr₂O₃ and/or ZnCr₂O₄, which results them the most stable TM-doped ZnO material against etching.     

Raman spectrum of U4O9: a new interpretation of damage lines in UO2

Characterisation of uranium oxides in different conditions is a challenge both in nuclear and environment sciences. We focus here on U₄O₉, which is a superstructure of UO₂. Homogeneous U₄O₉ powder was fabricated and characterised by Raman spectroscopy. The Raman spectrum of a pure‐phase U₄O₉ is, for the first time, completely described and interpreted. U₄O₉ Raman spectrum derives from the well characterised one of UO₂. Besides reminiscent modes of UO₂, a band at 630 cm^‐1, which has a specific response to polarised light, was interpreted as characteristic of clusters of interstitial oxygen atoms. These experimental data will help to rationalise the interpretation of UO₂ damaged samples. Copyright © 2011 John Wiley & Sons, Ltd.     

Use of micro‐Raman spectrometry coupled with scanning electron microscopy to determine the chemical form of uranium compounds in micrometer‐size particles

In the frame of nuclear safeguards, knowledge of the chemical form (stoichiometry) of the uranium compounds present in the micrometric particulate material sampled by wiping surfaces in an inspected nuclear facility may point out the industrial process implemented in the installation. Micro‐Raman spectroscopy (MRS) coupled with scanning electron microscopy (SEM) has been used for the first time to analyze micrometer‐size particles of various uranium oxides [UO₂, U₃O₈, UO₃, and UO₄ · 4(H₂O)] deposited on carbon disks. Uranium particles are detected by means of SEM, and Raman analysis is then directly carried out inside the SEM measurement chamber without moving the carbon disk from SEM to MRS. When particles are deposited on appropriate carbon disks (sticky carbon tapes), despite a loss of signal‐to‐noise ratio of about an order of magnitude with regard to the stand‐alone MRS, all uranium oxides are successfully identified in particles by in‐SEM Raman analysis, obtaining similar characteristic bands as the ones obtained with the stand‐alone MRS. Moreover, with the SEM–MRS coupling, particles as small as 1 µm can be analyzed, whereas, without the SEM–MRS coupling, only particles larger than ~5 µm are efficiently analyzed, after localization inside the SEM, transfer of the sample holder into the MRS, and relocation of the particles inside the MRS. Copyright © 2013 John Wiley & Sons, Ltd.     

Chemical and structural properties of ternary post-transition metal oxide thin films: InZnO,InGaO and GaZnO

The chemical states of ternary post-transition metal oxide thin films of InGaO, GaZnO and InZnO were investigated using X-ray photoelectron spectroscopy. Detailed binding energy (BE) analyses revealed certain evolution in chemistry in the ternary oxides compared to the reference binary oxides of In₂O₃, ZnO, or Ga₂O₃. In particular, O 1s BEs were changed with the compositions, which suggests that the charge transfer (CT) between In³⁺/Ga³⁺/Zn²⁺ and O²⁻ ions is significant. Results of extended X-ray absorption fine structure analyses further showed that the first shell coordination (cation–O bond) is roughly maintained even though the ternary oxide films were structurally disordered. This implies that the CT process via O²⁻ ions can influence the charge reconstructions in the ternary oxide systems.     

The electronic structures of Mg,Al and Si in octahedral coordination with oxygen from SCF Xα MO calculations

Molecular orbital calculations performed using the SCF Xα Scattered Wave Cluster method are presented for the octahedral oxyanions MgO₆^?10, AlO₆^?9 and SiO₆^?8. The AlO₆^?9 results are used to assign and interpret the X-ray photoelectron spectra (XPS), X-ray emission (XES) and u.v. spectra of Al₂O₃. Agreement between calculation and experiment is good for valence band and fair for conduction band orbitais. The SCF Xα results for MgO₆^?10 are also in good agreement with observed valence band energies in MgO, but in this case the lowest energy features in the u.v. spectrum are not assignable in terms of either the calculations or the X-ray spectral results. The substantial increase in covalency expected between the Mg and Si oxides is evidenced in the calculations by an increase in valence region width from 2.6 to 5.3 eV and an increase in valence-conduction band separation from 5.2 to 10.0 eV. The calculated trends are in reasonable agreement with u.v. spectral data and with absolute valence orbital binding energies derived from X-ray spectra. A comparison of the SiO₆^?8 calculation with the analogous tetrahedral SiO₄^?4 calculation shows the valence band in the octahedral oxyanion to be much simpler in structure and somewhat narrower than that in the tetrahedral oxyanion. Using the orbital structure calculated for the valence bands of tetrahedral and octahedral oxides, a method is presented for calculating atomization energies directly from X-ray spectral data for SiO₂, Al₂O₃ and MgO. Results are in good agreement with experiment but the method involves an empirical parameter which is not presently understood in detail. Studies of trends in p-type bonding orbital binding energies derived from experimental data provide a qualitative explanation for the preferred coordination numbers in the Mg, Al and Si oxides.     

Thermochemical and electrochemical study of sodium and zinc insertion into α-UO3 − y and α-U3O8

New sodium and zinc insertion compounds of the uranium oxides UO_2.95(α-UO₃) and UO_2.67(α-U₃O₈), were synthesized at ambient temperature. Enthalpies of formation were measured calorimetrically and corresponding free energies of formation were determined electrochemically. Electronic changes on insertion were monitored by measurements of magnetic susceptibilities.     

Behavior of Uranium oxides and oxyfluorides exposed to moisture and 244Cm alpha radiation

Alpha radiolysis studies have been performed on uranium oxides and oxyfluorides (UO₃, U₃O₈, and UO₂F₂) to evaluate the long-term storage characteristics of ²³³U. These uranium compounds (using ²³⁸U as the surrogate for ²³³U) were subjected to relatively high alpha radiation doses (235–634 MGy) by doping with ²⁴⁴Cm. The typical irradiation time for these samples was about 1.5 years, which would be equivalent to more than 50 years irradiation by a ²³³U sample. Both dry and wet (up to 10 wt% water) samples were examined in an effort to identify the gas pressure and composition changes that occurred as a result of radiolysis. This study shows that several competing reactions occur during radiolysis with the net effect of generating only very low pressures of hydrogen, nitrogen, and carbon dioxide from water, nitrate, and carbon impurities, respectively. In the absence of nitrate impurities, no pressures greater than 1000 Torr are generated. Usually, however, the oxygen in the air atmosphere over the oxides is consumed with the corresponding oxidation of the uranium oxide. In the presence of up to 10 wt% water, the oxides first show a small pressure rise followed by a net decrease due to the oxygen consumption and the attainment of a steady-state pressure at which the rate of generation of gaseous components is balanced by their recombination and/or consumption in the oxide phase.     

Interpretation of the x-ray photoemission spectra of cobalt oxides and cobalt oxide surfaces

CoO and Co₃O₄ have been studied by high-resolution X-ray photoemission. The characteristic binding energies in the Co 2p_{$\text{3}\text{2}$}, 2p_{$\text{1}\text{2}$}, and 3s regions, their band shapes and widths, the associated shake-up structure, the O(1s) and O(2s) BE's, and the valence band spectra have been examined. The two oxides are readily distinguished from their spectra though it is shown that the O(1s) BE's are identical at 529.50 ± 0.14 eV. Argon and oxygen ion sputtered surfaces were examined to establish the integrity of the oxides. A higher BE O(1s) component (530.7–531.6 eV), the intensity and BE of which vary with the treatments mentioned above, corresponds to non-stoichiometric surface oxygen. The results are discussed with respect to the electronic structures of the oxides and the often conflicting earlier studies of these oxides.     

X-ray emission study of the electronic structure of nanocrystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Valence states of metal ions and the phase composition of nanocrystalline Al₂O₃ (of the original oxide and the oxide irradiated by high-energy Fe⁺ ions) are studied by using x-ray emission Al L_{2, 3} and O Kα spectra. It is established that the shape of the Al L_{2, 3} spectra strongly changes as one goes from the original (bulk) Al₂O₃ to nanocrystalline oxide, while the O Kα spectra remain practically unchanged. Moreover, irradiation by high-energy Fe⁺ ions results in slight additional changes in the x-ray spectral characteristics of the aluminum oxides under study. The obtained experimental data are compared with the results of theoretical calculations of the electronic structure of α and γ phases of Al₂O₃ performed using the LDA formalism. Using the results of x-ray spectral studies, electronic structure calculations, and x-ray diffraction analysis, it is shown that the revealed spectral differences between the nanocrystalline state of aluminum oxide and the bulk material can be interpreted as a phase transition from the α phase to the γ phase of Al₂O₃ with an addition of bayerite.     

Raman scattering from phonons and electronic excitations in UO2 with different oxygen isotopes

Comparison of the phonons and electronic excitations of UO₂ with different oxygen isotopes (¹⁶O, ¹⁸O), two purely electronic excitations at 4162 and 5778 cm⁻¹ have been evidenced by their limited shift, when ¹⁶O is replaced by ¹⁸O. The bands observed at 570, 1144 and 2300 cm⁻¹ in U¹⁶O₂ shift to 545, 1091 and 2190 cm⁻¹ for U¹⁸O₂. The wavenumbers and the isotope shifts of these bands exhibit multiple relationships. It supports the assignments of the 1LO (~570 cm⁻¹) and 2LO (~1144 cm⁻¹) phonons for U¹⁶O₂. Both bands exhibit marked decrease for increasing O/U ratio. The observed phenomenon is regarded as a good estimation of the stoichiometry of a uranium oxide. In addition, by analyzing the intensity ratios of the 2LO to 1LO phonons for various UO₂ samples, the 2LO band is assigned to the Fröhlich interaction. Defect‐induced and/or deformation potential mechanisms play a dominant role for the 1LO band. Copyright © 2015 John Wiley & Sons, Ltd.     

The structural and spectroscopic properties for uranium oxides

The equilibrium structures, the charge population, and the spectroscopic properties of UO, UO 2 , UO 3 , and U 2 O 3 molecules are systematically investigated using the density functional theory (DFT) with the method of generalized gradient approximation (GGA). The bond lengths and the vibrational frequencies of the ground states of UO, UO 2 , and UO 3 molecules are all in agreement with available experimental data. For U 2 O 3 molecules, our calculations indicate that the ground state of the U 2 O 3 molecule is an X 7 A ′ 1 state with D 3h (trigonal bipyramid) symmetry (R 1 (U–O)=0.2113 nm, R 2 (U 1 –U 2 )=0.2921 nm, ∠U 1 OU 2 = 87.5 , dihedral angle Θ(U,O 1 ,O 2 ,O 3 )=62.40 ). The harmonic frequency, the IR intensity and the spin density of the U 2 O 3 molecule are all obtained for the first time in theory. For the ground state of U 2 O 3 molecules, the vibrational frequencies are 178.46 (A ′ 1 ), 276.79 (E ′′ 1 ), 310.77 (E ′ 1 ), 396.63 (A ′′ 2 ), 579.15 (E ′ 1 ), and 614.98 (A ′ 1 ) cm 1 . The vibrational modes corresponding to the IR maximum peaks are worked out for UO 3 and U 2 O 3 molecules. Besides, the results of Gophinatan–Jug bond order indicate that UO, UO 2 , and UO 3 molecules possess U=O double bonds and that the U 2 O 3 molecule possesses U–O single bonds and a U–U single bond.     

Estimation of the oxidation state of uranium in microparticles by using scanning electron microscope in the backscattered electrons mode

This paper considers the results of applying of scanning electron microscopes in the backscattered electron mode for the specifying of the contents of multielemental samples and minerals, which are presented in publications. This approach is used for estimation of oxidation state of uranium in microparticles. The possibility of this goal achieving is demonstrated on the example of the analysis of the UO₂ and U₃O₈ microparticles and the samples of In, Sn, Pt and Pb.     

Vibronic excitation spectrum of alkali aluminometaphosphate glasses containing uranium

The absorption, excitation and emission spectra of metaphosphate glasses MPO₃.Al(PO₃)₃ (M = Li, Na, K) doped with 1 or 2 w.% of uranium oxide have been measured at room temperature. The spectra were compared with those of solid uranyl nitrate and its water solution. The excitation band of glasses peaking at about 420 nm shows a great deal of structure like that of the solution. The diffuse structure would be due to the electron vibronic transitions in the uranium centre of a molecular form (probably uranyl (UO₂)²⁺).     

Multiplet splitting patterns exhibited by the first row transition metal oxides in X-ray photoelectron spectroscopy

High resolution photoelectron spectra from transition metal ions in TiO₂, V₂O₅, VO₂, V₂O₃, MnO, Mn₂O₃, MnO₂, Cr₂O₃, FeO, CoO, NiO, CuO, Cu₂O, FeSrO₃, and Cu doped CaTiO₃ were re-examined using a constrained curve fitting approach. Effective fits of the multiplet splitting present could be attained for the oxides with unfilled 3d bands if multiple final states were assumed. The type of transitions implied, also suggest the classification of these oxides during core level photoelectron emission as either; Mott–Hubbard (V₂O₅, VO₂, V₂O₃, Cr₂O₃, and FeO), intermediate (MnO, Mn₂O₃, and MnO₂) or charge transfer type compounds (FeSrO₃, CoO, NiO, CuO and Cu doped CaTiO₃). These transitions along with relationships defining the splitting energy with respect to the total spin and binding energy are discussed.     

The shape and extended fine structure of the manganese K X-ray absorption discontinuity in its oxides

The shape and extended fine structure of the manganese K absorption discontinuity have been studied in the pure metal and in its four oxides. viz. MnO, Mn₃O₄, Mn₂O₃ ad MnO₂ using a bent crystal Cauchois type spectrograph of 40cm diameter. The shape of the main discontinuity for the oxides is discussed on the basis of molecular orbital theory. Our results show a complimentarity between the absorption spectra and the emission spectra obtained earlier by Tsutsumi et al. for some of these oxides. The extended X-ray absorption fine structure (EXAFS) is discussed in the light of the recent proposed by Lytle et al.     

Raman spectroscopic study of the uranyl titanate mineral holfertite Ca x U2−x Ti(O8−x OH4x )·3H2O and the lack of metamictization

Raman spectra of the uranyl titanate mineral holfertite Ca _x U_2?x Ti(O_8?x OH_4x )·3H₂O were analyzed and related to the mineral structure. Observed bands are attributed to the TiO and (UO₂)²⁺ stretching and bending vibrations, U–OH bending vibrations, and H₂O stretching and bending. The mineral holfertite is metamict, as is evidenced by the order/disorder of the mineral. Unexpectedly, the Raman spectrum of holfertite does not show any metamictization. The intensities of the UO stretching and bending modes show normal intensity and the bands are sharp.     

First-principles FP-LAPW calculations and X-ray spectroscopy studies of the electronic structure of Zr3V3O and Zr3V3O0.6 oxides

Electronic properties of Zr₃V₃O oxide, a very promising hydrogen-storage material, were studied both from theoretical and experimental points of view employing the full potential linearized augmented plane wave (FP-LAPW) method as well as X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). Total and partial densities of states of the constituting atoms of Zr₃V₃O have been derived from the FP-LAPW calculations. These data indicate that, the O 2p-like states are the dominant contributors in the bottom of the valence band, whilst the top of the valence band and the bottom of the conduction band of Zr₃V₃O are dominated by contributions of the V2 3d-like states, with slightly smaller contributions of the V1 3d-like states as well. Significant contributions of the Zr 4d-like states throughout the whole valence-band region and near the bottom of the conduction band are also characteristic of the electronic structure of Zr₃V₃O. The XPS valence-band spectra and the XES Zr Lβ_2,15, V L_α and O K_α bands have been derived and compared on a common energy scale for Zr₃V₃O and Zr₃V₃O_0.6 oxides. This comparison of the experimental spectra was found to be in excellent agreement with the results of the FP-LAPW calculations. In addition, the XPS Zr 3d, V 2p and O 1s core-level binding energies have been measured for Zr₃V₃O and Zr₃V₃O_0.6 oxides.     

Chemically induced shifts of 235U and 234U neutron resonances

Transmission spectra were measured with the time-of-flight technique at the Dubna pulsed reactor for samples of metallic U, UO₂, U₃O₈, and UO₃ enriched in ²³⁵U, and were compared in the regions of nine low-energy resonances to observe chemically induced shifts. After elimination of contributions caused by different Doppler broadenings, the shifts are interpreted as changes of the mean-square charge radius for nuclei capturing neutrons. The 〈r²〉 of the compound-nucleus states show on average a weak diminution as compared with the ground-state value.