X-ray Absorption Data Research Articles

87Rb NMR investigations on binary and ternary fullerene compounds: Rb3C60, K2RbC60 and Rb2CsC60

Results of13C MAS NMR and87Rb NMR measurements on Rb3C60, K2RbC60 and Rb2CsC60 are presented. The87Rb NMR spectra show generally the same characteristics as already reported by other authors. For the ternary compound the degree of site preferential order of the alkali metal ions agrees with the analysis of X-ray absorption data. Furthermore, in the room temperature87Rb NMR spectra of all samples an additional broadened line is detected indicating that at least for the samples prepared from sublimed C60 a considerable amount of the Rb ions is located on sites with symmetry lower than cubic.

Soft-x-ray fluorescence study of buried silicides in antiferromagnetically coupled Fe/Si multilayers.

Soft x-ray fluorescence spectroscopy has been employed to obtain information about the Si-derived valence band states of Fe/Si multilayers. The valence band spectra are quite different for films with and without antiferromagnetic interlayer exchange coupling, demonstrating that these multilayers have different silicide phases in their spacer layers. Comparison with previously published fluorescence data on bulk iron silicides shows that the Fe concentration in the silicide spacer layers is substantial. Near-edge x-ray absorption data on antiferromagnetically coupled multilayers in combination with the fluorescence data demonstrate unambiguously that the silicide spacer layer in these films is metallic. These results on the electronic structure of buried layers in a multilayer film exemplify the wide range of experiments made possible by new high-brightness synchrotron sources.

X-ray Microradiographic Determination of Diffusion Coefficients within Porous Media Based on Fick's 2nd Law: Theory and Error Analysis

A method based on Fick’s 2nd Law is described for the determination of local effective diffusion coefficients, within porous media, from X-ray absorption measurements. Numerical methods were used to simulate concentrations and corresponding X-ray absorption data during diffusion of an X-ray absorbing species within the fluid phase. Errors arising from X-ray counting statistics were investigated analytically and by Monte Carlo simulations. The influence of experimental conditions on errors is discussed.

Soft X-Ray Emission Studies of the Electronic Structure in Silicon Nanoclusters

AbstractDensity of states changes in the valence and conduction band of silicon nanoclusters were monitored using soft x-ray emission and absorption spectroscopy as a function of cluster size. A progressive increase in the valence band edge toward lower energy is found for clusters with decreasing diameters. A similar but smaller shift is observed in the near-edge x-ray absorption data of the silicon nanoclusters.

First-principles study of excitonic self-trapping in diamond.

We present a first-principles study of excitonic self-trapping in diamond. Our calculation provides evidence for self-trapping of the 1s core exciton and gives a coherent interpretation of recent experimental X-ray absorption and emission data. Self-trapping does not occur in the case of a single valence exciton. We predict, however, that self-trapping should occur in the case of a valence biexciton. This process is accompanied by a large local relaxation of the lattice which could be observed experimentally.

Local order in sol-gel derived glassy TiO2

The short range order around Ti atoms in sol-gel derived bulk amorphous TiO2 is investigated by X-Ray Absorption Spectroscopy (EXAFS and XANES). Two TiO distances are found: the longer distance is attributed to the presence of anatase micro-crystallites, whereas the shorter one is due to a residual of the alkoxide precursor. XANES measurements give no evidence of tetrahedrally coordinated Ti, both in the organic precursor or in the amorphous phase. The X-Ray absorption data confirm that thermal treatments up to 350°C on bulk amorphous TiO2 give crystalline anatase and up to 750°C give crystalline rutile.

X-ray absorption studies of the purple acid phosphatase from red kidney beans (native enzyme, metal exchanged form)

Purple acid phosphatase from red kidney beans (KBP) catalyzes the hydrolysis of activated phosphoric acid monoesters and contains a heterodinuclear Fe(III)Zn(II) core in its active site. Iron K-edge X-ray absorption data have been obtained for the native enzyme and for a metal exchanged derivative, where Zn(II) was substituted by Fe(III). The environment of the native enzyme consists of 2.5 O/N at 1.91 Å, 3 O/N at 2.09 Å, and 1 Zn at 4.05 Å. For the metal exchanged form we obtained 2.5 O/N at 1.94 Å, 2.5 O/N at 2.09 Å, and 1 Fe at 3.79 Å.

Orientation of the oxygen-evolving manganese complex in a photosystem II membrane preparation: an X-ray absorption spectroscopy study.

X-ray absorption spectroscopy has been performed on oriented photosystem II membrane particles isolated from spinach. Structural features of the tetranuclear Mn cluster and the orientation of the cluster with respect to the lipid bilayer were determined in both the S1 and S2 states of the Kok cycle. Variation of the sample orientation with respect to the X-ray e-vector yields highly dichroic K-edge and extended X-ray absorption fine structure spectra (EXAFS), indicative of an asymmetric tetranuclear cluster. Mn-Mn vectors at 2.72 and 3.38 A can be resolved from these measurements using quantitative analysis. The 2.72-A vector, consisting of at least two component vectors, is oriented at an average angle of 60 degrees +/- 7 degrees to the membrane normal, with an average of 1.1 +/- 0.1 interactions per Mn atom. The 3.38-A vector, most probably an average of two vectors, makes an angle of 43 degrees +/- 10 degrees with respect to the membrane normal, with an average of 0.45 +/- 0.07 backscatterer per Mn atom. Upon advance to the S2 state, the orientation of these vectors and the average numbers of backscatterers are approximately invariant. Analysis of more subtle features of the EXAFS reveals changes accompanying this S-state advance that are consistent with the oxidation of Mn during this transition. However, the dominant structural features of the oxygen-evolving complex remain constant in the S1 and S2 states. The structure of the Mn complex and the orientation of the complex in the membrane within the context of dichroism of the X-ray absorption data are discussed.

An approximation of the Voigt I profile for the fitting of experimental X-ray absorption data

An analytical expression of the Voigt integral, derived by numerical methods, is proposed. It spans a wide range in X-ray spectral region in which the Lorentzian broadening of atomic origin and the Gaussian (instrumental) broadenings are of comparable magnitude. The indefinite integral of this expression is easy to evaluate and provide a good approximation for the X-ray threshold profiles. The proposed expressions are compared with previously proposed Voigt profiles and an example of experimental data analysis by fitting the lines and threshold profiles is given.

Zirconolite-Rich Ceramics for Actinide Wastes

AbstractNew X-ray diffraction and scanning electron microscopy data are given for the incorporation of Np and Pu in zirconolite, at levels of tens of percent. The actinide valences and the cations they replace are deduced from the microanalysis of the zirconolite compositions, and X-ray absorption data are used to obtain more direct information on the valences of Ce and Nd, which are used as simulants of Pu and trivalent actinides respectively. Trivalent rare earths and actinides have extensive solid solubility in zirconolite, mainly but not exclusively in the Ca site. Tetravalent rare earths and actinides have considerable solid solubility in the Zr site of zirconolite, and some solubility in the Ca site, but the strong tendency of zirconolite with ions substituted in the Zr site to undergo phase separation complicates structural interpretation. In zirconolite-rich Synroc-type ceramics designed to immobilise waste actinides, the target actinide waste loading has been set at 20 wt% and early leach results indicate the durability is at least as good as that of Synroc-C.

Measurement of Density and Water Content in Soils with X-ray Linescan and X-ray Computed Tomography

Potentials for using X-ray computed tomography for determining soil bulk density and water content were investigated using Cecil sandy clay and Lakeland sand soils. Soils were moistened and samples were compacted to several levels. These samples were sequentially dried and scanned using tomography reconstructions as well as the linescan mode available on a CT scanner. Mean X-ray absorption and pixel standard deviation data were collected for each soil sample at scanning in both modes. For each soil-mode combination, relationships between mean X-ray absorption and soil bulk density and water content were determined as were relations between pixel standard deviation and soil density and water content. The experiment was then repeated to provide validation data. In the tomographic reconstruction mode with validation data, optimal water content and bulk density values for samples were determined for each corresponding mean absorption and pixel standard deviation measurements which minimized the error sum of squares. Density was predicted in both soils to within 2%. Predicted water content was within 50% of observed values. Using known values of density, water content was predicted to within 5%. In the linescan mode, density was usually predicted to within 5%. Water content was not significant (P < 0.15%) thus a prediction for water was not attempted in the linescan mode.

Local structure determination of aluminum in Y zeolite: application of low energy X-ray absorption fine structure spectroscopy

The local Al structure and electronic properties in Na-Y, NH4-Y and H-Y zeolites have been determined by low energy AlXAFS. The Al-O bond distance of 1.700 A in H-Y is longer than that in NH4-Y and Na-Y, 1.636 and 1.620 A, respectively. In addition, the white line intensity of the Alion in H-Y is higher than in NH4-Y and Na-Y, indicating the electron density on the Alion is lower in H-Y than in the other two catalysts. The results of experimental Al-O bond distance and white line intensities are qualitatively in agreement with theoretically calculated Al-O bond distances in aluminosilicate clusters. However, further work is required in order to quantitatively evaluate the near edge structure of the X-ray absorption data.

EXAFS and EELS evidence for the synthesis of a new amorphous nickel oxythiophosphate NiPS 2O

The room temperature synthesis of NiPS 3 by mixing a nickel salt and [Li 4 +(P 2S 6) 4−] in water from which oxygen has not been removed leads to an oxygenated phase. Nickel K edge X-ray absorption data and nickel L 2,3, phosphorus and sulfur K edges electron energy loss spectroscopy experiments show that a pure oxythiophosphate phase with a formulation close to NiPS 2O is obtained. This composition is explained by a partial oxidation of the pristine (P 2S 6) 4− anion, before reaction to form (P 2S 4O 2) 4−. The higher oxidized anions are destroyed by the breaking of the phosphorus-phosphorus bond.

Study of some Ce intermetallics by core-level photoemission

Ce intermetallics CeAlNi, CeAlCu and CeNi2Si2 have been studied by core-level photoemission spectroscopy to investigate their electronic structure. The Ce 3d XPS data clearly indicate that the final state is a quantum mechanical admixture of 4f0, 4f1 and 4f2 configurations in these systems with an increasing 4f0 contribution as Cu is replaced by Ni. The two Ni-based systems are found to be mixed-valence systems with nf<1 while the ground state of the Cu-containing system appears to retain a purely trivalent character with nf approximately=1. These results are in agreement with the authors earlier reported X-ray absorption and resonant photoemission data. The present study highlights the fact that the replacing of Ni by Cu seems to be propitious for driving the Ce from an intermediate-valence (IV) to a non-IV ground state. The results Ce 3d and Ce 4d XPS are discussed.

EXAFS studies of uteroferrin and its anion complexes

Iron K-edge X-ray absorption data on the purple acid phosphatase from porcine uterus (uteroferrin, Uf) have been obtained for the native reduced enzyme and for the oxidized enzyme in its phosphate- and arsenate-bound forms. In all three complexes, the first sphere consists of 1.5 N/O at ∼1.94 A, 4 N/O at ∼2.1 A, and 0.5-1 N/O at ∼2.4 A; in no complex is found an Fe-O bond of ∼1.8 A which would derive from a μ-oxo bond. The ∼1.94-A shell corresponds to Fe-OAr and Fe-μ-OH(or R) bonds. The ∼2.1-A shell arises from histidine, carboxylate, oxoanion, and solvent ligation. The scatterer at ∼2.4 A is associated with a chelated carboxylate residue

X-ray absorption multiple-scattering study of angle distribution in high-T c superconductors

A new method, designed to perform data-analyis of X-ray absorption data and previously applied to molecular cases, has been used to investigate high- T c superconductros for the first time. This method includes properly multiple-scattering contributions to the EXAFS spectra. In this way, it is possible to derive unconventional structural information such as the angle distributions associated with the main scattering paths. We have analyzed well-known superconducting samples such as YBa 2Cu 3O ≈7 and Bi 2Sr 2CaCu 2O ≈8. In both cases, quantitative local structural information has been extracted form Cu K-edge EXAFS spectra by using a fitting procedure which performs the average of the multiple-scattering contributions. The dominant signal due to the inplane CuOCu angle distribution has been accurately studied taking proper account of the multiple-scattering signals due to the quasi-linear configurations. A good agreement with difraction results has been found. This application shows the capability of EXAFS to measure the local CuOCu angle distribution in this class of compounds.

Combined EXAFS and XRD for the in situ structural elucidation of solid catalysts under operating conditions

Two distinct kinds of experimental arrangement permit X-ray absorption and X-ray diffraction data to be recorded in a combined manner under in situ conditions. Results are shown for the conversion of a copper-zinc hydroxycarbonate precursor to a “live” water-gas shift catalyst and for a zinc oxide and a zeolite model catalyst heated to ca. 1000 ° C. The short-range and long-range structural changes accompanying the conversion of the zeolite into cordierite are charted at high temperature. EXAFS and XANES data can be recordable both in fluorescence or transmission mode with one of the arrangements, which also permits successive measurements (around more than one absorption edge) within a short space of time.

The structure of sodium adsorption phases on Al(111)

The structures formed by the adsorption of Na and Al(111) have been studied by normal incidence standing X-ray wavefield absorption using both the (111) and (1̄11) Bragg reflections in order to obtain not only the Na-Al layer spacings, but also the adsorption sites. In the case of the ordered (√3 × √3)R30°-Na phase, at a nominal coverage of 0.33 ML, and at lower coverages down to approximately 0.12 ML, Na is found to adsorb in a coverage independent site which involves a substitution of some of the top layer Al atoms; if the substrate layer spacings remain unchanged by this reconstruction, the effective radius of the adsorbed Na species is found to be 1.67 Å. The higher coverage (2 × 2)-Na phase is found to involve (at least) two distinct Na adsorption sites, but each of these has a very similar layer spacing relative to the extended (111) substrate scatterer lattice planes. A specific model of this phase, involving two reconstructed layers each of stoichiometry NaAl 2 is proposed, which accounts in a quantitative way for the present X-ray standing wave data and previous X-ray absorption data, and in a qualitative way for published high resolution soft X-ray photoelectron spectroscopy results. As Al and Na are essentially immiscible in the bulk, this two-layer structure appears to be an entirely new kind of surface phase.

High-energy paramagnetic spectral response of the valence fluctuation compound YbAl2

Abstract Measurements of the paramagnetic spectral response of YbAl2, reveal a broad Lorentzian inelastic hump closely similar to that of α-Ce in spectral shape, in integrated intensity (within ± 20% uncertainty), as well as in characteristic energy ω o (=180 ± 10meV). Relating the latter to the linear specific heat coefficient y via a Fermi-liquid relation requires that the 4f occupancy (n f) be close to unity (i.e. 0.9 ± 0.1). With the same values of (n f) and ω o the Fermi-liquid relation for the low-temperature bulk susceptibility χ(0) yields a reduced effective moment of magnitude close to that for the crystal-field Γ7, state (N = 2) or the J = 5/2; spin-orbit state (N = 6), the latter implying an inversion of Hund's rule states of Yb. The nearly integral valence of Yb in YbAl2, suggested by the Fermi-liquid analysis is in conflict with the lattice constant, photoemission and L111 X-ray absorption data which all yield (n f)≈ 0·4.

The electronic structure of actinide dioxides

The f-electron elements in actinide dioxide crystals have cubic site symmetry. Nevertheless, our understanding of their electronic structure has advanced rather slowly. Now, in addition to the magnetic susceptibility, magnetic resonance, optical and X-ray absorption data, there is information from both elastic and inelastic neutron magnetic scattering experiments. This report compares experimental data with results from two types of calculations: discrete-variational X α molecular-orbital studies and traditional f n crystal-field studies. In this way, we not only gain an insight into the single-site crystal-field parameters but we learn something about the significance and the approximate magnitude of the interactions between the f electrons on neighboring actinide sites.