Showing papers on "Extended X-ray absorption fine structure published in 1988"

X-ray absorption : principles, applications, techniques of EXAFS, SEXAFS, and XANES

Abstract: Theory of EXAFS. Theory of XANES. INSTRUMENTAL AND DATA ANALYSIS. Design of an EXAFS Experiment. EXAFS with Synchrotron Radiation. Laboratory EXAFS Facilities. Data Analysis. APPLICATIONS. Biochemical Application of X-ray Absorption Spectroscopy. Catalysis. Amorphous and Liquid Systems. SEXAFS. XANES Spectroscopy.

Cavity ring‐down optical spectrometer for absorption measurements using pulsed laser sources

Abstract: We have developed a technique which allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity significantly greater than that attained using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. The decay of the light intensity within the cavity is a simple exponential with loss components due to mirror loss, broadband scatter (Rayleigh, Mie), and molecular absorption. Narrowband absorption spectra are recorded by scanning the output of a pulsed laser (which is injected into the optical cavity) through an absorption resonance. We have demonstrated the sensitivity of this technique by measuring several bands in the very weak forbidden b1Σg−X3Σg transition in gaseous molecular oxygen. Absorption signals of less than 1 part in 106 can be detected.

Structure of Mn and Fe oxides and oxyhydroxides: A topological approach by EXAFS

Abstract: The structure of Mn and Fe oxides and oxyhydroxides has been probed by EXAFS. It is shown that EXAFS spectroscopy is sensitive to the nature of interpolyhedral linkages relying on metal-two nearest metal distances. Spectra recorded at 290 K and 30 K indicate that intercationic distances can be determined by EXAFS with a good accuracy (0.02 A) assuming a purely Gaussian distribution function, even at room temperature. Although the accuracy on atomic numbers determination is fair for these disordered systems, EXAFS can differentiate structures with contrasted edge- over corner-sharing ratio like pyrolusite, ramsdellite, todorokite and lithiophorite or lepidocrocite and goethite. A direct application of this result has shown that the proportion of pyrolusite domains within the lattice of nsutite from Ghana is equal to 35±15 percent. The systematic study of Mn dioxides also put forward the sensitivity of EXAFS to the presence of corner-sharing octahedra, with a detection limit found to be less than 8 percent. In spite of their similar XRD patterns, the EXAFS study of todorokite and asbolane confirms that they possess a distinct structure; that is, a tunnel structure for the former and a layered structure for the second. Such a topological approach has been used to probe the structure of ferruginous vernadite; a highly disordered iron-bearing Mn oxide. Fe and Mn K-edges EXAFS spectra are very dissimilar, traducing a different short range order. The Mn phase is constituted by MnO2 layers. Its large local structural order contrasts with the short range disorder of the iron phase. This hydrous Fe oxyhydroxide is constituted by face-, edge- and corner-sharing octahedra. This iron phase possesses the same local order as feroxy-hyte, but is long range disordered. The presence of face-sharing Fe(O,OH)6 octahedra prevents its direct solid-state transformation into well crystallized oxyhydroxides, and explains the necessary dissolution-reprecipitation mechanism generally invoked for the hydrous ferric gel → goethite transformation.

X-ray absorption (EXAFS/XANES) study of supported vanadium oxide catalysts. Structure of surface vanadium oxide species on silica and γ-alumina at a low level of vanadium loading

Abstract: X-Ray absorption spectroscopy has been employed to clarify the environmental structure around vanadium atoms in silica-supported and γ-alumina-supported vanadium oxide catalysts. Catalysts containing 2.8% of vanadium by weight were prepared with NH4VO3 and VO(acac)2 as impregnation agents. X.p.s. (V 2p3/2) of the catalysts showed that the vanadium atoms in these catalysts are pentavalent. E.s.r. signals from V4+ in the reduced catalysts indicated that paramagnetic VO4–4 and (VO)2+ ions are generated by the reduction on silica and alumina, respectively. Although the dispersion of vanadium oxide in the catalysts prepared with VO(acac)2 solution was found to be higher than those prepared with NH4VO3, the XANES and EXAFS spectra do not exhibit a significant difference for the two types of the catalyst, indicating that the dominant surface species are the same at such a low level of vanadium loading. Analysis of the XANES spectra suggests that VO4 tetrahedra are the dominant species on alumina and the vanadates on silica are square pyramidal; part of the vanadium species is present as V2O5 microcrystallites. EXAFS spectra of the catalysts and their Fourier-transforms show that VO4 units are isolated on alumina, and that the majority of vanadates on silica are polymeric. These findings by EXAFS/XANES spectroscopy indicate that VO4 on silica and VO5(or VO6) on γ-alumina, as detected by e.s.r., are only a minority species on each support even at a low level of loading of vanadium.

Total-electron-yield current measurements for near-surface extended x-ray-absorption fine structure.

Abstract: A total-electron-yield technique is described in which near-surface extended x-ray-absorption fine-structure (EXAFS) data are obtained from direct measurements of specimen current. Experiments with several model systems---amorphous germanium and crystalline germanium, nickel, and cobalt; and arsenic ion implanted into silicon---demonstrate that this technique can reproduce EXAFS \ensuremath{\chi}(k) functions obtained from transmission and fluorescence measurements. Experiments also reveal that EXAFS amplitudes from total-electron-yield data can be 5--10 % smaller than those from transmission measurements for samples where the very-near-surface structure, at depths of tens to hundreds of angstroms, differs from the bulk structure. Measurements with buried layers confirm that the sampling depth for this total-electron-yield technique is determined primarily by the penetration ranges of Auger electrons emitted from the absorbing atoms. For the model systems listed above, LMM Auger electrons have ranges of hundreds of angstroms and KLL Auger electrons have ranges of thousands of angstroms. Expressions are derived for the sampling depth for total-electron-yield EXAFS experiments. The total-electron-yield technique described here is particularly useful for studying impurities within a few thousand angstroms of the surface of single crystals, where Bragg diffraction complicates the use of fluorescence measurements.

An extended x‐ray absorption fine structure study of aqueous rare earth perchlorate solutions in liquid and glassy states

Abstract: Extended x‐ray absorption fine structure (EXAFS) measurements were performed for concentrated aqueous rare earth perchlorate solutions (R=28; R is the moles of water per mole of salt) in the liquid state at room temperature and in the glassy state at liquid nitrogen temperature. The quantitative analysis of the EXAFS data has revealed that the hydration number changes from about nine for light rare earth ions to about eight for heavy rare earth ions through the intermediate ions of Sm3+ ∼Eu3+ in both liquid and glassy states. The average Ln3+ –OH2 distances were determined and they are in agreement with previously reported values from x‐ray and neutron diffraction. The Debye–Waller factor of the average Ln3+ –OH2 bonds for the light rare earth ions was larger than that for the heavy ions, suggesting that the hydration shell of the light rare earth ions is statically disordered, consisting of different Ln3+ –OH2 bonds.

Structural studies of argon-sputtered amorphous carbon films by means of extended x-ray-absorption fine structure.

Abstract: The temperature-dependent microscopic structure of plasma-deposited a-C:H and magnetron-sputtered a-C films, in situ sputter cleaned by argon bombardment, has been investigated by near-edge (NEXAFS) and extended (EXAFS) x-ray-absorption fine-structure studies. We find that the microscopic structure of the two films becomes indistinguishable after sputtering with a loss of hydrogen for the a-C:H sample. The structure of the sputtered films at 30 \ifmmode^\circ\else\textdegree\fi{}C is characterized by a first-neighbor C-C bond length of 1.445(10) A\r{}. Upon annealing the bond length approaches that of graphite (1.421 A\r{}) with a value of 1.427(10) A\r{} at 1050 \ifmmode^\circ\else\textdegree\fi{}C, the highest annealing temperature used. Analysis of the EXAFS amplitude of the first-neighbor shell leads to a two-phase structural model consisting of a ``graphitelike'' network and a statically and dynamically disordered ``random matrix.'' The fraction of carbon atoms in the ``graphitelike'' network increases from 60(6)% at 30\ifmmode^\circ\else\textdegree\fi{}C to 92(9)% at 1050\ifmmode^\circ\else\textdegree\fi{}C. Analysis of the higher-neighbor-shell EXAFS signals leads to a model for the ``graphitelike'' regions, consisting of a network of conjugated odd- and even-membered rings, without long-range order. In contrast, the ``random matrix'' is suggested to be a mostly chainlike network of double and single bonds. Our results suggest that the ``graphitelike'' matrix is a precursor state for crystallization.

Extended X-ray absorption fine structure studies of platinum-tin catalysts

Abstract: Extended X-ray absorption fine structure (EXAFS) studies at 140 K, along with studies of X-ray absorption edges themselves, were conducted on a Pt-Sn/Al/sub 2/O/sub 3/ catalysts that had previously been exposed to hydrogen at high temperatures (775-795 K). For comparison, similar studies were conducted on a Pt-Sn/SiO/sub 2/ sample. For the Pt-Sn/Al/sub 2/O/sub 3/ catalyst the extent of coordination of tin to platinum atoms or to other atoms of tin is small, but the extent of coordination to oxygen is substantial. An analysis of the energy shift of the K absorption edge of the tin indicates that the tin is not present as Sn/sup 4 +/. The EXAFS data, coupled with the absorption edge data, suggest that the tin is largely present as Sn/sup 2 +/. There appears to be a significant interaction between the platinum and tin, since the platinum is more highly dispersed on alumina when tin is present. After consideration of all the data, it seems reasonable to regard the catalysts as if it consisted essentially of platinum clusters dispersed on an alumina that has Sn/sup 2 +/ present at the surface. The EXAFS data on the Pt-Sn/SiO/sub 2/ sample lead to very different conclusions. In thismore » case, most of the tin has an oxidation state of zero, and the material consists predominantly of bimetallic entities of platinum and tin on silica.« less

In-Plane Structure of Underpotentially Deposited Copper on Gold (111) Determined by Surface EXAFS (Extended X-Ray Absorption Fine Structure).

Abstract: : The Surface Extended X Ray Absorption Fine Structure (SEFAFS) spectrum of an electrochemically deposited ad-layer of copper on a gold (111) electrode immersed in solution and under potential control was obtained by employing fluorescence detection and grazing incidence geometry. The x-rays were polarized parallel to the plane of the gold surface. Two peaks were observed in the radial distribution function, indicating two different near neighbors for copper. The near neighbors were determined to be gold at 2.58 plus or minus 0.03 angstrom and copper at 2.92 plus or minus 0.03 angstroms. The copper-copper distance is identical to the gold-gold lattice spacing showing that at full coverage, the copper ad-layer forms at 1 x 1 commensurate layer on the gold (111) substrate. The intensity of the gold-copper scattering relative to the copper-copper suggests the copper atoms sit in 3 fold hollows sites.

Spin-dependent photoabsorption at theL-edges of ferromagnetic Gd and Tb metal

Abstract: The spin-dependent absorption of circularly polarized x-rays is studied at theL-edges of ferromagnetic Gd and Tb metal. At theL1-edge a spin-dependent part of the absorption coefficient of 10−3–10−2 is observed. Strong resonance absorption known as “white line” occurs at theL2- andL3-absorption onset. Correlated with it one finds large spin-dependent absorption effects with amplitudes of a few percent. The spin-dependent absorption spectra reflect the profiles of the spin densities of the states populated in the absorption process. Thep-states show spin densities correlated with the first two flat bands above the Fermi level. The spin density of thed-like states is concentrated in the energy range of the white line. In Gd a splitting of ∼(0.5–0.6) eV of the unoccupied 5d spin up and spin down bands is indicated for both spin-orbit partners. In Tb a large dependence of the 5d spin density on the spin-orbit configuration is observed. The experimental results on the spin densities in Gd are compared with band structure calculations for the ferromagnetic ground-state. The theoretical and experimental spin density profiles agree well for thep-states but not for thed-states. The discrepancy concerning thed-states may be attributed to core-hole polarization effects in the absorption process.

Glancing-angle extended x-ray-absorption fine structure and reflectivity studies of interfacial regions

Abstract: Glancing-angle extended x-ray-absorption fine structure (EXAFS) and x-ray reflectivity measurements have been made for a Au surface and several Cu/Al interfaces. The Cu/Al samples consisted of 1000 A\r{} Al on Cu, and the measurements demonstrate the potential of these techniques for determining interface structure and morphology. In particular, distinct differences were observed in both the reflectivity and interface EXAFS signals for Cu/Al samples prepared under different conditions. The information obtained on these samples is compared with information obtained by transmission electron microscopy, Rutherford backscattering, and Auger sputter profiling, and excellent agreement is found. The glancing-angle measurements are distorted by anomalous dispersion effects and a simple analytic correction scheme is presented. This correction has been tested on the Au data and found to work well for the entire range of incident angles. For the Cu/Al interfaces the corrections allowed quantitative analysis of the Cu-Al bonding at the interface, and a substantial difference is found for samples prepared under UHV and non-UHV (${10}^{\mathrm{\ensuremath{-}}6}$ Torr) conditions. These glancing-angle techniques offer some unique characteristics and are compared to more standard interface studies.

Depth dependence for extended x-ray-absorption fine-structure spectroscopy detected via electron yield in He and in vacuum

Abstract: X-ray absorption spectra have been measured using electron detection with both vacuum and helium gas surrounding the samples. The samples consisted of thin iron films covered with various thicknesses of aluminum to determine the contribution versus depth. The height of the iron K--absorption-edge jump decreases exponentially with aluminum covering thickness, with a 1/e depth of 1600 A. The addition of helium gas forms an ionization detector for the electrons, which have an average energy of about 2500 eV. The effects of electrode geometry and bias voltage are evaluated. When operated in a linear-response region, the signal-to-noise ratio for this method is excellent and the extended x-ray-absorption fine-structure (EXAFS) amplitudes agree with transmission measurements to better than 3%.

The structure and reactivity of chemisorbed aromatics: Spectroscopic studies of benzene on Mo(110)

Abstract: The reactions of benzene on Mo(110) have been investigated under ultrahigh vacuum conditions using temperature programmed reaction spectroscopy (TPRS), x‐ray photoelectron spectroscopy (XPS), and near‐edge x‐ray absorption fine structure (NEXAFS) measurements. Benzene undergoes competing decomposition and desorption processes during temperature programmed reaction; saturation exposures of C6H6 made at a crystal temperature of 120 K yield a small amount of benzene desorption at 360 K as well as substantial H2 evolution at 400 and 530 K. Additionally, a trio of molecular benzene desorptions attributed to weakly bound and multilayer states is observed below 200 K. Isotopic exchange experiments demonstrate that the 360 K molecular desorption arises from intact chemisorbed benzene. For surface temperatures up to 300 K, carbon 1s photoelectron spectra show only a single photoemission peak at a binding energy of 284.0 eV, attributed to molecular benzene. Above 300 K, increasing amounts of atomic carbon are obser...

Characterization of the manganese O2-evolving complex and the iron−quinone acceptor complex in photosystem II from a thermophilic cyanobacterium by electron paramagnetic resonance and X-ray absorption spectroscopy

Abstract: The Mn donor complex in the S1 and S2 states and the iron-quinone acceptor complex (Fe2+-Q) in O2-evolving photosystem II (PS II) preparations from a thermophilic cyanobacterium, Synechococcus sp., have been studied with X-ray absorption spectroscopy and electron paramagnetic resonance (EPR). Illumination of these preparations at 220-240 K results in formation of a multiline EPR signal very similar to that assigned to a Mn S2 species observed in spinach PS II, together with g = 1.8 and 1.9 EPR signals similar to the Fe2+-QA- acceptor signals seen in spinach PS II. Illumination at 110-160 K does not produce the g = 1.8 or 1.9 EPR signals, nor the multiline or g = 4.1 EPR signals associated with the S2 state of PS II in spinach; however, a signal which peaks at g = 1.6 appears. The most probable assignment of this signal is an altered configuration of the Fe2+-QA- complex. In addition, no donor signal was seen upon warming the 140 K illuminated sample to 215 K. Following continuous illumination at temperatures between 140 and 215 K, the average X-ray absorption Mn K-edge inflection energy changes from 6550 eV for a dark-adapted (S1) sample to 6551 eV for the illuminated (S2) sample. The shift in edge inflection energy indicates an oxidation of Mn, and the absolute edge inflection energies indicate an average Mn oxidation state higher than Mn(II). Upon illumination a significant change was observed in the shape of the features associated with 1s to 3d transitions. The S1 spectrum resembles those of Mn(III) complexes, and the S2 spectrum resembles those of Mn(IV) complexes. The extended X-ray absorption fine structure (EXAFS) spectrum of the Mn complex is similar in the S1 and S2 states. Simulations indicate O or N ligands at 1.75 +/- 0.05 A, transition metal neighbor(s) at 2.73 +/- 0.05 A, which are assumed to be Mn, and terminal ligands which are probably N and O at a range of distances around 2.2 A. The Mn-O bond length of 1.75 A and the transition metal at 2.7 A indicate the presence of a di-mu-oxo-bridged Mn structure. Simulations indicate that a symmetric tetranuclear cluster is unlikely to be present, while binuclear, trinuclear, or highly distorted tetranuclear structures are possible. The striking similarity of these results to those from spinach PS II suggests that the structure of the Mn complex is largely conserved across evolutionarily diverse O2-evolving photosynthetic species.

EXAFS study of yttria stabilized cubic zirconia

Abstract: EXAFS (Extended X-ray Absorption Fine Structure) studies of cubic (ZrO2)1−x(Y2O3)x are reported for the concentration range 0.09

Structure of rhodium/titania in the normal and the SMSI state as determined by extended x-ray absorption fine structure and high-resolution transmission electron microscopy

Abstract: Extended X-ray absorption fine structure (EXAFS) and high-resolution transmission electron microscopy (HRTEM) have been used to study the structure of a Rh/Ti02 catalyst. After reduction in H, at 473 K (when the catalyst is in the normal state) the metal particles contain on the average five rhodium atoms and are situated preferably on edges of the Ti0, crystallites but also on [loll and to a lesser extent on [OOl] anatase crystal faces. Reduction in H2 at 723 K leads to the SMSI state. Besides oxygen neighbors from the support, the rhodium metal atoms in the metal-support interface have Ti“’ neighbors at 3.4 and 4.3 A. These distances and their coordination numbers fit well with a model in which the metal particles rest on a TiO, suboxide. This indicates that the supporting oxide near the metal particle has been reduced to a suboxide of TiO,. In the SMSI state no indication for coverage has been found with either EXAFS or HRTEM. On the contrary, exposing the catalyst in the SMSI state to oxygen at 100 K resulted in changes in the EXAFS spectrum due to physisorption of oxygen. Consequently, in the SMSI state the particles are either not covered or are incompletely covered with TiO,. Since a Rh/AI2O3 catalyst under the same conditions became partly oxidized, it is evident that for the Rh/Ti02 catalyst oxidation has been suppressed. This is most probably the result of an electronic influence from the reduced supporting oxide. Even after oxygen admission at room temperature, the rhodium particles on the TiO, support remain in the metallic state. The TiO, suboxide in the vicinity of the metal particles starts to reoxidize and the metal-support interaction becomes weaker.

A structural basis for the corrosion resistance of lead-iron-phosphate glasses: an X-ray absorption spectroscopy study

Abstract: Lead metaphosphate glass is characteristically prone to aqueous attack but becomes chemically resistant on the addition of iron oxide. We have measured the K-edge extended X-ray absorption fine structure (EXAFS) of Fe and the LIII-edge EXAFS of Pb and find the atomic environments of the two sites to be very different. Whereas Fe is octahedrally coordinated with an oxygen distancce of 1•9(5) A, the coordination of Pb is 8 (±1) and the sites display considerable disorder with an average lead—oxygen distance of 2•4(7) A. X-ray absorption near-edge structure (XANES) measurements of the P K-edge confirm the presence of phosphate as (PO4)n, chains. These results suggest that, by pinning the structure, Fe sites not only shorten the average polyphosphate chain length but also block the percolation of Pb ions. Both effects increase the chemical resistance of the structure to aqueous attack and also reduce the tendency of these glasses to crystallize.

The copper sites of dopamine beta-hydroxylase: an X-ray absorption spectroscopic study.

Abstract: X-ray absorption edge and extended X-ray absorption fine structure (EXAFS) spectra are reported for the Cu(I) and Cu(II) forms of bovine dopamine beta-hydroxylase (DBH; EC 1.14.17.1) and for the Cu(I) form of DBH bound either to tyramine substrate or to a multisubstrate inhibitor [Kruse, L. I., DeWolf, W. E., Jr., Chambers, P. A., & Goodhart, P. J. (1986) Biochemistry 25, 7271-7278]. A significant change in the structure of the copper sites occurs upon ascorbate-mediated reduction of Cu(II) DBH to the Cu(I) form. While the average Cu(II) site most likely consists of a square-planar array of four (N,O)-containing ligands at 1.98 A, the average Cu(I) site shows a reduction in (N,O) coordination number (from approximately 4 to approximately 2) and the addition of a S-containing ligand at 2.30 A. No change in the average Cu(I) ligand environment accompanies binding of tyramine substrate, whereas binding of a multisubstrate inhibitor, 1-(3,5-difluoro-4-hydroxybenzyl)-1H-imidazole-2(3H)-thione, causes an increase in the Cu-S coordination, consistent with inhibitor binding to the Cu(I) site through the S atom. Although excellent signal-to-noise ratio in the EXAFS spectra of ascorbate-reduced DBH facilitated analysis of outer-shell scattering for a Cu..Cu interaction, the presence of a binuclear site could not be proven or disproven due to interference from Cu...C scattering involving the carbons of imidazole ligands.

Extended x-ray-absorption and electron-energy-loss fine-structure studies of the local atomic structure of amorphous unhydrogenated and hydrogenated silicon carbide.

Abstract: Extended x-ray-absorption (EXAFS) and electron-energy-loss fine-structure (EXELFS) measurements have been performed on amorphous unhydrogenated silicon carbide, a-SiC, and amorphous hydrogenated silicon carbide, a-SiC:H. Two hydrogenated samples with hydrogen concentrations corresponding, respectively, to H flows of 4 sccm (20% of argon flow) and 8 sccm (40% of argon flow) during the reactive sputtering process, were analyzed (sccm denotes standard cubic centimeters per minute at STP). It is found that short-range order (SRO), consisting of the same tetrahedrally coordinated units present in cubic crystalline c-SiC (zinc-blende structure), where a Si atom is surrounded by nearly four C atoms and vice versa, does exist in all the amorphous samples. This SRO, however, is detected only at a level of the first C and Si coordination shells in a-SiC and a-SiC:H. The structural disorder of the first Si and C coordination shells in all forms of amorphous SiC is somewhat greater than c-SiC, and it decreases appreciably as hydrogen is added. The a-SiC sample exhibits large Si and C coordination numbers, almost identical to c-SiC, a low atomic density, and virtually the same Si-C bond length as c-SiC. These results indicate that a relatively small concentration of large voids exist in a highly disordered a-SiC matrix. The a-SiC:H samples, on the other hand, exhibit a decrease in the C coordination number relative to a-SiC, which is independent of H concentration, low Si and C atomic densities, comparable to a-SiC, and virtually the same Si coordination number as a-SiC. These EXAFS-EXELFS results are consistent with a model where part of the H is substituting for Si in the local tetrahedra surrounding C atoms, while the rest is located inside internal voids in the a-SiC:H samples. The surface of the voids is composed of C atoms which have at least one bond to H, and of Si atoms. Finally, a straightforward computational procedure is applied to estimate the size of these voids. It is found that their average size \ensuremath{\ge}21 A\r{}, thus excluding the possibility that the voids might be point defects.

Extended X-ray absorption fine structure studies of nanocrystalline materials exhibiting a new solid state structure with randomly arranged atoms

Abstract: Nanocrystalline materials are polycrystals in which the diameter of the individual crystallites is of the order of a few (typically 5–15) nanometres. Structurally, these materials consist of the following two components with comparable volume fractions: a crystalline component and a grain boundary component. Extended X-ray absorption fine structure studies indicate that the grain boundary component of nanocrystalline materials represents a new solid state structure with random atomic arrangement.

EXAFS Study on Selenium-Tellurium Mixed Chains

Abstract: Extended X-ray absorption fine structure (EXAFS) has been taken on the Se K-edge for isolated Se-Te mixed chains confined in the mordenite channels, as well as for trigonal and amorphous Se-Te mixtures. The Se-Se and Se-Te bond lengths in the Se-Te chain molecules have been determined by utilizing large difference in the wave vector dependence of backward scattering between Se and Te. The Se-Se and Se-Te bonds in the isolated chains are found to be shorter than those in the trigonal mixtures, which suggests that the intrachain covalent bonds are strengthened by reducing the interchain coupling. The Se-Se bond in the mixed chain molecule elongates with addition of Te, while the Se-Te bond changes little.

The structure of the cobalt sulfide phase in carbon-supported Co and Co-Mo sulfide catalysts as studied by EXAFS and XANES

Abstract: An X-ray absorption spectroscopy study has been carried out on a sulfided Co/C and a Co-Mo/C catalyst, consisting of a fully sulfided Co-Mo-S phase. By comparing the EXAFS and XANES spectra of the catalysts with those of pure Co9S8 and CoS2 reference compounds, it is shown that the cobalt ions in the Co-Mo-S phase have an octahedral-like sulfur coordination while the cobalt ions in the Co/C catalyst have a larger fraction of octahedral cobalt than Co9S8.

Topological short-range disorder in Au1-xNix solid solutions: An extended x-ray-absorption fine-structure spectroscopy and computer-simulation study.

Abstract: The present extended x-ray-absorption fine-structure (EXAFS) investigation aims at determining the atom-pair displacements on a local scale in the Au1-xNix system, for the three types of pairs Au-Au, Au-Ni, and Ni-Ni. X-ray-absorption spectra have been obtained above both edges (Au L3 and Ni K), over the whole composition range. For the purpose of the separation of the Au and Ni scattering contributions, a detailed analysis of EXAFS over a wide photoelectron wave-vector range beginning close to the edge has been performed. The amplitude and phase functions for unlike-neighbor pairs have been obtained in two ways: first by using empirically refined theoretical partial phase shifts, which enabled us to calculate the total phase functions, and secondly by making use of fluorescence recorded spectra of dilute alloys from which appropriate amplitude functions have been extracted. Many low-k amplitude effects such as the photoelectron wave curvature, the experimental energy resolution, the limited core-hole lifetime, and the many-electron excitation factors have been analyzed, and taken into account, as well as the difference between p- and d-symmetry photoelectron scattering. Although the alloys are crystalline, a large positional disorder was found, resulting in a distribution of the three pair distances. An appropriate model for the EXAFS function was selected by use of several constraints relating the structural parameters determined by EXAFS to the ones of the regular fcc lattice, deduced from x-ray diffraction measurements. The three nearest-neighbor partial radial distribution functions are thus described in terms of mean distance and second and third momenta. They are broad and display a marked asymmetry, especially in concentrated alloys, these features increasing in the order Au-Au, Au-Ni, and Ni-Ni. Indeed, the Ni-Ni radial distribution function is very wide, at the extent that it is comparable to those encountered in amorphous metal-metal alloys. The three partial mean distances are distinct from each other by as much as 0.1 A, and exhibit different dependence on composition. In dilute alloys, the relaxation around the impurity atoms was found to be appreciable. The most striking feature of the local structure, irrespective of the composition, is that the larger Au atoms are more confined (statically and dynamically) on the nodes of the regular lattice than the Ni ones. The positive bowing of the lattice parameter variations with composition is discussed in the light of the present results. The experimental work was completed by a computer simulation of the topological structure of these alloys, based on a minimization of the total elastic energy. The experimental results were quantitatively well reproduced, by use of a Morse interaction pair potential. Qualitative features were found to have small dependence on the particular pair potential chosen to model the interactions. In addition, as a further proof of the adequacy of the function chosen for EXAFS fitting, model EXAFS spectra calculated from the computed radial distribution functions were fitted in the same manner as the experimental ones, yielding an estimate of the results accuracy.

Structural characterization of bismuth molybdates by X-ray absorption spectroscopy and powder neutron diffraction profile analysis

Abstract: By use of synchrotron radiation, high-quality X-ray absorption data have been recorded for three bismuth molybdate phases with the general composition Bi/sub 2/O/sub 3/ x nMoO/sub 3/, for n = 3 (..cap alpha..-phase) n = 2 (..beta..-phase), and n = 1 (..gamma..-phase). The results of these bismuth L/sub 3/-edge and molybdenum K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies provide structural insights about the local environments of the Bi/sup 3 +/ and Mo/sup 6 +/ ions. Also, the structure of ..beta..-Bi/sub 2/O/sub 3/ x 2MoO/sub 3/ was investigated by the use of time-of-flight powder neutron diffraction. The profile analysis of the neutron data for ..beta..-bismuth molybdate (monoclinic, P2/sub 1//n, a = 11.9515 (4) A, b = 10.7993 (4) A, c = 11.8805 (4) A, ..beta.. = 90.142 (5)/sup 0/, and Z = 8) revealed that the bismuth and molybdenum cations are present in a fluorite-like sublattice.