Analysis of the Ni K-edge X-ray absorption near edge structure in Ni((C2H5O)2PS2)2

The Ni((C₂H₅O)₂PS₂)₂ complex has been investigated using the X-ray absorption near edge structure (XANES) analysis. Nickel K-edge XANES spectra of the complex have been measured and theoretical calculations of the spectra have been carried out using the finite difference method; the calculation of molecular potential has been carried out both in the full potential and in the muffin-tin approximation. The analysis of results obtained has shown that a good agreement between theoretical and experimental spectra in the low-energy region is achieved only in the case of full potential calculations (beyond the muffin-tin approximation for the potential shape).     

Nearest surroundings of aluminum atoms and features of the electron structure of Al90FexCe10−x alloys (x=3, 5, 7)

The spectra of x-ray absorption near-edge structure (XANES) were measured at the K-edge of aluminum in Al₉₀Fe_xCe_{10 ?x} alloys (x=3, 5, 7). A theoretical XANES analysis at the Al K-edge was performed for Al₉₀Fe₅Ce₅ by using the multiple scattering algorithm. A comparison of the experimental and theoretical results allowed establishment of the most probable model of local surroundings of aluminum atoms in the alloy system studied. The density of free Al and Fe electron states in the conduction band was determined. It was found that the interaction between Al p states and Fe d states leads to pushing of the former states by the latter ones.     

Antiferromagnetic stripe ordering at the tetragonal-orthorhombic transition in LnOFeAs probed by X-ray absorption spectroscopy

Measurements of the Fe K-edge X-ray absorption near edge (XANES) spectra of LnOFeAs high T_c superconductors exhibit small but measurable changes across the tetragonal to orthorhombic phase structural phase transition. Calculations, based on the local structure centered at the Fe site obtained by crystallographic investigations, reproduce the observed changes in the spectra under the change of the crystal structure, indicating variations in the Fe local unoccupied electronic states. Calculated Fe density of states including stripe spin ordering in the orthorhombic phase show significant modifications with respect to Fe 3d density of states in the tetragonal phase or the density of states in the orthorhombic phase calculated ignoring spin ordering. This implies that the striped spin ordering in the orthorhombic phase is necessary to describe its electronic properties not only in the ground state but also for excited states above the Fermi energy.     

Theoretical and experimental study of the electronic structure of tin dioxide

The electronic structure of tin dioxide has been theoretically studied within the linearized augmented plane wave method using the Wien2k program package. The total and local partial electron densities of states have been calculated. The X-ray emission K-spectrum of oxygen has been calculated. The X-ray absorption spectra of the tin M _4,5-edge and oxygen K-edge have been calculated by simulating the supercell and core hole. The calculated results have been compared with the experimental data obtained using synchrotron radiation.     

Crystal structure and electronic states of Co and Gd ions in a Gd<Subscript>0.4</Subscript>Sr<Subscript>0.6</Subscript>CoO<Subscript>2.85</Subscript> single crystal

X-ray diffraction and X-ray absorption near edge structure (XANES) spectra have been measured at the Со K-edge and Gd L ₃-edge in GdCoO₃ and Gd_0.4Sr_0.6CoO_2.85 cobaltites. The effect of Sr substitution on the crystal structure and electronic and magnetic states of Co³⁺ ions in a Gd_0.4Sr_0.6CoO_2.85 single crystal has been analyzed. The XANES measurements at the Co K-edge have not showed a noticeable shift of the absorption edge with an increase in the concentration of Sr. This indicates that the effective valence of cobalt does not change. An increase in the intensity of absorption at the Gd L ₃-edge is due to an increase in the degree of hybridization of the Gd(5d) and O(2p) states. The effect of hole doping on the magnetic properties results in the appearance of the ferromagnetic component and in a significant increase in the magnetic moment.     

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.     

Effect of local atomic phase separation in the x-ray absorption near edge structure spectroscopy of FeSexTe1−x

Ab initio X-ray absorption near edge structure (XANES) calculations for FeSe_xTe_1−x, using a structural model that combines FeSe and FeTe phases at the nanoscale, are compared with Fe K-edge XANES measurements in the “pre-edge” region. The important aspects of this model are (i) magnetic order in the FeTe phase; (ii) Se and Te atoms placed randomly in both FeSe and FeTe crystallographic positions and; (iii) the two distinct distances for Fe–Se and Fe–Te of the bulk phases. The calculated spectra reproduce the observed increase of spectral weight in the experiments on FeSe_xTe_1−x with Se concentration. This is consistent with an inhomogeneous local electronic structure of FeSe_xTe_1−x. Additionally, we have calculated projected electronic density of d-states for the Fe atom, revealing increased spectral weight in the “pre-edge” region of the XANES spectra, which correlates with the increase in Se concentration. The decrease of calculated Fe d-density of states for the Fermi level, N(ε_F), for high Te content is consistent with the suppression of superconductivity in the title system.     

X-ray absorption near edge structures (XANES) in simple and complex Mn compounds

The X-ray Absorption Near Edge Structures (XANES) of Mn compounds at the Mn K-edge have been measured with high resolution at the Frascati Synchrotron Radiation Facility “PULS”. Molecular effects, such as “shape resonances”, have been identified in molecular complexes. Experimental evidence of solid state (or long-range order) effects have been found in spectra of simple oxides.The application of XANES for determining the local structure and chemical bonding of Mn ion in unknown compounds is discussed.     

Characterization of yellow and colorless decorative glasses from the Temple of the Emerald Buddha, Bangkok, Thailand

Yellow and colorless ancient glasses, which were once used to decorate the Temple of the Emerald Buddha, Bangkok, Thailand, around 150 years ago, are studied to unravel the long-lost glass-making recipes and manufacturing techniques. Analyses of chemical compositions, using synchrotron x-ray fluorescence (SRXRF), indicate that the Thai ancient glasses are soda lime silica glasses (60 % SiO₂; 10 % Na₂O; 10 % CaO) bearing lead oxide between 2–16 %. Iron (1.5–9.4 % Fe₂O₃) and manganese (1.7 % MnO) are present in larger abundance than the other 3d transition metals detected (0.04–0.2 %). K-edge x-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure spectroscopy (EXAFS) provide conclusive evidence on the oxidation states of Fe being 3+ and Mn being 2+ and on short-length tetrahedral structures around the cations. This suggests that iron is used as a yellow colorant with manganese as a decolorant. L ₃-edge XANES results reveal the oxidation states of lead as 2+. The results from this work provide information crucial for replicating these decorative glasses for the future restoration of the Temple of the Emerald Buddha.     

X-rayK-absorption edge interpretation and energy determination

The low energy features of XANES (X-ray absorption near edge structure) in the X-rayK-absorption edge of transition metal systems have long been detected and attempts have been made to explain their origin. Only recently have some theoretical approaches based upon multiple scattering (MS) effects been applied in explaining the shape of theK-edge features. But the pre-edge shoulder, often labelled asB, has hitherto defied satisfactory explanation for its origin. The present work is an attempt in explaining the possible origin of this feature by using MS-theory to theK-edge XANES of some first row transition metal systems. The calculated energies using MS-formalism were found to be closer to the experimentally observed values for some systems of cobalt and copper. Using these results, theK-edge featuresA,B,C have been assigned to be originating from the resonance effects of scattering of the ejected photo-electron with the molecular potential set up by the central absorbing atom and its near neighbours.     

On the shape of iron K absorption edges for monoferrites with a Me(Mg, Mn, Ni, Zn)Fe2O4 spinel structure

Calculations of the K-edge x-ray absorption near-edge structure (XANES) in a pure metal and in monoferrites of stoichiometric compositions MgFe₂O₄, MnFe₂O₄, NiFe₂O₄, and ZnFe₂O₄ are carried out using a FEFF8 program. It was confirmed that the Fe K-edge energy shift found experimentally occurs upon the transition from pure iron to monoferrites. It is demonstrated that this shift is identically directed for ferrites with a structure of normal (MnFe₂O₄, ZnFe₂O₄) and inverted (MgFe₂O₄, NiFe₂O₄) spinels, but numerical values of theoretical Fe K-edge shifts agree well with experimental data only for normal spinels.     

X-ray detected magnetic resonance at the Fe K-edge in YIG: Forced precession of magnetically polarized orbital components

X-ray detected magnetic resonance (XDMR) has been measured for the first time on exciting the Fe K-edge in a high-quality yttrium iron garnet film epitaxially grown on a gadolinium gallium garnet substrate. This challenging experiment required resonant pumping of yttrium iron garnet at high microwave power, i.e., in the foldover regime. X-ray magnetic circular dichroism (XMCD) was used to probe the change in the longitudinal component of the magnetization M _Z induced by the precession of magnetic moments located at the iron sites. Since XMCD at the Fe K-edge refers mostly to the equilibrium contribution of magnetically polarized 4p orbital components, XDMR at the Fe K-edge should reflect the precessional dynamics of the latter orbital moments. From the measured precession angle, we show that there is no dynamical quenching of the polarized orbital components at the iron sites in yttrium iron garnet.     

Fluorescent X-ray transport in microchannel plates

The X-ray absorption spectra of microchannel plates have been measured at grazing incidence of the primary radiation. The Si L components of X-ray absorption near-edge structure (XANES) spectra exhibit specific features for incident angles below a certain critical value. In order to explain the experimental data, a mechanism of selective X-ray fluorescence transport via microchannels is proposed, which is based on the notion of anomalous dispersion in the vicinity of the Si L absorption edge.     

Electronic structure of ErS and PrS: Analysis of the sulfur K-edge XANES spectra

K-edge XANES spectra of sulfur in ErS and PrS have been measured. A detailed analysis of the electronic structure of ErS and PrS was carried out by the full-scale multiple scattering method, and a good agreement with experimental spectra was obtained. The fine structure of unoccupied sulfur states in the conduction band was determined. The unoccupied d states of praseodymium in PrS are shown to interact with the sulfur p states to expel the sulfur delocalized states from the energy region where the states of the rare-earth elements are localized.     

Structural characterizations of sol–gel synthesized TiO2 and Ce/TiO2 nanostructures

Mixed phase TiO₂ and Ce/TiO₂ samples were synthesized by a sol–gel method using different hydrolysis conditions. In pure TiO₂ samples, traditional X-ray diffraction (XRD) and Ti K-edge synchrotron X-ray absorption near edge structures (XANES) independently revealed their anatase/rutile phase ratios. XANES results further revealed a substantial amount of Ti atoms existed in other forms beside anatase and rutile TiO₂ in the sample synthesized by the low hydrolysis condition. An increase in the extent of the hydrolysis during the synthesis leads to an increased rutile ratio and a reduction in other forms. In Ce/TiO₂ samples, the crystal sizes were too small for XRD characterization. Only XANES could be used to characterize their phase ratios. It is found that adding Ce impedes rutile formation; leading to increased anatase ratio. The difference in the fundamental aspects of XRD and XANES techniques in providing the phase ratios is discussed.     

K‐edge XANES analysis of sulfur compounds: an investigation of the relative intensities using internal calibration

Sulfur K‐edge XANES (X‐ray absorption near‐edge structure) spectroscopy is an excellent tool for determining the speciation of sulfur compounds in complex matrices. This paper presents a method to quantitatively determine the kinds of sulfur species in natural samples using internally calibrated reference spectra of model compounds. Owing to significant self‐absorption of formed fluorescence radiation in the sample itself the fluorescence signal displays a non‐linear correlation with the sulfur content over a wide concentration range. Self‐absorption is also a problem at low total absorption of the sample when the sulfur compounds are present as particles. The post‐edge intensity patterns of the sulfur K‐edge XANES spectra vary with the type of sulfur compound, with reducing sulfur compounds often having a higher post‐edge intensity than the oxidized forms. In dilute solutions (less than 0.3–0.5%) it is possible to use sulfur K‐edge XANES reference data for quantitative analysis of the contribution from different species. The results show that it is essential to use an internal calibration system when performing quantitative XANES analysis. Preparation of unknown samples must take both the total absorption and possible presence of self‐absorbing particles into consideration.     

Titanium K-edge absorption structure in Ti1−x NbxO2

The fine structure in the titanium x-ray K-edge absorption has been measured in Ti_1−x Nb_xO₂ mixed dioxides (x=0–0.1) with rutile structure in a laboratory-type spectrometer by total electron quantum-yield measurement. The position of the XANES lines is shown to be in good agreement with classical x-ray absorption spectra obtained in transmission. The structure and main features of the XANES spectra, including the effects of impurities and manyelectron excitations, are discussed. It is suggested that the intensity of the B peak characteristic of the titanium K edge depends on the Nb concentration and correlates with the charge state of titanium ions. Fiz. Tverd. Tela (St. Petersburg) 41, 894–896 (May 1999)     

Structural study of Co/Pd(1 1 1) and CO/Co/Pd(1 1 1) by surface X-ray absorption fine structure spectroscopy

The structure of the Co thin films on Pd(1 1 1) and the effect of the CO adsorption on Co thin films were studied by Co K-edge surface X-ray absorption fine structure (XAFS). The polarization dependences of the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra indicate that Co thin films grow in the fcc stacking mode on Pd(1 1 1) up to 12 ML. The analysis of the nearest neighbor shell shows little mechanical strain at the interface, indicating that Co atom does not grow pseudomorphically on Pd(1 1 1). There is no alloy-like structure at the interface. CO adsorption causes no structural change of the Co thin films but modifies the Co surface electronic state. These structural studies provide deep insight in the magnetic property of the Co thin films on Pd(1 1 1).     

A XANES study of the structural role of lead in glazes from decorated tiles, XVI to XVIII century manufacture

Aged lead-rich, tin-opacified glazes from polychrome tiles manufactured in the 16th–18th century were studied to ascertain the structural role of lead. Glaze fragments with white, blue, yellow, brown and green colouring were analysed using non-destructive X-ray techniques, both laboratorial – X-ray diffraction to identify crystalline components – and synchrotron-based. Elemental analyses by synchrotron radiation X-ray fluorescence were performed at the former LURE photon microprobe (line D15A at DCI, in Orsay). The instrumental set-up of beamline BM29 at the ESRF, in Grenoble, was applied to collect X-ray absorption spectra at the Pb L₃-edge. Natural minerals and synthetics with known crystal structure were used as model oxy-compounds to configure different formal valences and coordinations of lead ions by oxygen anions, and to interpret the effects upon details of X-ray absorption near-edge spectroscopy (XANES) spectra. Experimental evidence supports the general conclusion that lead is hosted by the glassy matrix, irrespective of the glaze colour. Furthermore, it was concluded that lead ions assume coordinations higher than usual for silica glasses, acting as network modifiers in the silica-lime-alkali glasses of ancient tile glazes. PACS 61.43.Fs; 41.60.Ap; 61.10.Ht     

A XANES study of chromophores in archaeological glass

We applied X-ray absorption near edge spectroscopy (XANES) to obtain information on the origin of glass colour of several archaeological samples and on the oxidation conditions employed during their production. We studied a series of selected glass fragments—mainly from excavated primary and secondary production centres and dated to the first millennium AD—containing iron and manganese in a wide compositional range. In most of the studied samples iron is rather oxidised, while Mn K-edge XANES data show that, in all the studied glasses, Mn is mainly present in its reduced form (predominantly 2+), with the possible subordinate presence of Mn³⁺. The most oxidised samples are the HIMT (high iron manganese titanium) glasses, while the less oxidised ones belong to the primary natron glass series from the early Islamic tank furnaces at Bet Eliezer (Israel), and to the series coming from a Roman glass workshop excavated in Basinghall Street, London. In these glasses, iron is approximately equally distributed over the 2+ and 3+ oxidation states. The XANES analyses of two glasses which had been deliberately decolourized using Sb- and Mn-based decolourizers demonstrate that Sb is more effective than Mn as oxidant.