Search

The charge compensation mechanisms in U y Nd 1− y O 2± x and its consequence on the overall O stoichiometry (or O/M ratio where M = Nd + U) have been studied through the uranium valence state mixture evolution as a function of Nd content up to y = 0.62 by means of high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) at the U M 4 -edge. Our results clearly demonstrate the formation of U 5+ at low Nd content (y < 0.15). Upon increasing the Nd content, oxygen vacancies and the formation of U 6+ appear as competing mechanisms for intermediate Nd concentrations, leading to the co-existence of U 4+ /U 5+ /U 6+ mixed valence and an overall hypostoichiometry (O/M < 2.00). Finally, the formation of U 6+ associated with strongly distorted U local environment is observed for high Nd concentrations (y = 0.62), leading to an overall hyperstoichiometry (O/M < 2.00). [ABSTRACT FROM AUTHOR]

We report here the first direct observation of U(V) in uranium binary oxides and analyze the gradual conversion of the U oxidation state in the mixed uranium systems. Our finding clarifies previous contradicting results and provides important input for the geological disposal of spent fuel, recycling applications, and chemistry of uranium species. [ABSTRACT FROM AUTHOR]

Cu Kβ2,5 X-ray emission and resonant inelastic X-ray scattering measurements were performed on monovalent and divalent copper compounds. The data were compared with the results of local-density-approximation calculations. The methods were found to be efficient tools for studies of Cu 4p states in the valence band and for distinguishing between different monovalent copper compounds. This is of particular importance for the debate concerning copper corrosion in oxygen-free water. [ABSTRACT FROM AUTHOR]

We study both theoretically and experimentally the photon-in and photon-out spectra of CeO, which are caused by the Ce 2 p to Ce 5 d excitation followed by the three different de-excitation channels: (i) Ce 3 d to Ce 2 p (denoted by 3 d-RXES), (ii) O 2 p to Ce 2 p ( v-RXES), and (iii) Ce 5 d to Ce 2 p (RIXS). In 3 d- and v-RXES, the 5 d electron plays a role of a spectator, but in RIXS it is a participator. By extending our single impurity Anderson model (SIAM), which was used recently for our calculations of v- and 3 d-RXES spectra of CeO, we study the polarization dependence in the spectator and participator spectra, and we perform more detailed calculations for 3 d- and v-RXES spectral features, as well as new calculations for the RIXS spectrum with charge transfer excitations. The polarization dependence is different for the spectator and participator spectra; we have no polarization correlation between the incident and emitted photons for the spectator spectra but a strong polarization correlation for the participator spectrum. The theoretical calculations predict that the charge transfer excitations in RIXS occur in the transfer-energy range overlapped with v-RXES, but the RIXS and v-RXES spectra can be discriminated by taking advantage of the different polarization dependence. The overlapped RIXS and v-RXES spectra are observed successfully by our experiments and well reproduced by our SIAM calculations. [ABSTRACT FROM AUTHOR]

The article describes the structure of the Ce L3 x-ray absorption spectra (XAS) of CeFe2. The single impurity Anderson model (SIAM) and the first-principles band calculations based on the density-functional theory (DFT) were utilized to describe the structure. It was discovered that the mixed valence 4f character revealed by the core-hole potential effect influence the structure of Ce L3 XAS.

Mn-doped UO2 is under consideration for use as an accident tolerant nuclear fuel. We detail the synthesis of Mn-doped UO2 prepared via a wet co-precipitation method, which was refined to improve the yield of incorporated Mn. To verify the Mn-doped UO2 defect chemistry, X-ray absorption spectroscopy at the Mn K-edge was performed, in addition to X-ray diffraction, Raman spectroscopy and high-energy resolved fluorescence detection X-ray absorption near edge spectroscopy at the U M4-edge. It was established that Mn2+ directly substitutes for U4+ in the UO2 lattice, accompanied by oxygen vacancy (Ov) charge compensation. In contrast to other divalent-element doped UO2 materials, compelling evidence for U5+ in a charge compensating role was not found. This work furthers understanding of the structure and crystal chemistry of Mn-doped UO2, which could show potential advantages as a novel efficient advanced nuclear fuel. [ABSTRACT FROM AUTHOR]

We present resonant x-ray emission spectroscopic data from the uranium intermetallics UPd3, USb, USn3, and URu2Si2 at the M4,5 edges of uranium, and we compare the data to those from the well-localized 5f² 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 5f³. We have found a small energy shift between UO2 and UPd3, both known to have localized 5f² configurations, which we ascribe to the effect of conduction electrons in UPd3. The spectra from UPd3 and URu2Si2 are similar, strongly suggesting a predominant 5f² configuration for URu2Si2. The valence-band resonant inelastic x-ray scattering 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 of these low-energy transitions reasonably well. [ABSTRACT FROM AUTHOR]

The microstructures of amorphous and polycrystalline ferroelectric Hf0.5Zr0.5O2 films are studied by X-ray spectroscopy and ellipsometry. EXAFS spectra demonstrate that the amorphous film consists of an “incompletely mixed” solid solution of metallic oxides HfO2 and ZrO2. After rapid thermal annealing, the mixed Hf0.5Zr0.5O2 oxide films have a more ordered polycrystalline structure, and individual Hf and Zr monoxide islands are formed in the films. These islands are several nanometers in size and have a structure that is similar to the monoclinic structure of HfO2 and ZrO2. The presence of the HfO2 and ZrO2 phases in the Hf0.5Zr0.5O2 films is also detected by ellipsometry. [ABSTRACT FROM AUTHOR]

New germanides with composition RE3Pt4Ge13 (RE = Y, Pr, Sm, Gd, Tb, Tm) have been prepared by high-pressure, high-temperature synthesis. Their crystal structures have been refined, and the relationship of this new rhombohedral and monoclinic structure types with the primitive cubic Yb3Rh4Sn13 prototype is discussed. Band structure calculations within density functional theory confirm the distorted rhombohedral and monoclinic structural arrangements to be energetically more favorable than the simple cubic one. X-ray absorption spectroscopy and magnetic susceptibility measurements indicate that the RE-atoms are in the +3 oxidation state in all studied compounds. [ABSTRACT FROM AUTHOR]

We present a study of the Co spin-state changes in Pr0.5Ca0.5Co CoO3 across the metal-insulator transition by means of the analysis of two complementary synchrotron x-ray spectroscopic techniques. Recorded experimental and charge transfer multiplet-based calculated Co L2,3 absorption spectra, in combination with Co Kß main emission lines, indicate that Co3+ ion spin state evolves from low in the insulating phase to a low:high mixed state in the metallic regime, close to a 1:1 ratio at ambient temperature. A pure high spin state can be discarded, while the occurrence of an intermediate state is also questioned. [ABSTRACT FROM AUTHOR]

The present study investigates possible use of Layered Double Hydroxides (LDH) for Tc(VII) remediation. Mg/Al– and Mg/Fe–LDH were obtained by a hydrothermal route and thermally activated at 450 °C, which was shown to significantly improve the Tc(VII) removal efficiency. Based on XRD investigation of Tc–LDH phases, the uptake Tc(VII) from the solution follows the restoring of a LDH structure. X-ray absorption spectroscopy demonstrates that Tc ions interact solely via the Tc O bond, leaving no evidences of farther atomic interactions with, e.g., layers of LDH. The presence of competing anions, like NO 3 −, or CO 3 2– in the solution decreases Tc(VII) uptake by LDH. Maximum uptake capacity of thermally activated Mg 0.67 /Al 0.33 -LDH and Mg 0.75 /Fe 0.25 –LDH were derived from fitting the experimental data into the modified Langmuir equation and correspondingly comprise 0.227 mol/kg and 0.213 mol/kg. In agreement with these findings, theoretical simulations predicted incorporation energies for Mg 0.67 /Al 0.33 –LDH and Mg 0.75 /Fe 0.25 –LDH of −128 kJ/mol and − 110 kJ/mol, respectively. Investigation of stability of Tc–LDH in different aqueous solutions demonstrated a rather low release Tc(VII) in contact with diluted solutions containing Cl− and OH−, however, in a high saline solution, like Q-brine a rather fast release of TcO 4 − occurs due to anion exchange with Cl−. • Thermally "activated" at 450 °C Mg/Al– and Mg/Fe–LDH can efficiently remove soluble 99Tc(VII). • Theoretical prediction (DFT-simulation) of a higher stability of Tc(VII) incorporated in the Mg/Fe–LDH compared to Mg/Al–LDH. • 99Tc–LDHs reveal rather high stability in diluted solutions containing Cl− and OH−, and on ambient conditions. • Fast release of 99Tc from LDHs in the highly saline solutions. [ABSTRACT FROM AUTHOR]

The effect of fractionation on the phase behavior of colloidal goethite dispersions with different polydispersities (17%, 35%, and 55% in length) has been studied by small angle x-ray scattering and transmission electron microscopy. All systems show at least nematic and smectic phases. The occurrence of the latter phase at such a high polydispersity is remarkable. It is shown that in the highly polydisperse systems strong fractionation occurs, which is able to reduce the local length polydispersity up to a factor of 2. A columnar phase was only found in the 35% and 55% polydisperse systems. It seems that the columnar phase accommodates the particles that do not fit into the smectic layers and, thus, reduces the length polydispersity within the smectic phase even further. The fact that a columnar phase was not found in the system of lowest polydispersity indicates that the smectic phase is the most stable phase at higher concentrations. [ABSTRACT FROM AUTHOR]

Using X-ray absorption near edge structure spectroscopy, extended X-ray absorption fine structure spectroscopy, atomic force microscopy, and Rutherford backscattering spectroscopy, the features of the microstructure and elemental composition of Si/GeMn magnetic systems obtained by molecular beam epitaxy and containing quantum dots are studied. Intense mixing of Ge and Si atoms is found in all samples. The degree of mixing (diffusion) correlates with the conditions of synthesis of Si/GeMn samples. For these systems, direct contacts of germanium atoms with manganese atoms are characterized and the presence of interstitial manganese with tetrahedral coordination and substitution of manganese for germanium and silicon in the lattice sites is found. The presence of stoichiometric phases Ge8Mn11, Ge3Mn5 is not detected. The correlations of the Ge, Si, and Mn coordination numbers in the Ge environment are determined both with the Mn flux value (evaporator temperature) and with the temperature at which quantum dots are grown, as well as with other synthesis conditions. The manganese concentration in the samples is determined. [ABSTRACT FROM AUTHOR]

We report that by using crystal optics and laboratory-based X-ray source, one may perform easily X-ray Absorption Spectroscopy (XAS) state-of-the-art measurements at the U L 3 -edge. Our results are in excellent agreement with synchrotron-based studies for UO 2 , KUO 3 and β -UO 3 . The laboratory scaled approach constitutes an inexpensive solution to the limited availability of the XAS beamlines for actinide research. Moreover, our results allow for the development of many future applications addressing actinide f-electron chemistry and nuclear energy-related physico-chemistry, such as advanced nuclear fuel development and safe long term disposal of nuclear wastes. [ABSTRACT FROM AUTHOR]

Ge K edge EXAFS (Extended X-Ray Absorption Fine Structure) spectra have been measured for multilayer semiconducting heterosystems containing interacted groups of quantum dots ('molecules from quantum dots') ordered in rings on different stages of their growth depending on topologic parameters and growth conditions. In accordance with our results obtained previously for the quantum dots of SiGe, for the molecules of quantum dots it was found that deformation at the interface leads to decrease in the interatomic distance of Ge-Ge by ~0.03 Å. Effect of heterosystem topology and temperature at different stages of their growth on interlayer diffusion was investigated. It was found that at the first growth stage (growth of 'seeded islands' serving as a basis for obtaining the molecules) at 700°C a concentration of Ge atoms in the system is ~38%. With further growth of the vertically-matched quantum dots groups the concentration of Ge increases up to ~43-47% depending on the growth conditions. Comparable analysis of different modes of EXAFS measurements was performed to determine precisely structural parameters of heterosystem SiGe with different thickness grown on Si(100) surface. [ABSTRACT FROM AUTHOR]

Layered double hydroxides (LDH) containing Al 3+ , Mg 2+ and Zn 2+ cations with the ratios of Zn/(Mg + Zn) = 0, 0.05, 0.1, 0.2, 0.5, 0.7, and 1.0 were synthesized. The effect of zinc content on the phase composition of LDH and on the structural parameters, textural characteristics and acid–base properties of the corresponding mixed oxides were studied. This type of supports was used to obtain the non-acid platinum Pt/Mg(Zn)AlO x catalysts. The formation of supported platinum particles, their composition, dispersion and electronic state were examined by means of TPR, TEM, XPS, and EXAFS. The possibility to obtain the bimetallic PtZn particles, whose structure and strength of interaction with the support depend on the zinc content of the support, was demonstrated. It was found that the presence of zinc atoms in the platinum environment decreases the particle size of active metal and stabilizes platinum in the active metallic state ensuring a high activity of the catalyst in dehydrogenation of propane with the selectivity for propylene above 99%. [ABSTRACT FROM AUTHOR]

The Au L X-ray absorption fine structure (XAFS) spectra have been measured for gold nanoparticles with the calibrated size d ∼ 1 nm in the cavities of cucurbit[7]uril molecules. The features of their electronic structure and microstructure have been characterized. It has been found that gold clusters in cavities of cucurbit[7]uril are characterized by smaller (by 0.01-0.02 Å) interatomic distances and a noticeably larger (by a factor of 3 at 12 K) Debye-Waller factor as compared to bulk gold. Analysis of the experimental results and their comparison with the model calculations show that the special properties of atoms on the surface of small metal (gold) particles are not determined by their position at the vertices and edges of small clusters; they are likely attributed to the structural disorder, deformations, and stresses, which increase with a decrease in the size of the particles. [ABSTRACT FROM AUTHOR]

In the present work local surrounding of vanadium atoms in layered copper-chromium disulfides CuCrVS is investigated using high-resolution X-ray absorption spectroscopy above vanadium K-edge. Based on experimental and theoretically simulated spectra comparison it is shown that vanadium atoms replace chromium ones even at high concentrations of vanadium and that they are in 3+ oxidation state. [ABSTRACT FROM AUTHOR]

We present a method suitable for the measurement of a high-energy resolved absorption spectrum in the emission mode upon target excitation with an intense monochromatic pulse of x-ray light. The approach is based on a measurement of a single resonant Raman x-ray spectrum at a fixed off-resonant excitation energy by a dispersive-type high-resolution emission spectrometer. Using a simple Lorentzian tail correction function and an energy-scale transformation, we reconstructed the L3 absorption edge of Xe from the 3d-2p3/2 resonant Raman x-ray spectrum recorded at excitation energy 25 eV below the L3 edge. The energy resolution is well below the L3 core-hole lifetime broadening and allows for a straightforward extraction of the [2p3/2]nℓ resonance energies and emission strengths. The reconstructed spectrum is nearly equivalent to the high-energy resolution fluorescence detected absorption spectrum, which is recorded by scanning the photon excitation energy across the absorption edge. [ABSTRACT FROM AUTHOR]

The size and lattice constant evolution of Pb nanoparticles (NPs) synthesized by high fluence implantation in crystalline Si have been studied with a variety of experimental techniques. Results obtained from small-angle x-ray scattering showed that the Pb NPs grow with increasing implantation fluence and annealing duration. The theory of NP growth kinetics can be applied to qualitatively explain the size evolution of the Pb NPs during the implantation and annealing processes. Moreover, the lattice constant of the Pb NPs was evaluated by conventional x-ray diffraction. The lattice dilatation was observed to decrease with increasing size of the Pb NPs. Such lattice constant tuning can be attributed to the pseudomorphism caused by the lattice mismatch between the Pb NPs and the Si matrix. [ABSTRACT FROM AUTHOR]

The distribution of the scattering intensity in the reciprocal space for natural and artificial opals has been reconstructed from a set of small-angle X-ray diffraction patterns. The resulting three-dimensional intensity maps are used to analyze the defect structure of opals. The structure of artificial opals can be satisfactorily described in the Wilson probability model with the prevalence of layers in the fcc environment. The diffraction patterns observed for a natural opal confirm the presence of sufficiently long unequally occupied fcc domains. [ABSTRACT FROM AUTHOR]