Chemical shift of Mn and Cr K-edges in X-ray absorption spectroscopy with synchrotron radiation

Mn and Cr K X-ray absorption edges were measured in various compounds containing Mn in Mn^2?+?, Mn^3?+? and Mn^4?+? oxidation states and Cr in Cr^3?+? and Cr^6?+? oxidation states. Few compounds possess tetrahedral coordination in the 1st shell surrounding the cation while others possess octahedral coordination. Measurements have been carried out at the energy dispersive EXAFS beamline at INDUS-2 Synchrotron Radiation Source at Raja Ramanna Centre for Advanced Technology, Indore. Energy shifts of ~ 8–16 eV were observed for Mn K edge in the Mn-compounds while a shift of 13–20 eV was observed for Cr K edge in Cr-compounds compared to values in elemental Mn and Cr, respectively. The different chemical shifts observed for compounds having the same oxidation state of the cation but different anions or ligands show the effect of different chemical environments surrounding the cations in determining their X-ray absorption edges in the above compounds. The above chemical effect has been quantitatively described by determining the effective charges on Mn and Cr cations in the above compounds.     

Contribution de la spectrometrie x a l'etude d'oxydes de fer a valence mixte. analyse de la structure fine du seuil d'absorption k

Chemical shifts of Lα_1,2, Kβ_1,3, Kβ₅ emission bands and X-ray absorption spectra near the K edge have been measured in several iron spinels oxides with the metal in the formal oxidation states +2 and +3. The first high resolution XANES spectra for these materials were performed at the DCI storage ring at LURE, Orsay. Two transitions in the range 15–20 eV above the 1s → 3d observed on the derivative XANES spectra are very sensitive to the iron oxidation state. The analysis of the X-ray absorption fine structure in the near edge region and the measurements of the chemical shift of FeKβ₅ are more adapted than Mössbauer effect to disclosing FeII in mixed ferrites.     

“Pattern recognition” from the U<Emphasis Type="Italic">L</Emphasis> emission spectra as a method for identifying the likeness and difference of uranium state in compounds

The UL X-ray emission spectra of a wide range of uranium compounds were systematically studied. The chemical and isotope (^{233, 235, 238}U) shifts in the L transitions were estimated. The suitability of these data for identifying the U state in real compounds and minerals was assessed. The experimental results were compared with the theoretical predictions based on the Hartree-Fock-Dirac method.Translated from Radiokhimiya, Vol. 46, No. 6, 2004, pp. 494–502.Original Russian Text Copyright © 2004 by Makarov, Solomennikov.     

Oxidation state and coordination surrounding of iron and uranium in sodium aluminum iron phosphate glasses

The oxidation state of Fe and U and the coordination surrounding of Fe in uranium-containing sodium aluminum iron phosphate glasses were determined by analysis of the FeK and UL₃ X-ray absorption nearedge structure (XANES), X-ray photoelectron spectroscopy (XPS), and Mössbauer spectroscopy on ⁵⁷Fe nuclei. Uranium is present in the glasses in the form of U(V) and U(VI), and iron, in the form of Fe(III) and Fe(II), mainly in the distorted octahedral surrounding. The fraction of U in various oxidation states depends on the form of untroducing uranium (UO₂ or UO₃) and on the oxide concentration. With an increase in the UO₃ concentration in glasses, the fraction of U(VI) increases and the fraction Fe(III) relative to Fe(II) decreases.     

X-ray absorption near edge structure (XANES) and X-ray Kβ emission spectra in mixed valence manganese spinels

The X-ray MnKβ spectra and XANES of Mn₃O₄, LiMn₂O₄ and NiMn₂O₄ are reported. The chemical shifts (MnKβ_1.3 and MnK edge) and the X-ray absorption fine structure in the near edge region of NiMn₂O₄ compared with those of reference compounds are indicative of the charge distribution: Ni_x²⁺MN_1−x²⁺[NI_1−x²⁺MN_2x³⁺MN_2x³⁺MN_1−x⁴⁺]O₄ (O < x < 1)     

Excited states in yttrium orthovanadate YVO4 measured by soft X-ray absorption spectroscopy

We present the electronic structure of vanadium in YVO₄ compound obtained by X-ray absorption (XAS) at the L_2,3 edge. We also performed a charge transfer multiplet calculation (CTM) to analyze the experimental results. YVO₄ compound was synthesized by the polyacrylamide sol–gel method. For a comparative evaluation, YVO₄ compound was prepared by solid-state reaction. XAS results show that the increase of heat treatment favored the presence of one oxidation state V⁵⁺. We observed a distinctive broadening at L₂ edge of vanadium due to the L₂L₃v Coster–Kronig process. We identify the excitations of 3d bonding orbitals by CTM on the XAS results. Tetragonal parameters (D _s and D _t) in D _4h symmetry reveal a compression of the 3d orbitals in the z direction.     

Reduction of U(VI) in laser liquids POCl3-SnCl4-235UO 2 2+ -Nd3+

U(IV) is irreversibly accumulated during synthesis of laser liquids POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ prepared from various initial Nd(III) and U(VI) compounds, irrespective of the way of their introduction. The rate of U(IV) accumulation in POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions increases with increasing UO ₂ ²⁺ and Nd³⁺ concentrations; for laser liquids with the Nd³⁺ luminescence lifetime τ > 150 μs the observed rate constant of U(IV) accumulation by the second-order reaction k ₂[U⁴⁺] is equal to (3 ± 1) × 10^?5 1 mol^?1 s^?1 at T = 380 K. U(IV) is accumulated during storage of POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions in hermetically sealed glass cells at room temperature and upon irradiation of solutions by xenon lamp light in the spectral region of UO ₂ ²⁺ absorption. The U(VI) reduction proceeds by chemical and photochemical activation of uranyl with formation of stable U⁴⁺ complexes with dichlorophosphate ions and also with Nd³⁺. Deactivation of the uranyl ion excitation with proton-and chlorine-containing impurities prevents U(VI) reduction.     

Room temperature ferromagnetism in Fe-doped CeO2 thin films grown on LaAlO3 (001)

Ce_1 − xFe_xO_2 − δ/LaAlO₃(001) thin films (x = 0.01 and 0.03) have been prepared by pulsed laser deposition method and thoroughly characterized using X-ray diffraction (XRD), dc magnetization, near edge X-ray absorption fine structure (NEXAFS), and X-ray magnetic circular dichroism (XMCD). XRD data reveal a single-phase cubic structure with a strong crystallographic orientation along the (200) plane. Room temperature ferromagnetism is confirmed through isothermal hysteresis as well as temperature dependent magnetization measurements, which clearly show the ferromagnetic Curie temperature occurring at least above 350 K. The Fe L_3,2 edge NEXAFS spectra for both Fe-doped thin films exhibit mixed valent Fe²⁺/Fe³⁺ states, whereas Ce M_5,4 edge shows the 4+ state of Ce, throughout the doping. With the increase in Fe doping, Fe²⁺ state increases and a simultaneous decrease in magnetization value is also observed. The XMCD signal of both samples reveals the ferromagnetic ordering of substituted Fe ions in the ceria matrix. Our results indicate that ferromagnetism is intrinsic to the ceria system and is not due to any secondary magnetic impurity.     

Low-temperature synthesis and magnetic properties of the mixed-valence lanthanide intercalation compounds EuxTiS2 and YbxTiS2

The lanthanide (L) intercalation compounds Eu_xTiS₂ (x=0.007,0.014, and 0.066) and Yb_xTiS₂ (x=0.060) have been synthesized by thermal deintercalation of NH₄⁺ and NH₃ from the corresponding L-NH₃ precursors at 270°C and characterized by thermogravimetric analysis, powder X-ray diffraction, and SQUID magnetometry. Both L²⁺ and L³⁺ are present in the van der Waals gap, and the deintercalation process is accompanied by the conversion of some of the L³⁺ to L²⁺. The resulting compounds contain comparable concentrations of L²⁺ and L³⁺. Magnetic exchange interactions are negligible due to the low concentrations of lanthanide cations.     

Investigation on the growth and structure of Fe-doped near-stoichiometric LiNbO3 crystal

The Fe-doped near-stoichiometric LiNbO₃ crystal has been grown from the Li-rich melt by Czochralski method. The UV-Vis absorption spectra, IR transmittance spectra and X-ray diffraction were measured and used to study the defect structure of the crystal. Compared with that of congruent Fe:LiNbO₃, in near-stoichiometric Fe:LiNbO₃, the lattice parameter reduces, the basal absorption edge shifts towards shorter wavelength, and the OH stretching vibration absorption band shifts towards the lower wavenumber side (3466 cm^–1) in the Fe-doped near-stoichiometric LiNbO₃.     

Soft XAFS study on the 4d electronic structure of ruthenium in complex perovskite oxide

The bond covalency effect on the electronic structure of the perovskite oxides, La₂MRuO₆ (M=Zn, Mg, and Li) and Ba₂YRuO₆, has been investigated by the complementary methods of X-ray absorption spectroscopy and X-ray diffraction. Ru K-edge and Ru L_III-edge XANES analyses clarify that the ruthenium ions with oxidation states of IV (d⁴) and V (d³) are stabilized in a nearly regular RuO₆ octahedron. Comparison of L-edge XANES spectra for the compounds with isovalent ruthenium ion has shown that the t_2g level is mainly influenced by the A site cation, whereas the e_g level is mainly affected by the neighboring B site cation, which can be understood on the basis of the bond competition scheme. The present Ru K-edge EXAFS analysis results reveal that the first neighbor coordination (Ru–O shell) is nearly isotropic even in monoclinic perovskites, which is consistent with the XRD and XANES results. The experimental EXAFS spectra in the range R≤∼4.5 Å are well reproduced by ab initio calculation based on crystallographic data, which supports the long-range structure presented by Rietveld refinement. The EXAFS curve-fitting analyses allow us to distinguish the differences in (Ru–O) bond lengths for isovalent ruthenates, which is supported by the XANES spectral features and the chemical interpretation of bond covalency.     

X-ray absorption spectroscopic study of the mixed valence system CePd3

Mixed valency in CePd₃ has been examined by a study of the L_III (Ce) absorption edge in this compound as well as other model compounds. In CePd₃, peaks characteristic of 3+ and 4+ states of Ce are found to be separated by 2 eV.     

X-ray absorption near edge structure investigation of vanadium-doped ZnO thin films

X-ray absorption near edge structure spectroscopy has been used to investigate the electronic and atomic structure of vanadium-doped ZnO thin films obtained by reactive plasma. The results show no sign of metallic clustering of V atoms, +4 oxidation state of V, 4-fold coordination of Zn in the films, and a secondary phase (possibly VO₂) formation at 15% V doping. O K edge spectra show V 3d-O 2p and Zn 4d-O 2p hybridization, and suggest that V⁴⁺ acts as electron donor that fills the σ* band.     

Disproportionation of nonoxygenated U(V) bound in the complex with heteropoly anions P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Stack><Subscript>61</Subscript><Superscript>10−</Superscript></Stack>

The effect of experimental conditions on U^VL₂ (1–2 mM) disproportionation was studied spectrophotometrically through the U^IVL₂ accumulation (L = P₂W₁₇O ₆₁ ^10? ). In 1 M NaNO₃ solution containing 0.01 M HAc and 0.01 M NaAc, the rate of U^VL₂ disappearance is described by the equation V = k ₁[U^VL₂]. The k ₁ value is almost constant with pH decreasing from 4.5 to 1.7, but increases with increasing acetate concentration; the presence of 1 mM U^IVL₂, U(VI), or L does not affect k ₁. In the solutions of 0.1–1.0 M HClO₄ (ionic strength 1), the reaction rate is described by the equation V = 2k ₂[H⁺]^2.5[U^VL₂]². Probable disproportionation mechanism is discussed. The first stage is substitution of L by water molecules in the U^IVL₂ complex and appearance of the reactive U(V) complex with mixed coordination sphere.     

Coulometric methods for uranium and plutonium determination

Data on coulometric methods for U and Pu determination in various nuclear materials are presented, analyzed, and summarized. Coulometric determination methods at controlled current and controlled potential are considered. The main principles of the U and Pu determination procedures based on the use of the UO₂ ²⁺/U⁴⁺, Pu⁴⁺/Pu³⁺, and PuO₂ ²⁺/Pu⁴⁺ redox couples are presented. Various methodical factors influencing the error of the U and Pu determination, such as sample weight; supporting electrolyte composition; oxidation and reduction potentials; current; electrode material, shape, and activation procedure; impurity elements, etc., are discussed. Procedures for joint coulometric determination of U and Pu are considered separately. The main methods used by the authors for eliminating the mutual effect of U and Pu in their joint determination are described.     

Investigations of Sr3Fe2−xMnxO7−δ the n=2 Ruddlesden–Popper phases with d3/d4 interactions

We have prepared a series of Mn substituted strontium ferrates, Sr₃Fe_2−xMn_xO_7−δ where x=2/3, 1 and 4/3, and investigated their properties with X-ray diffraction, X-ray absorption near edge spectroscopy (XANES), Mössbauer spectroscopy, variable-temperature resistivity and magnetic susceptibility. These compounds are metastable and have to be prepared at high temperatures, >1250°C, with solid state techniques followed by quenching in air to room temperature. As a consequence the compounds are oxygen deficient and contain some Fe³⁺. They are insulators and exhibit spin-glass like behavior at low temperatures due to frustrated magnetic interactions between the disordered array of transition metal ions on the B site. Mössbauer and XANES spectra show that the B cations are not fully oxidized and that the Fe⁴⁺ in these compounds are charge-disproportionated into Fe³⁺ and Fe⁵⁺ at low temperature.     

Effect of α-Fe2O3 addition on the morphological,optical and decolorization properties of ZnO nanostructures

Visible light sensitive photocatalysts of Fe₂O₃/ZnO nanocomposites were prepared by a simple solid-state reaction method, using zinc acetate, α-Fe₂O₃ and sodium hydroxide at room temperature. The products were characterized by scanning electron microscopy, powder X-ray diffraction, N₂ adsorption–desorption measurement, UV–vis absorption, and photoluminescence spectroscopy and used for photodecolorization of Congo red. The characterization results showed that the morphology, crystallite size, BET surface area and optical absorption of the samples varied significantly with the Fe³⁺ to Zn²⁺ ratios. The nanocomposites show two absorption edges at ultraviolet and visible region. The optical band gap values of these nanocomposites were calculated to be about 3.98–3.81 eV and 2.88–2.98 eV, which show a red shift from that of pure ZnO. These red shifts are related to the formation of Fe s-levels below the conductive band edge of ZnO and effectively extend the absorption edge into the visible region. The growth mechanisms of the samples are proposed. These nanocomposites showed high decolorization ability in visible light with wavelength up to about 400 nm. Among the samples, Fe₂O₃/ZnO nanoflower (molar ratio of Fe³⁺ to Zn²⁺ is 1:100) exhibited higher decolorization efficiency than the other nanocomposites. It could be considered as a promising photocatalyst for dyes treatment.     

Effect of carbon impurity in irradiated mixed uranium–plutonium nitride on the phase composition upon β<Superscript>−</Superscript>-decay of fission products

As shown by thermodynamic modeling, accumulation of fission products in the course of irradiation of mixed uranium–plutonium nitride containing 0.25 wt % C impurity leads to the formation of a multicomponent carbonitride solid solution containing U, Pu, Am, Np, Zr, Y, lanthanides, and also separate carbide (UMoC₂, U₂RuC₂, URu₃C_0,7, BaC₂, SrC₂) and nitride (U₂N₃) phases and intermetallic compounds U(Ru, Rh, Pd)₃. The β^?-decay of metallic radionuclides in separate carbide phases of spent nuclear fuel leads to changes in their chemical compositions. The transformation kinetics of separate dicarbide (⁸⁹SrC₂ → ⁸⁹YC₂, ⁹⁰SrC₂ → ⁹⁰YC₂ → ⁹⁰ZrC + C, ¹⁴⁰BaC₂ → ¹⁴⁰LaC₂ → ¹⁴⁰CeC₂) and complex carbide (U⁹⁹MoC₂ → UC₂ + ⁹⁹Tc) phases was calculated.     

An X-ray photoelectron study of the interaction of uranyl group with sulfosuccinyl chitosan

Interaction of uranium with low-molecular-weight sulfosuccinyl chitosan (SSLMWC) from sulfate solutions was studied by X-ray photoelectron spectroscopy. The interaction involves formation of complexes with the uranyl group on the SSLMWC surface. The equatorial plane of these complexes can contain nitrogen atom of the amino group and oxygen atoms of the sulfato and sulfo groups SO₄²⁻ and -SO₃H, carboxy groups, and free hydroxy groups. On keeping the complex in a vacuum, two chemically nonequivalent states of U ions, U⁴⁺ and UO₂²⁺, are observed. The U⁴⁺ formation in solid U-containing samples of chitosan and SSLMWC is attributed to the radical reaction of the UO₂²⁺ reduction with the primary hydroxy group of chitosan in a vacuum of the spectrometer under the action of X-ray radiation.