XANES Spectra (XANE + spectrum)

Distribution by Scientific Domains

Selected Abstracts

Relationship between the structural distortion and the Mn electronic state in La1,xCaxMnO3: a Mn K -edge XANES study

Jesús Chaboy
A theoretical study of the X-ray absorption near-edge structure (XANES) spectra at the Mn K -edge in the La1,xCaxMnO3 series is reported. The relationship between the edge shift, the Ca,La substitution and the distortion of the MnO6 octahedra in these systems has been studied. It is shown that, by correctly considering these effects simultaneously, the experimental XANES data are consistent with the presence of two different Mn local environments in the intermediate La1,xCaxMnO3 compounds. By taking into account the energy shift associated with the modification of the MnO6 distortion as Ca substitutes for La, it is possible to reproduce the XANES spectra of the intermediate-doped compounds starting from the experimental spectra of the end-members LaMnO3 and CaMnO3. These results point out the need to re-examine the conclusions derived in the past from the simple analysis of the Mn K -edge XANES edge-shift in these materials. In particular, it is shown that the modification of the Mn K -edge absorption through the La1,xCaxMnO3 series is well reproduced by considering the simultaneous presence of both distorted and undistorted octahedra and, consequently, that the existence of charge-ordering phenomena cannot be ruled out from the XANES data. [source]

Calcium L -edge XANES study of some calcium compounds

S. J. Naftel
The Ca L3,2 -edge XANES spectra of six calcium salts have been measured in both total electron and fluorescence yields using a high-resolution spherical grating monochromator. The compounds investigated were; CaF2, CaCO3, CaCl2,2H2O, calcium phosphate, calcium glycerophosphate and calcium gluconate. We find that the fine structure of the Ca L3,2 -edges for each compound is unique and relates to the local structure of the Ca atom. The implications of these results to the study of the structure of calcium binding sites in systems of biologically interest will be discussed. [source]

4d Electronic structure analysis of ruthenium in the perovskite oxides by Ru K - and L -edge XAS

Jong-Young Kim
The 4d electronic structure of ruthenium in the perovskite oxides, La2MRuIVO6 (M = Zn, Mg, and Li) and Ba2YRuVO6, has been investigated by the Ru K-and L-edge XANES and EXAFS analyses. Such X-ray absorption spectroscopic results clarify that the RuIV (d4) and RuV (d3) ions are stabilized in nearly regular Oh site. Comparing the Ru L-edge XANES spectra of perovskites containing isovalent ruthenium, it has been found that the t2g state is mainly influenced by A site cation, whereas the eg is mainly affected by neighboring B site cation. The experimental EXAFS spectra in the range of 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. [source]

Piezo-QEXAFS with fluorescence detection: fast time-resolved investigations of dilute specimens

Dirk Lützenkirchen-Hecht
First X-ray absorption spectroscopy experiments with a vibrating piezo-driven double-crystal monochromator (piezo-QEXAFS) and fluorescence detection are reported. It is shown that high-quality XANES spectra can be recorded on a time scale of about 50,ms per spectrum, even for very low concentrations of <10,mmol,l,1 using fluorescence detection. The quality of the spectra, possible applications, as well as present limits of the technique will be discussed. [source]

Chemical Speciation of Trace Zinc in Ordinary Portland Cement Using X-ray Absorption Fine Structure Analysis

Isao Tsuyumoto
Chemical change of trace zinc in ordinary portland cement (205.1 ppm) was investigated in hydration process using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Intensities of the peaks appearing at the same energy of ZnO in XANES spectra were decreased with cement hydration. The interatomic distances and the coordination numbers of the first and the second shells calculated from EXAFS spectra indicated that ZnO hydrolyzed to zincate ion [Zn(OH)4]2, with cement hydration keeping their fundamental structure of ZnO4 tetrahedra. [source]

Organic matter from comet 81P/Wild 2, IDPs, and carbonaceous meteorites; similarities and differences

S. Wirick
Sections were analyzed using a scanning transmission X-ray microscope (SXTM) and carbon X-ray absorption near edge structure (XANES) spectra were collected. We compared the carbon XANES spectra of these Wild 2 samples with a database of spectra on thirty-four interplanetary dust particles (IDPs) and with several meteorites. Two of the particles analyzed are iron sulfides and there is evidence that an aliphatic compound associated with these particles can survive high temperatures. An iron sulfide from an IDP demonstrates the same phenomenon. Another, mostly carbon free containing particle radiation damaged, something we have not observed in any IDPs we have analyzed or any indigenous organic matter from the carbonaceous meteorites, Tagish Lake, Orgueil, Bells and Murchison. The carbonaceous material associated with this particle showed no mass loss during the initial analysis but chemically changed over a period of two months. The carbon XANES spectra of the other four particles varied more than spectra from IDPs and indigenous organic matter from meteorites. Comparison of the carbon XANES spectra from these particles with 1. the carbon XANES spectra from thirty-four IDPs (<15 micron in size) and 2. the carbon XANES spectra from carbonaceous material from the Tagish Lake, Orgueil, Bells, and Murchison meteorites show that 81P/Wild 2 carbon XANES spectra are more similar to IDP carbon XANES spectra then to the carbon XANES spectra of meteorites. [source]

Iron oxidation state in impact glass from the K/T boundary at Beloc, Haiti, by high-resolution XANES spectroscopy

Gabriele Giuli
The samples have been analyzed by Fe K-edge high-resolution X-ray absorption near edge structure (XANES) spectroscopy to obtain data on both the Fe oxidation state and the coordination number. The pre-edge peak of our high-resolution XANES spectra display noticeable variations indicative of significant changes in the Fe oxidation state spanning a wide range from about 20 to 75 mol% trivalent Fe. All data plot along the same trend, falling between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of four- and five-coordinated Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra can be interpreted as a mixture of [4]Fe2+, [5]Fe2+, [4]Fe3+, and [5]Fe3+. There is no evidence for six-fold coordinated Fe; however, its presence in small amounts cannot be excluded from XANES data alone. Our observations can be explained by two possible scenarios: either these impact glasses formed under very reducing conditions and, because of their small size, were easily oxidized in air while still molten, or they formed under a variety of different oxygen fugacities resulting in different Fe oxidation states. In the first case, the oxidation state and coordination number would imply similar formation conditions as splash-form tektites, followed by progressive oxidation. [source]

Impact of Annealing on the Conductivity of Amorphous Carbon Films Incorporating Copper and Gold Nanoparticles Deposited by Pulsed Dual Cathodic Arc

Jose Luis Endrino
Abstract The influence of annealing in argon at 300,°C on the conductivity, phase stability and electronic structure of hydrogen-free amorphous carbon (a-C) films containing copper (a-C:Cu) and gold (a-C:Au) nanoclusters was investigated. The motivation of this work is twofold: (1) to study the thermal stability of a-C:Cu and a-C:Au films and (2) to point out the relevance of X-ray absorption near edge structure (XANES) technique to study the structural evolution of metal-doped a-C nanocomposites. The films were produced at room temperature using a selective-bias pulsed dual-cathode arc deposition technique. Compositional analysis was performed with secondary neutral mass spectroscopy whereas grazing incidence X-ray diffraction (GIXRD) was used to monitor phase transformation and identify the dispersion or agglomeration of the crystallites within the carbon matrix. XANES spectra at the C-K was used to investigate the effect of annealing in argon on the electronic structure of the a-C matrix, while Cu K and Au L-edges were investigated on a-C:Cu and a-C:Au samples, respectively, to study the nanocluster evolution. XANES showed that the a-C host matrix increased its graphitic character and that stress was relieved upon annealing. No relevant changes were observed in the Au arrangements in a-C:Au films. In the case of the a-C:Cu samples, the Cu-K XANES spectra indicated the formation of Cu2O crystals which correlated well with GIXRD spectra and the decrease in conductivity. [source]

The Valence Problem of Pd4Br4Te3

Manuel Janetzky
Abstract Pd4Br4Te3 was prepared from Pd, Te, and PdBr2 at 700,K. Its structure was determined by single-crystal X-ray diffraction to be triclinic, P, Pearson symbol aP22; a=842.5(2), b=845.0(3), c=864.8(3),pm; ,=82.55(3), ,=73.36(2), ,=88.80(2)°; Z=2. The Br and Te atoms are arranged according to the motif of cubic closest-packed spheres in which every 15th position is vacant; the Pd atoms occupy 8/15 of the octahedral voids. The symmetry relations with the packing of spheres are derived. Prominent structural units are hollow cuboctahedral [(PdBrTe)6] units, the Pd atoms are positioned near the centers of the square faces of the Br6Te6 cuboctahedra; the cuboctahedra and double-octahedral Pd2Br4Te6 units are connected to strands by sharing triangular Te3 faces. The strands are condensed by common Br atoms into layered assemblies. Conspicuously close TeTe contacts in the Te3 triangles indicate attractive TeTe interactions. The valence puzzle is resolved by the formula Pd(+II)4Br(,I)4Te(,4/3)3. Positive TeTe Mulliken orbital populations and the PdK, BrK, and TeLIII XANES spectra of Pd4Br4Te3 referenced to the spectra of PdBr2, K2PdBr6, PdTe, and PdTe2 are in accord with attractive TeTe interactions. The measured semiconducting and diamagnetic properties are compatible with the derived picture of chemical bonding in Pd4Br4Te3. Pd4Br4Te3 wurde aus Pd, Te und PdBr2 bei 700,K hergestellt. Nach der Strukturanalyse mittels Einkristall-Röntgenbeugung kristallisiert es triklin, P, Pearson-Symbol aP22; a=842,5(2), b=845,0(3), c=864,8(3),pm; ,=82,55(3), ,=73,36(2), ,=88,80(2)°; Z=2. Die Br- und Te-Atome sind nach dem Muster einer kubisch-dichtesten Kugelpackung angeordnet, in der jede 15-te Position unbesetzt ist; die Pd-Atome besetzen 8/15 der Oktaderlücken. Die Symmetriebeziehungen zur Kugelpackung werden hergeleitet. Im Zentrum unbesetzte, kuboktaedrische [(PdBrTe)6]-Baugruppen sind mit Pd2Br4Te6 -Doppeloktaedern über gemeinsame dreieckige Te3 -Flächen zu Strängen verknüpft, die ihrerseits über gemeinsame Br-Atome zu Schichten angeordnet sind. Auffällig kurze TeTe-Kontakte in den Te3 -Dreiecken deuten auf attraktive TeTe-Wechselwirkungen hin. Die zunächst rätselhafte Valenzsituation wird durch die Formulierung Pd(+II)4Br(,I)4Te(,4/3)3 gedeutet. Positive TeTe-Mulliken-Orbital-Populationen und die PdK -, BrK - und TeLIII -XANES-Spektren von Pd4Br4Te3 im Vergleich zu den Spektren von PdBr2, K2PdBr6, PdTe und PdTe2 bestätigen die attraktiven TeTe-Wechselwirkungen. Die gemessenen halbleitenden und diamagnetischen Eigenschaften von Pd4Br4Te3 passen zu dem abgeleiteten Bindungsmodell. [source]