Edge Extended X-ray Absorption Fine Structure Spectra Research Articles

An EXAFS study of phenanthroline-holmium complex

A holmium complex, $$[\hbox {Ho}(\hbox {phen})_{\mathrm {2}}(\hbox {H}_{\mathrm {2}}$$ O) $$_{\mathrm {2}} (\hbox {Cl})_{\mathrm {2}}$$ ]Cl.H $$_{\mathrm {2}}$$ O, as a luminescent/bio probe has been made and characterized using extended X-ray absorption fine structure (EXAFS) as well as IR spectroscopy, UV-Vis spectroscopy, and elemental analysis. Structural data of the formed Ho(III)–phen species are determined from the Ho $$L_{\mathrm {III}}$$ -edge EXAFS spectra as well as its $$k^{\mathrm {2}}$$ -weighted and $$k^{\mathrm {3}}$$ -weighted spectra. The coordination numbers and distances between the holmium and the first shell atoms, i.e., O, N, and Cl, are measured. The EXAFS spectra are also analyzed by Fourier transform of the normalized absorption coefficient into the molecular distance coordinates to further verify the determined Ho(III) structural model.

Structure, electronic, and vibrational properties of glassy Ga11Ge11Te78: Experimentally constrained density functional study

The atomic structure and electronic and vibrational properties of glassy Ga${}_{11}$Ge${}_{11}$Te${}_{78}$ have been studied by combining density functional (DF) simulations with x-ray (XRD) and neutron diffraction (ND), extended x-ray absorption fine structure (EXAFS), and Raman spectroscopies. The final DF structure (540 atoms) was refined using reverse Monte Carlo methods to reproduce the XRD and ND data as well as Ge and Ga $K$-edge EXAFS spectra, while maintaining a semiconducting band gap and a total energy close to the DF minimum. The local coordination of Ga is tetrahedral, while Ge has twice as many tetrahedral as defective octahedral configurations. The average coordination numbers are Ga, 4.1, Ge, 3.8, and Te, 2.6. The chemical bonding around Ga involves Ga $4s$, Ga $4p$, Te $5s$, and Te $5p$ orbitals, and the bond strengths show bonding close to covalent, as in Ge. There are fewer Te chains and cavities than in amorphous Te, and a prepeak in the structure factor at 1.0 \AA{}${}^{\ensuremath{-}1}$ indicates medium-range order of the Ga/Ge network. Density functional calculations show that contributions of Te-Te, Ga-Te, and Ge-Te bonds dominate the experimental Raman spectra in the $110--150$ cm${}^{\ensuremath{-}1}$ range.

Coordination of uranium(vi) with functional groups of bacterial lipopolysaccharide studied by EXAFS and FT-IR spectroscopy

The complexation of uranyl ions with lipopolysaccharide (LPS), the main component of the cell wall of Gram-negative bacteria, was investigated on a molecular level with U L(III)-edge extended X-ray absorption fine structure (EXAFS) and attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy over a wide pH range (2.6 to 7.0). For the first time, structural determinations of uranyl complexes with cell wall compounds were extended from acidic up to neutral pH. The main functionalities responsible for uranyl binding are phosphoryl and carboxyl groups. At an excess of LPS, related to environmental conditions, the uranyl ion is mainly complexed by phosphoryl groups four-fold monodentately coordinated in the equatorial plane of the uranyl dioxo cation UO(2)(2+) showing great homologies to the uranyl mineral phase meta-autunite in the EXAFS spectra. At equimolar ratios of uranyl and functional groups of LPS, according to a slight deficit of phosphoryl groups, additional carboxyl coordination in a bidentate manner becomes important as it is shown by IR spectroscopy. From the vibrational spectra, a mixed coordination of UO(2)(2+) with both phosphoryl and carboxyl groups is derived. The coordination of uranyl ions to the LPS molecule is obviously mainly controlled by the U/LPS concentration ratio, and the influence of pH is only of minor significance at the investigated range.

Zn-K edge EXAFS study of human nails

Extended X-ray absorption fine structure (EXAFS) spectroscopy at the Zn - K edge is applied for the study of the bonding geometry of Zn in human nails. The studied nail clippings belong to healthy donors and donors who suffer from lung diseases. Fitting of the first nearest neighboring shell of Zn reveals that it is bonded with N and S, at distances that take values in the ranges 2.00–2.04 Å and 2.23–2.28Å, respectively. Zn is four - fold coordinated and the ratio of the number of sulfur and nitrogen atoms (NS/NN) in the first coordination shell ranges from 0.52 to 1. The sample that belongs to the donor who suffers from lung fibrosis, a condition that is related to keratinization of the lung tissue, is characterized by the highest number of NS/NN. Simulation, using the FEFF8 code, of the Zn - K edge EXAFS spectra with models of tetrahedrally coordinated Zn with 1 (or 2) cysteine and 3 (or 2) histidines is satisfactory.

Characterization of Reaction of Green Rust with Foreign Ions using X-ray Absorption Fine Structure

Measurements of X-ray absorption fine structure (XAFS), that is X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were performed for characterizing the reactions of hydrosulphate green rust (GR) with copper or nickel sulphate ions. Fe K edge XANES spectra showed that a part of Fe(II) in GR2 was oxidized by the addition of copper sulphate ions, while Fe(II) in GR was not oxidized by the addition of nickel sulphate ions. Cu K edge and Ni K edge XANES spectra showed that copper sulphate ions were reduced to zero charge copper in the GR suspension, whereas nickel sulphate ions were not reduced. Radial structural functions for iron obtained from Fe K edge EXAFS spectra indicated that the structure of GR consisted of corner sharing of structural units formed by FeO6 octahedra. Radial structural functions for copper and nickel also indicated that copper sulphate ions were transformed to metallic copper with the face-centered cubic structure in the GR suspension, whereas the local structure of nickel sulphate ions were unchanged. XANES and EXAFS data of GR oxidized by a H2O2 solution were also investigated for comparison.

AmorphousAl90FexCe10−xalloys: X-ray absorption analysis of the Al, Fe and Ce local atomic and electronic structures

X-ray-absorption fine structure (XAFS) above the Fe K edge, the Ce ${L}_{3}$ edge, and the Al K edge in amorphous ${\mathrm{Al}}_{90}{\mathrm{Fe}}_{x}{\mathrm{Ce}}_{10\ensuremath{-}x}$ $(x=3,$ 5, and 7) alloys have been measured and analyzed. Quantitative analyses of the Fe K-edge and Ce ${L}_{3}$-edge extended XAFS spectra are limited to local structure parameters of the first coordination sphere. Comparison of experimental x-ray-absorption near-edge structure (XANES) spectra with theoretical multiple-scattering (MS) XANES spectra for model compounds, such as crystalline ${\mathrm{FeAl}}_{6}$ and ${\mathrm{CeAl}}_{4},$ allows one to determine the local structure around the aluminum, iron, and Ce sites in ternary amorphous alloys. Using the theoretical MS approach, we show that the Fe and Al K-edge XANES are sensitive to the structure of coordination spheres, which extend up to nearly 4.4 and 3.3 \AA{}, respectively. The Ce ${L}_{3}$-edge XANES, on the other hand, is sensitive to the structure that extends up to 3.15 \AA{}.

Extended X-ray absorption fine structure and time differential perturbed angular correlation study of hafnium(IV) sorbed onto amorphous silica

Sorption of Hf(IV) onto amorphous silica at pH values between 1 and 7 is investigated by extended X-ray absorption fine structure (EXAFS) and time differential perturbed angular correlation (TDPAC) spectroscopies. Hafnium(IV) is studied as a chemical analogue for understanding the speciation of tetravalent actinides. In addition, 181Hf is a frequently investigated TDPAC probe nuclide. Spectra obtained from both techniques show that there is a change in Hf-species with sample preparation pH. Results from TDPAC measurements show that two similar surface binding sites exist for Hf(IV) on the silica surface at low pH values. At a pH above 3, a third, unspecific surface binding site for Hf(IV) begins to dominate. EXAFS investigations do not detect the presence of polynuclear surface species. Both TDPAC and Hf L3 edge EXAFS spectra of the sorption samples differ from that of a Hf(IV) solution or a Hf(IV) hydrolysis product. This indicates that the sorbed species are different from solution or simple hydrolysis species. Short Hf-O distances are obtained from theoretical fits to the EXAFS oscillations for all samples. This suggests that the primary interaction of Hf(IV) with the silica surface occurs via silanol groups. Differences in the electric field gradient surrounding sorbed Hf(IV) cations as a function of pH, observed in the TDPAC spectra, must result from changes in overall species geometry rather than a change in the primary Hf-silica surface interaction.

Multiple-edge EXAFS refinement: Short-range structure in liquid and crystalline Sn.

L-edge extended x-ray-absorption fine structure (EXAFS) spectra of solid and liquid tin have been collected by using synchrotron radiation. Information on local structure has been obtained by simultaneous analysis of Sn ${\mathit{L}}_{3}$-, ${\mathit{L}}_{2}$-, and ${\mathit{L}}_{1}$-edge EXAFS spectra by using an original data-analysis scheme based on the ab initio multiple-scattering GNXAS method. The use of the three edges allowed the extension of the energy range of the useful data increasing the accuracy of the derived structural parameters. Details of the multiple-edge data-analysis methodology are provided. The general interest of the method for multiple-edge structural studies of monatomic and multiatomic systems is discussed. A very good agreement with diffraction data is found for crystalline Sn. The first-neighbor distribution is studied as a function of temperature. Statistical errors on the structural parameters are provided. The model pair distribution function g(r) obtained by x-ray-diffraction data is refined in liquid Sn at T=245 \ifmmode^\circ\else\textdegree\fi{}C. The EXAFS structural signal is shown to be very sensitive to the short-range correlations. The foot of the first peak of the g(r) is found to be steeper and the mean distance is found slightly shorter than previously indicated. Error bars on the g(r), due to random errors in the EXAFS data, are also shown. The signature of a three-body signal assigned to short-range covalent tetrahedral configurations is evidenced. \textcopyright{} 1996 The American Physical Society.