EXAFS Experiments Research Articles

Zr-EXAFS study of fluoride ion-conducting ZrF 4-BaF 2-CsF glasses

Fluorine coordination environment around zirconium in fluoride ion-conducting 55ZrF 4·(45- x)BaF 2· xCsF glasses was investigated by means of Zr-EXAFS spectroscopy using SOR. EXAFS experiments were carried out on glasses of x=0, 7, 14, 20, 27, 34 and 40, and crystalline Li 2 ZrF 6. The Zr-F peaks in the Fourier transform magnitude curves of glasses shifted to shorter distances with the substitution of CsF for BaF 2. In the Zr-F peaks of the x=27, 34 and 40 glasses, at the same time, small but discernible shoulders emerged at the right sides of the peaks, growing more prominently in this order. The Fourier backfiltering and curve fitting treatments of these Zr-F peaks revealed that, although the F coordination numbers of Zr hardly vary with the substitution of CsF for BaF 2, the Zr-F interatomic distances split into two largely different ones with the substitution of CsF more than about 20 mol%. For instance, the Zr-F interatomic distances and the F numbers at the distances were 2.07 Å/6.5 in the 55ZrF 4·45BaF 2 glass, 2.06 Å/6.4 and 2.27 Å/0.6 in the 55ZrF 4·18BaF 2·27CsF glass, and 2.05 Å/5.1 and 2.23 Å/1.9 in the 55ZrF 4·5BaF 2·40CsF glass. The present Zr-EXAFS result may give an interpretation for the anomalous compositional dependence of the activation energy for conduction in electrical conductivity that was previously observed for ZrF 4-BaF 2-CsF glasses.

Spectral transformation of EXELFS data — NiO on Ni(100) and Ni(110)

Extended electron energy loss fine structure (EXELFS) spectra have been obtained from the K-level of oxygen in NiO films grown on both the Ni(100) and Ni(110) surfaces. The spectra indicate that the oxide films grown on both surfaces have Ni-O bond lengths of 2.08 Å, identical to those of bulk NiO. These analyses agree with previous measurements on the Ni(100) surface. As in previous experiments the actual measurements made is of N( E) rather than direct measurement of the electron energy loss distribution N( E). The EXELFS spectra from these two surfaces are used to illustrate the influence of this collection scheme on the radial distribution function obtained by Fourier transformation of the raw data. To obtain the direct analog of the radial distribution function found from the EXAFS experiment one must either resort to spectral integration or appropriate scaling of the distribution function found from the EXELFS experiment.

Pressure calibration in high pressure EXAFS experiments

Abstract To avoid glitches due to Bragg reflection we use boron carbide anvils for our high pressure EXAFS experiments. Since boron carbide precludes the ruby fluorescence pressure calibration we use the EXAFS of selected materials to measure the reduction with pressure of the nearest neighbor distance and infer the pressure from an isothermal equation of state. The requirements for a good pressure calibrant and the difficulties in EXAFS data analysis are discussed. Cu, NaBr and RbCl have successfully been tested as pressure calibrants.

Spectral Transformation of Exelfs Data and a Structural Examination of Nitrogen on Cu(100) and Cu(110) Surfaces

ABSTRACTEXtended Electron Energy Loss Fine Structure (EXELFS) spectra have been obtained above the K-edge of nitrogen atoms adsorbed on Cu. The radial distribution functions obtained from the fine structure indicate N-Cu bond lengths of 1.84(±.03) Å and 1.81(±.03) Å for nitrogen on the Cu(100) and Cu(110) surfaces respectively. As in previous EXELFS measurements, the actual measurements made are of N"(E) rather than direct measurement of N(E), the electron energy loss distribution. The EXELFS spectra from these two surfaces are used to illustrate the influence of this collection scheme on the radial distribution function obtained by Fourier transformation of the raw data. To obtain the direct analog of the radial distribution function found from the EXAFS experiment one must either resort to spectral integration or appropriate scaling of the distribution function found in the EXELFS experiment.

Imaging system with an amorphous silicon linear sensor

An amorphous silicon (a-Si) linear image sensor with a long active length and good spatial resolution has been applied to two-dimensional x-ray imaging. An area-to-line converter using plastic optical fibers (250 μm φ) with a Gd2 O2 S phosphor sheet has been constructed and attached to the sensor. This 2D detector has only 32×32 channels, but there is no technical limit for constructing a larger area detector. An EXAFS experiment is also presented for demonstration of the utility of the detector.

EXAFS determination of the Fe and Zr environments in amorphous Fe31Zr69

EXAFS experiments have been performed at low temperature above the Fe and Zr K absorption edges in different amorphous Fe Zr samples: a sputtered Fe31Zr69 alloy in the as-made state and after annealing at 250°C, and a melt-spun Fe39Zr61 alloy. The local order has been determined around the Fe and Zr atoms and evidence for FeFe first neighbours obtained.

A novel Z-axis diffractometer for X-ray diffraction and fluorescence studies

Recently several X-ray techniques have been demonstrated to have high potential for providing needed structural information to understand the many physical and chemical phenomena that occur at surfaces and interfaces. In this article we will describe an instrument that facilitates the performing of these types of measurements and allows for the investigation of the gas-liquid interface using a variety of synchrotron X-ray scattering techniques. This instrument employs the z-axis diffraction geometry in which the detector has two degrees of freedom while the sample (the gas-liquid interface) has only one degree of freedom — rotation around the vertical axis. A distinctive feature of this diffractometer is that it leaves a fixed illuminated footprint on the liquid interface and allows for an accurate determination of the relative scattering or fluorescence intensity at very shallow angles. The unique capabilities of this novel instrument have been demonstrated at synchrotron facilities in a variety of X-ray techniques: grazing incidence diffraction (GID), shallow angle reflection (SAR), near total external fluorescence (NTEF) and EXAFS experiments from the surface of liquids

A model for the chemical modification of electrical properties of chalcogenide glasses by bismuth

A mechanism is proposed to account for the ability of Bi to change the electrical properties of certain chalcogenide glasses. EXAFS experiments have shown that there is a significant change in the local structural environment of the Bi at compositions close to the critical concentration of Bi(10 at.%) at which the p-n transition takes place. These structural changes are interpreted in terms of a model in which the type of bonding at the Bi sites changes from covalent to partially ionic. The presence of charged impurity centres upsets the equilibrium between native charged defects in the glass, and a p-n 'doping' transition subsequently occurs at a critical impurity concentration.

Local Structure of Heme-Iron Studied by High-Resolution XANES; Thermal Spin Equilibrium in Myoglobin

The local structure of heme-iron of alkaline met-myoglobin (MbOH) during the thermal spin equilibrium between high and low spin states has been studied by a high-resolution XANES. The spin-state sensitive near edge features have been found in the pre-edge ls-3d transition peak and absorption threshold region. These characteristic features are dependent on the high spin concentration and are related to the d-electron configuration and local structure of heme-iron, i. e. the distortion of octahedral ligand field caused by a displacement of heme-iron from the heme plane. The spin-state sensitive near-edge structures were also found for other myoglobin derivatives and can be used as a spin state marker. From a systematic high resolution XANES study on MbCO, MbCN and oxy-Mb), near-edge features which are sensitive to the angle between the diatomic ligand molecule and heme normal were found. These characteristic features can provide the elaborate local structure of heme-iron such as the displacement of iron atom from the hemeplane or the bond angle of ligand molecule when combined with EXAFS experiments.

EXAFS-STUDY OF THE Zn2+ COORDINATION IN AQUEOUS HALIDE SOLUTIONS

Aqueous solutions of ZnCl2, ZnBr2 and ZnI2 in the concentration range from 0.1 molal up to saturation have been studied by EXAFS experiments at the zinc K-edge. EXAFS spectra of the crystalline compounds of ZnCl2, ZnBr2, ZnI2 and ZnSO4*7H2O have been used as references. I n the low concentration range up to 0.5 molal a complete hydration of the zinc ions by 6 - 7 water molecules is observed. At high concentrations above 5 molal the mean local coordination around the zinc ions consists of halide ions and water molecules.

ATOMIC RELAXATION IN MIXED-VALENT SYSTEMS DEDUCED FROM EXAFS SPECTROSCOPY

We will present how it is possible, from EXAFS spectroscopy, to estimate atomic relaxation effects even in homo.geneous mixed valent compounds, for which these relaxations are known to be very small. EXAFS experiments, analysed by our procedure, can then be used to decide if the ground state is homogeneous or inhomogeneous. At least, the procedure will be applied to the Tm Se system. We conclude that the homogeneous character of the mixed valent stzte is preserved in the non-stoichiometric compounds.

X-RAY ABSORPTION SPECTROSCOPY ON AMORPHOUS MIXED-VALENT ALLOYS

X-ray absorption measurements were performed on Ybx Pd1-x and Eux Pd1-x amorphous alloys. In both systems, LIII edges clearly demonstrate a concentration-induced valence change. The Eu valence admixture is found to be inhomogeneous in nature. XANES and EXAFS experiments show the existence of well-defined local environments which are typical of those encountered for Eu2+ and Eu3+ in the Eu/Pd system.

SPHERICAL WAVES EXAFS AND MULTIPLE SCATTERING EFFECTS IN XANES OF THE K-EDGE SPECTRUM OF SILICON

A theoretical calculation of the K-edge spectrum of silicon was performed. A good fit with the experimental EXAFS spectrum has been obtained by using the spherical wave formalism, the actual values of the mean free path and the Debye-Waller terms.The relevant multiple scattering contribution in the first 70 eV of the spectrum has been obtained by subtraction of a simulated EXAFS spectrum from the experimental one. INTRODUCTION. - In order to establish the role of the basic physical processes in the EXAFS and XANES pans of K-edge absorption spectra such as inelastic losses, thermal vibration and multiple scattering we have performed a careful quantitative analysis of the silicon K-edge absorption spectrum. A good fitting of the EXAFS oscillations from 50 to 450 eV above the absorption threshold (in the range of wave vector 4-11 A-l, with ET=1838 eV) has been obtained using spherical The spectrum of the mean free path h(E) of the photoelectron has been determined from photoemission experiments(3) and it has been included in the EXAFS calculations. The EXES Debye-Waller factors (4) have been also considered. They have been calculated for each shell using a dispersive model of lattice vibrations of Walker and eati in^(^) starting from X-ray diffraction measures on vibrational amplitudes in Silicon (6J). An investigation on multiple scattering effects has been carried out by subtracting the calculated spectrum from the experimental one; the difference spectrum shows an oscillation which can be explained considering the first path of multiple scattering. E-

ChemInform Abstract: EXAF S Study of Chromium Ions in the Mixed‐Solvent System of Formamide and Ammonium Formate.

AbstractThe coordination structures of Cr(VI) and Cr(III) ions in the HCONHz‐HCOONH′, system have been investigated by EXAFS experiments.

EXAFS and X-ray diffraction of RM 2X 2 compounds (R = rare earths, M> = transition metal, X = Si, Ge)

EXAFS is often used to obtain local distances which are difficult to extract from conventional powder X-ray diffraction. The class of rare- earths compounds with the Th Cr 2Si 2 structure presents very interesting properties ranging from superconducting heavy fermions to strongly mixed valence compounds. Precisely in such compounds, the local distances between the Th and Si sites can only be deduced from X-ray diffraction on single crystals. We will show in this paper, that EXAFS experiments can be safely used to extract these local distances within the usual EXAFS accuracy ∼ ± 0.02 Å, nevertheless a great deal of precaution must be taken in order to avoid spurious effects and false distances.

Scattering studies of photostructural changes in chalcogenide glasses

Structural changes which occur in chalcogenide glasses upon absorption of bandgap light have been investigated using neutron diffraction. The first part of this paper describes studies by small-angle neutron scattering of the irreversible photostructural changes which occur in obliquely evaporated amorphous chalcogenide films. The second part deals with the reversible photostructural effect observed in chalcogenide glasses, investigated by conventional neutron diffraction and EXAFS experiments. The various models which have been proposed for the photostructural effect are discussed in the light of these experimental results.

The electronic structure of model lead glasses

Abstract Tight-binding band structures of PbO, SnO and of model lead silicates are presented. The ordering of bands and the electronic charge densities are discussed in terms of the major interactions. Differences between the bonding in octet compounds (Sn IV and Pb IV) and super-octet compounds (Sn II and Pb II) are illustrated. The bonding at two types of Pb site in lead silicate glasses is considered; a symmetric (ionic) PbO8 site and an asymmetric (nominally covalent) PbO4 site. The PbO4 site is finally favoured because its band gap is closer to that observed experimentally, and because of the direct evidence of EXAFS experiments. The core exciton spectra of Pb in lead silicates, which has been interpreted in terms of an ionic model, is reconciled with the present model by noting that the shift of the Pb 5d→6p lines is a function of Pb 6p conduction-band width. The bonding at a PbO4 site is resonant-covalent, so that lead silicate glasses constitute one of the few known examples of resonantly bonded g...

Development of a laboratory EXAFS facility

The EXAFS technique is a powerful new structural tool, particularly useful for studies of disordered or otherwise complex materials for which x-ray diffraction techniques are difficult or unfeasible. At the present time, most EXAFS experiments are carried out at a synchrotron facility because of the larger fluxes available. We have developed an in-laboratory apparatus utilizing a focusing crystal technique which increases available fluxes two to three orders of magnitude over previous laboratory facilities, so that EXAFS measurements can be carried out quickly and accurately in the laboratory. We will discuss the principles of the focusing monochromator and we will also illustrate the experimental method with examples, including studies of chemical solutions, defect crystalline solids, and high-temperature superconductors.

Atomic origin of structure in EXAFS experiments

X-ray absorption edges in solids show extended fine structure up to 30-60 Hartrees (1 or 2 keV) above the edge which is known to be caused by electron scattering from other atoms surrounding the absorbing atom. In addition other structure is sometimes observed, of a slower period, especially nearer to the edge, which are shown to be caused by scattering in the periphery of the absorbing atom. The new structure is closely related to the derivative of the p-phase shift in the case of K-shell excitation, and improved values of the phase shift can be extracted from experiment.

Novel x‐ray spectroscopy probes complex molecules

When the Stanford Synchrotron Radiation Project began operating in May experimenters soon were able to demonstrate the great potential that such sources have for studying local atomic arrangements in complex molecules. The technique used goes by the forbidding name of “extended x‐ray absorption fine‐structure spectroscopy” or EXAFS. At the Fourth International Conference on Vacuum Ultraviolet Radiation Physics held in Hamburg, Germany, late in July, Peter Eisenberger (Bell Labs) discussed the EXAFS experiments done by a group that included himself, Brian Kincaid and Sally Hunter (Stanford), Dale Sayers and Edward A. Stern (University of Washington) and Farrel Lytle (Boeing Co.). Eisenberger told the meeting that the unique element‐specific nature of EXAFS shows promise of monitoring changes of environment in complex solids and in systems of interest to chemists and biologists.