Extended X-ray Absorption Fine Structure Results Research Articles

Extended X-ray absorption fine structure of interfaces of layered structures

This research is concerned with characterizing impurity level elements in interface conditions. Impurity elements such as gallium and arsenic, when in ppm to less than 1.0 wt.% concentrations are not easily investigated without exposing the interface as a free surface. With extended X-ray absorption fine structure (EXAFS) spectroscopy, one can characterize the impurity level elements as to their local structure in the in situ condition. Samples of alternating layers of AlGa and AlAs were made using a molecular beam epitaxy system where gallium and arsenic layers were half monolayer thickness. The gallium in the AlGa samples was characterized to be in solution in the aluminum. The arsenic was well ordered with a near neighbor AlAs structure. The impurity interfaces were identified to be locally diffuse. The EXAFS results were complemented by Auger electron spectroscopy analysis.

Study of metglas 2605 CO by extended and near edge X-ray absorption fine structure

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) studies of the transition metal sites in Metglas 2605 CO (Fe 67Co 18B 14Si 1) illustrate differences in the two sites. The average transition metal distance, obtained by standard EXAFS analysis, is larger for iron than for cobalt sites in this material. The XANES for Co and Fe are nearly identical except for a shift of the Co fine structure to high energy relative to that of Fe; the sign and magnitude of this shift is commensurate with the EXAFS results. Finally, there was no detected polarization dependence of the EXAFS, indicating that the structural manifestations of the magnetic anisotropy in this material are smaller than the detection limits of these experiments.

Supported metals and supported organometallics

Organometallic chemistry and heterogeneous catalysis are developing into common territory, illustrated by catalysts incorporating structures ranging from mononuclear metal complexes to metal aggregates (crystallites) on metal oxide supports. Reactions of precursors such as metal carbonyls and metal allyls with metal oxide surfaces have been understood on the basis of solution organometallic chemistry combined with functional group chemistry of the supports. The supported species are transformed by processes including reductive carbonylation (whereby metal complexes and salts are converted into metal clusters) and oxidative fragmentation (whereby metal clusters and aggregates are converted into mononuclear metal complexes). Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used in conjunction with other techniques to characterize bonds between mononuclear metal complexes and metal oxide surfaces and also to provide structural information about metal-support interfaces in highly dispersed supported metal aggregates. EXAFS results are also helping to define the limitations of standard methods for estimating surface areas of highly dispersed metals by measurements of chemisorption of H 2 and CO. Some supported ‘molecular’ organometallics are structurally and catalytically novel, indicating possibilities for design of surface catalytic sites. Ensembles consisting of three Re complexes on MgO are active for the structure-sensitive cyclopropane hydrogenolysis, whereas a catalyst consisting of the same complexes (evidently isolated on the support surface) is inactive. ‘Molecular’ osmium carbonyl clusters stabilized on the basic MgO surface and in basic zeolites catalyze CO hydrogenation, and 10-atom aggregates of osmium, formed by removal of CO ligands from a ‘molecular’ precursor, are active for n-butane hydrogenolysis. These structurally well-defined catalysts offer good opportunities for understanding metal-support interactions and structure sensitivity in metal catalysis.

Lattice vibrational studies of superconducting YBa2Cu3O7 by polarized extended x-ray-absorption fine-structure measurements.

We report polarization-dependent extended x-ray-absorption fine-structure (EXAFS) measurements of the superconducting Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. Measurements made at the Cu and Y $K$ edges with the x-ray polarization vector oriented perpendicular and parallel to the $c$ axis have enabled us to probe scattering atoms in the horizontal layer plane as well as in the vertical layer plane. Three different Cu-O vibrational modes which are associated with the Cu(1)-O pair and the Cu(2)-O pair along the $c$ axis and the planar Cu-O pair are distinguished. The lowest vibrational frequency of the Cu(2)-O pair along the $c$ axis indicates that its bond strength is much weaker than those of other Cu-O pairs. No anomalies are observed in the temperature dependence of the mean-square relative displacements ${\ensuremath{\sigma}}^{2}(T)$ for Cu-O and $\mathrm{Y}\ensuremath{-}X$ ($X=\mathrm{O}, \mathrm{Cu}, \mathrm{Ba}, \mathrm{and} \mathrm{Y}$) pairs. There appears to be no significant softening of any phonons which modulate Cu-O bond lengths due to coupling with electrons, thus casting some doubt on the importance of phonons for superconductivity in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The relation between EXAFS results and other vibrational investigations is presented. The discrepancy between current data and other EXAFS reports is also discussed. Furthermore, a careful examination of orientation-dependent Y spectra has ruled out the possibility of extensive Cu and Y antisite disorder.

Evidence for microcrystallites in icosahedral AlMnSi

Abstract An analysis of extended X-ray absorption fine structure (EXAFS) measurements at the Mn K-edge of the α-crystalline and icosahedral (i) phases of MnAlSi indicates that the icosahedral cage of Mn atoms that is part of the common structural unit in both phases retains its cubic distortion in the i-phase. Neither a quasicrystalline nor a randomly connected icosahedron (icosahedral glass) model is in agreement with this experimental result. A model that is consistent with the new EXAFS results and other properties of the i-phase of MnAlSi is one in which ∼40 Å crystallites of the α-phase are oriented icosahedrally along the threefold directions of the structural unit.

Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) studies of Ag–Cu and Au–Cu clusters

Extended x-ray absorption fine structure (EXAFS) studies were conducted on silica supported silver–copper and gold–copper bimetallic clusters to obtain information on their structures. The atomic ratio of copper to either silver or gold was close to one in the materials investigated. The EXAFS results, which were obtained in the presence of hydrogen, indicate extensive segregation of the components in both the silver–copper and gold–copper clusters, although it is much more pronounced in the former. The greater segregation in the silver–copper clusters is readily understandable, since silver and copper are only slightly miscible in the bulk, whereas gold and copper are completely miscible. The EXAFS results on the silver–copper clusters suggest that the copper-rich region is in the interior of the clusters, with the silver concentrating at the surface. The location of the gold-rich region in the gold–copper clusters is less clear from the EXAFS data, but there is a slight indication that it is present at the surface. However, for air exposed gold–copper clusters, x-ray diffraction data suggest that the gold-rich region is in the interior.

Model of the local structure of random ternary alloys: Experiment versus theory.

We have performed an extended x-ray-absorption fine-structure (EXAFS) measurement of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te solid solutions for various concentrations x in the single-phase range 0\ensuremath{\le}x\ensuremath{\le}0.7. Data have been collected on the Mn K, Cd ${L}_{\mathrm{III}}$, and Te ${L}_{\mathrm{III}}$ edges. We have found well-defined different nearest-neighbor Cd-Te and Mn-Te distances almost independent of x. A model of the microscopic structure of the zinc-blende-type ${A}_{1\mathrm{\ensuremath{-}}x}$${B}_{x}$C ternary alloys based on a random distribution of cations has been developed. The model describes the bimodal distribution of near-neighbor distances in terms of distortion of the anion sublattice (the cation sublattice is assumed to remain fixed) with use only of the lattice constants of the alloy and the bond-stretching constants of each binary component. Its application to ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te and ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ga}}_{\mathrm{x}}$As alloys is proved to be in good agreement with the EXAFS results. Within the framework of this model we also consider the problem of the structural stability of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te.

Time-Resolved X-Ray Absorption of An Amorphous Si Foil During Pulsed Laser Irradiation

ABSTRACTWe report time-resolved X-ray absorption and extended X-ray absorption fine structure (EXAFS) measurements on amorphous silicon under nanosecond pulsed-laser irradiation. Each measurement was performed with one laser shot in the X-ray energy range from 90 to 300 eV. An X-ray absorption spectrum for induced liquid Si (liq*Si) was first observed above an energy density of 0.17 J/cm2. It differs significantly from the spectrum for amorphous Si and characteristically shows the disappearance of the Si-L(II,III) edge structure at around 100 eV. This phenomenon is interpreted in terms of a significant reduction in the 3s-like character of the unfilled part of the conduction band of liq*Si compared to that of amorphous Si. This is the first direct evidence that liq*Si has a metallic-like electronic structure. Timeresolved EXAFS results are also discussed briefly.

A proposal for the metal geometry in yeast superoxide dismutase based on results from EXAFS spectroscopy

Extended x-ray absorption fine structure (EXAFS) spectra have been recorded at the Cu edge and Zn edge in native yeast superoxide dismutase and at the Cu edge and Cd edge in the yeast superoxide dismutase derivative, where Zn has been substituted with Cd. Two different metal ligand distances in the range 1.9-2.0 A and 2.3-2.4 are determined for the Cu and Zn metals. For Cd in the Zn site two different metal ligand distances about 2.2 A and 2.6 A, respectively, were found. The striking feature is the similarity between the amplitude and radii determined for both the Cu and Zn sites. The increased distances for Cd can be explained by the increased ionic radius of Cd relative to Cu and Zn. Based on these EXAFS results and other relevant knowledge about the metal geometries, we propose that histidine 61 (63) positioned between the Cu and Zn metals are in one subunit bound to Zn and in the other to Cu. This model explains the recently observed difference between the two metal sites in each subunit.

EXAFS study of zeolite-supported Rh catalysts for methanol carbonylation

In order to determine the structural basis for the activity of transition metals exchanged into zeolite frameworks, a series of extended X-ray absorption fine structure (EXAFS) measurements have been made. In the particular case of Rh, it has been shown that solutions of [RhCl(NH 3) 5]Cl 2 exchanged with Na-X form a highly active catalyst (“RhA”) for the carbonylation of methanol when used with an organic iodide promoter. On the other hand, systems prepared from RhCl 3 are far less active (“RhC”). EXAFS spectroscopy from the Rh K-edge has been used to follow the fate of the Rh species for the two preparation techniques from the initial impregnation through the catalyst calcination to the final activation with the promoter. Differences in structure are readily apparent at each stage of the catalyst preparation. The results indicate that the species present in RhA after impregnation is a mobile aquo complex while the Rh of RhC is in the form of Rh 2O 3 crystallites. Upon calcination, the RhA system forms highly dispersed Rh metallic crystallites while the structure of the Rh species in RhC is only slightly affected by calcination. The activation step in RhA involves modification of the metal particles at the surface atoms in a manner similar to classical metal catalysis. The EXAFS results indicate that the intermediate oxide formed in RhC during calcination is too stable for effective activation since it fails to convert fully to the metal form.

Variations in the classical model of staging in graphite intercalates: EXAFS results

Extended X-ray absorption fine structure (EXAFS) spectra havee been obtained at the potassium K-edge of KC 8 and KC 12 n , n = 2, 3, and 4 over the temperature range 80–300K. Quantitative results indicate that the carbon-intercalate-carbon layer sandwich is not of constant thickness for a given intercalant species and that in KC 12 n , n = 2, 3, and 4 the disordered potassium layers form a lattice gas at room temperature. Also indicated are puckering distortions of the bounding carbon layers due to the disordered potassium layer.