Edge Extended X-ray Absorption Fine Structure Research Articles

Exafs Spectroscopic Studies of Uranium (VI) Oxide Precipitates

AbstractWe have investigated the structures of U(VI) oxides precipitated from room temperature aqueous solutions at low ionic strength as a function of pH. Using the uranium LIII - edge extended x-ray absorption fine structure (EXAFS) as a probe of the local structure around the uranium, a trend is observed whereby the axial oxygen bond lengths from the uranyl groups increase from 1.80 Å at pH=7 to 1.86 Å at pH=11. A concomitant decrease in the equatorial oxygen and nearest-neighbor uranium bond lengths also occurs with increasing pH. Expansion of the linear O=U=O group is seen directly at the L111 absorption edge where multiple scattering resonances systematically shift in energy. EXAFS curve-fitting analysis on these precipitates and a sample of synthetic schoepite indicate that the structure of the species formed at pH=7 is similar to the structure of schoepite. At pH=11, the precipitate structure is similar to that of a uranate.

New synthetic route to WSF4 and its solution-phase structure as determined by tungsten L(III)-edge extended X-ray absorption fine structure studies

The compound WSF4 was synthesized in one step by the reaction of WF6 with (Me3Si)2S in solution at ambient temperatures. Solution-phase structural data have been obtained for the first time for it and related tungsten chalcogenide tetrahalides via a tungsten L(III)-edge EXAFS (extended X-ray absorption fine structure) study. This demonstrates that WOF4, WOCl4 and WSF4 in acetonitrile or dichloromethane solution are monomeric. For WSF4 analysis of the EXAFS data yielded bond lengths of 2.026(8)(WS) and 1.863(3)A(W–F). The WS bond length is shorter than previously determined, by gas-phase electron diffraction, but is more consistent with that found in related structures.

Local lattice instability of CuO 2 plane in La 1.85Sr 0.15CuO 4 by polarized Cu K edge absorption

The temperature dependence of the local structure of the CuO 2 plane in La 1.85Sr 0.15CuO 4 has been studied by in-plane polarized Cu K edge extended X-ray absorption fine structure (EXAFS). The high signal-to-noise ratio in the E‖ ab EXAFS allows us to separate the contributions of CuO(planar), CuOCu and CuLa/Sr shells and to study their temperature dependence in the low-temperature orthorhombic (LTO) phase. We have observed an anomalous behavior of the local structure below 100 K with clear anomalies at ∼ T c (35 K) and T ∗ (∼1.6T c ) that are not expected on the basis of the average structure. These results suggest that the structure of the CuO 2 plane is non-homogeneous and it shows anomalous changes at T ∗ and T c.

Thermal Induced Evolution of Chlorine-Containing Precursors in Impregnated Pd/Al2O3 Catalysts

The results of a combined study using diffuse reflectance spectra, temperature-programmed reduction, and extended X-ray absorption-edge fine structure of chlorine-containing precursors formed during preparation of Pd/Al 2 O 3 catalysts are reported. The catalyst dried (385 K) after impregnation contains both PdCl 4 2- species adsorbed electrostatically (about 60-70%) and strongly interacted PdCl x O y species. Calcination at 775 and 925 K results in decomposition of the initial impregnated species, formation of highly distorted PdCl x O y structures, and nucleation of disordered PdO clusters. Growth of large PdO particles occurs during calcination at 1075 K. The reduction of the oxide phase is more facile than that of chlorinated species in intimate interaction with alumina. The concept of geometrical constraints during adsorption of square planar PdCl 4 2- species on various configurations of positively charged hydroxyl groups on different crystal planes of alumina is discussed. It is proposed that electrostatic adsorption is favored on (100) and (110) planes, where the adsorbate matches the configuration of oppositely charged surface sites. The misfit between adsorbed species and surface hydroxyls on the (111) planes leads to ligand-substituted species.

Total electron yield exafs studies of (001) Au/Co monocrystalline multilayers

Total Electron Yield (TEY) Co K edge Extended X-ray Absorption Fine Structure (EXAFS) experiments have been performed on (001) monocrystalline Au/Co multilayers. The interface is oriented parallel to the synchrotron polarization vector in order to probe preferentially the in-plane local environment. This study shows that the thin Co layers have a body centered tetragonal structure (bct), i.e. a flattened [001] bcc structure. A Co-Au mixing at the interface has also been put in evidence.

Selenol binds to iron in nitrogenase iron-molybdenum cofactor: an extended x-ray absorption fine structure study.

The biological N2-fixation reaction is catalyzed by the enzyme nitrogenase. The metal cluster active site of this enzyme, the iron-molybdenum cofactor (FeMoco), can be studied either while bound within the MoFe protein component of nitrogenase or after it has been extracted into N-methylformamide. The two species are similar but not identical. For example, the addition of thiophenol or selenophenol to isolated FeMoco causes its rather broad S = 3/2 electron paramagnetic resonance signal to sharpen and more closely approach the signal exhibited by protein-bound FeMoco. The nature of this thiol/selenol binding site has been investigated by using Se-K edge extended x-ray absorption fine structure (EXAFS) to study selenophenol ligated to FeMoco, and the results are reported here. EXAFS data analysis at the ligand Se-K edge was performed with a set of software, GNXAS, that provides for direct calculation of the theoretical EXAFS signals and least-squares fits to the experimental data. Data analysis results show definitively that the selenol (and by inference thiol) binds to Fe at a distance of 2.4 A. In contrast, unacceptable fits are obtained with either Mo or S as the liganded atom (instead of Fe). These results provide quantitative details about an exchangeable thiol/selenol binding site on FeMoco in its isolated, solution state and establish an Fe atom as the site of this reaction. Furthermore, the utility of ligand-based EXAFS as a probe of coordination in polynuclear metal clusters is demonstrated.

Characterization of a split axial-oxygen site in TlBa2Ca3Cu4O11 by extended x-ray-absorption fine-structure spectroscopy.

The Cu {ital K}-edge extended x-ray-absorption fine structure (EXAFS) of TlBa{sub 2}Ca{sub 3}Cu{sub 4}O{sub 11} ({ital T}{sub {ital c}}=118 K) is analyzed in the range 10{le}{ital T}{le}156 K. Nonlinear-least-squares fits to the EXAFS Cu--axial-oxygen contributions show the presence of two equally populated axial-oxygen sites at {ital R}{sub 1}=2.57 A and {ital R}{sub 2}=2.74 A for temperatures above {ital T}{sub {ital c}} and at 10 K. The corresponding Tl--axial-oxygen distances are 2.18 and 2.01 A, which are similar to those found in other thallium superconducting phases. In a fluctuation region below {ital T}{sub {ital c}} ({ital T}=100, 117 K), the separation between the two sites decreases to less than 0.10 A. This effect is analogous to the one previously reported in YBa{sub 2}Cu{sub 3}O{sub 7}, suggesting a similar coupling between elastic degrees of freedom and superconductivity. We note, however, that both the separation between the sites and the decrease in this separation within the fluctuation region are greater than in YBa{sub 2}Cu{sub 3}O{sub 7}.

X-ray diffraction and EXAFS studies of sputter-deposited TiPd films

The structure of amorphous Ti 1− x Pd x (0.34 ⩽ x ⩽ 0.48) films produced by RF sputtering has been studied by X-ray diffraction and EXAFS (extended X-ray absorption-edge fine structure) measurements. The peak parameters and the coordination numbers derived from the radial distribution function analysis of X-ray diffraction have no distinct composition dependence. EXAFS data also show no specific compositional dependence of the pair distances of TiTi, TiPd and PdPd correlations. Thus Ti 1− x Pd x amorphous films produced by sputtering are considered to have nearly identical structures in this limited composition range.

An EXAFS study of Gd, Er and Lu site location in the epidote structure

L III edge extended X-ray absorption fine structure (EXAFS) spectra of Gd, Er and Lu environments in synthetic epidotes of composition CaLa0.9 X 0.1Al2MgSi3O13H (X=Gd or Er or Lu) were recorded using synchrotron radiation. The Fourier transforms of the Gd-, Er- and Lu-EXAFS are clearly different from one another indicating wholly or partially different site occupancy. Model fitting of the Fourier-filtered partial EXAFS and comparison of pair distribution functions with those calculated for natural epidote leads to the conclusion that three different sites are probably involved in the accommodation of these elements in the epidote structure, and that site preference is a function of the rare earth ionic size. Gd is located in A2-type sites, whereas the local atomic environment of Er is consistent with A1 site occupancy and the Lu environment has been modelled on an M3-type octahedral site.

STRUCTURE INVESTIGATIONS ON HALIDE GLASSES (MnF2)x(BaCl2)100-x WITH THE EXAFS AND WAXS TECHNIQUES

A short range structure investigation of halide glasses (MnF2)x(BaCl2)100-x(x = 65, 60, 55) has been made using the extended X-ray absorption fine structure (EXAFS) and the wide angle X-ray scattering (WAXS) techniques. The local structure information around the Mn is derived from the study of the MnK absorption edge EXAFS and the X-ray scattering intensity of the materials, respectively. The results show that the F and Cl atoms in the first neighbour shell of the Mn atom construct a octahedra (MnCl2F4) with the bond-length rMn-Cl = 0.246 nm and rMn-F= 0.200 nm in the glass of x = 65, which share the atom F with the neighbouring one to form zigzag chains linked by Ba atoms distributed between them. With decreasing of the amount of MnF2 below x = 65, the bond-length rMn-Cl and the Debye-Waller factor σMn-Cl2 of Mn-Cl increase, and that of Mn-F is almost invariant. The disappearance of the Mn-Mn peak has been found from x = 60 to x = 55 with an abrupt increase in rMn-Cl and σMn-Cl2, which can be explained as the chains being broken to form the solitude octahc-dra (MnCl2F4).

Extended x-ray-absorption fine-structure study of amorphous (NixPt100-x)75P25 alloys.

The Pt ${L}_{3}$ edge and the Ni K edge extended x-ray-absorption fine-structure (EXAFS) measurements have been made on amorphous (${\mathrm{Ni}}_{\mathrm{x}}$${\mathrm{Pt}}_{100\mathrm{\ensuremath{-}}\mathrm{x}}$${)}_{75}$${\mathrm{P}}_{25}$ alloys at three low temperatures as well as at room temperature. With a limited k range, the Fourier transforms of the Pt ${L}_{3}$ and the Ni K EXAFS for amorphous Ni-Pt-P alloys exhibit extremely broad peaks which manifest an unusually large disorder of the system. As the maximum k range of the transformation is extended with an enhanced signal-to-noise ratio, these broad major peaks in the Fourier transforms still remain broad and begin to split into two and finally into three. The relative magnitudes and the thermal variation of these three split peaks strongly indicate the existence of subshells for different atomic pairs such as Pt-P, Pt-Ni, and Pt-Pt. Though it is not possible, at present, to determine the degree of asymmetry in the radial distribution of each subshell, a rough simulation of the split peaks in the Fourier transforms of the Pt ${L}_{3}$ EXAFS data at 7 K for two compositions of amorphous Ni-Pt-P provides further evidence for the existence of separate subshells for different atomic pairs. Moreover, while the thermal disorder is apparently greater for a Pt-Pt pair subshell than those of the two other subshells, the structural disorder seems to be greater for a Pt-Ni pair subshell than that for a Pt-Pt subshell.

Thickness effect on the extended-x-ray-absorption-fine-structure amplitude

The thickness effect, which is caused by inevitable leakage radiation accompanying the desired radiation, can cause significant decreases of extended-x-ray-absorption-fine-structure (EXAFS) amplitude when the sample is thick enough. The effect is illustrated by measurements of the $K$-edge EXAFS on a series of copper foils of varying thicknesses. Significant distortions in EXAFS amplitudes occur when $\ensuremath{\Delta}{\ensuremath{\mu}}_{0}x\ensuremath{\ge}1.5$, where $\ensuremath{\Delta}{\ensuremath{\mu}}_{0}$ is the $K$-edge step in the absorption coefficient and $x$ is the sample thickness. Therefore, the optimum total sample thickness of ${\ensuremath{\mu}}_{T}x=2.6$ as determined by statistical considerations will introduce errors in EXAFS amplitudes in concentrated samples due to the thickness effect. The measurements presented here determine the most accurate values of EXAFS amplitude for copper metal which agree well with theory as corrected for the many-body overlap effect.

Curved-crystal (LiF) x-ray focussing array for fluorescence EXAFS in dilute samples

A focussing monochromator array for detection of fluorescence EXAFS (Extended X−ray Absorption Edge Fine Structure) is described. The array is made of LiF crystals, specially prepared for large mosaic spread. Design criteria, performance specifications, and details of fabrication and crystal preparation are given.

Short Range Order in Noncrystalline Iron(III) Sulfide

AbstractThe short range order in noncrystalline iron (III) sulfide has been studied by a radial distribution analysis of the diffuse scattered intensity from X‐ray diffraction. Supplementary information was obtained from the Mayn oscillation in the Extended X‐ray Absorption Edge Fine Structure (EXAFS) of the FeK edge. The average iron‐sulfur distance determined was d(FeS) = 2.285(30) Å. The data infer the presence of FeFe interactions at distances between 2.7–2.9 Å.

Theory of extended x-ray absorption edge fine structure (EXAFS) in crystalline solids

Theory of extended x-ray absorption edge fine structure (EXAFS) in crystalline solids