Extended X-ray Absorption Fine Structure Signal Research Articles

Assessment of structural order indices in kaolinites: A multi-technique study including EXAFS

This work presents a study of the structural properties on four representative kaolinite samples. Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Thermochemical Analyses (DTA/TGA), and X-ray Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy measurements were carried out, and different empirical indices that quantify the structural order were assessed. Also, XRD patterns and EXAFS signals were compared to those corresponding to idealized defect-free structures calculated theoretically, to analyze the effects related to the structural disorder on real samples. The structural order at different scales (crystallographic, superficial, and local) was comprehensively examined for the four samples, and correlations between the order indices were performed and discussed, including the Debye-Waller factor obtained from Si K-edge EXAFS measurements. The proposed methodology shows that EXAFS is a suitable complementary technique that can provide valuable information about the Si local environments in natural kaolinites.

High pressure atomic structure of Zr–Cu metallic glass via EXAFS spectroscopy and molecular dynamics simulations

ABSTRACTIn this work, we use extended X-ray absorption fine structure (EXAFS) data collected using nano-polycrystalline diamond anvil cell to study the atomic arrangement in Zr–Cu metallic glass in high pressure (HP) conditions. To reveal the microscopic details of stress accommodation mechanism, we performed molecular dynamics (MD) simulations of the HP atomic arrangement. By comparing the experimental and the calculated Zr and Cu K-edge EXAFS signal we prove the realistic character of the computer simulations. A detailed geometrical analysis of the simulated atomic configurations shows that with increasing hydrostatic pressure the local structure of Zr–Cu amorphous alloy becomes gradually dominated by Cu-centred icosahedral structural motifs involving fivefold symmetry incompatible with crystalline ordering. The variation of the short-range order is attributed to preferential straining of mechanically soft bonds between zirconium atoms.

Fluorescence Extended X-Ray Absorption Fine Structure Study on Local Structures of Rare-Earth-Doped InGaGdN

Local structures around gadolinium atoms in rare-earth (RE)-doped InGaGdN thin films were studied by means of fluorescence extended X-ray absorption fine structure (EXAFS) measured at the Gd LIII-edges. The samples were doped with Gd in-situ during growth by plasma-assisted molecular beam epitaxy (PAMBE). Gd LIII-edge EXAFS signal from the GaGdN, GdN and Gd foil were also measured as reference. The X-ray absorption near edge structure (XANES) spectra around Gd LIII absorption edge of InGaGdN samples observed at room temperature indicated the enhancement of intensities with the increase of Gd composition. Further EXAFS analysis inferred that the Gd atoms in InGaN were surrounded by similar atomic shells as in the case of GaGdN with the evidence indicating majority of Gd atoms substituted into Ga sites of InGaGdN. A slight elongation of bond length for the 2nd nearest-neighbor (Gd–Ga) of sample with higher Gd concentration was also observed.

Evidence for nanoscale two-dimensional Co clusters inCoPt3 films with perpendicular magnetic anisotropy

The length scale of the local chemical anisotropy responsible for thegrowth-temperature-induced perpendicular magnetic anisotropy of face-centered cubicCoPt3 alloy films was investigated using polarized extended x-ray absorption fine structure(EXAFS). These x-ray measurements were performed on a series of four (111)CoPt3 films epitaxially grown on (0001) sapphire substrates. The EXAFS data show a preferencefor Co–Co pairs parallel to the film plane when the film exhibits magnetic anisotropy, andrandom chemical order otherwise. Furthermore, atomic pair correlation anisotropy wasevidenced only in the EXAFS signal from the next neighbors to the absorbing Co atomsand from multiple scattering paths focused through the next neighbors. This suggests thatthe Co clusters are no more than a few atoms in extent in the plane and onemonolayer in extent out of the plane. Our EXAFS results confirm the correlationbetween perpendicular magnetic anisotropy and two-dimensional Co segregation inCoPt3 alloy films, and establish a length scale on the order of 10 Å for the Co clusters.

Wavelet data analysis of EXAFS spectra

The application of wavelet transform to the analysis of the extended X-ray absorption fine structure (EXAFS) from perovskite-type compounds is presented on the example of the Re L 3-edge in ReO 3 and Co K-edge in LaCoO 3. We propose a modified wavelet transform procedure, which allows better discrimination of the overlapped contributions into the EXAFS signal.

Extended X-Ray Absorption Fine Structure Study of Arsenic in HgCdTe

Extended x-ray absorption fine structure (EXAFS) experiments using the As K-edge have been carried out at the European Synchrotron Radiation Facility on two molecular beam epitaxy (MBE) grown arsenic-doped HgCdTe samples. Arsenic (As) is provided by a radio-frequency plasma cell and was incorporated to a level of a few 1018 at·cm−3. Both samples were analyzed as grown and after a high-temperature anneal (410°C/1 h) under saturated mercury pressure. The EXAFS signature of as-grown and annealed samples are strikingly different, indicating a drastic change in the environment of the As atom. In any case, the EXAFS signal originates from at least two different contributions and is found to be dominated by As clusters. The other contribution for as-grown samples comes from tellurium neighbors indicating that As incorporates partially in the (Cd,Hg) site.

A Coupled Car-Parrinello Molecular Dynamics and EXAFS Data Analysis Investigation of Aqueous Co2+

We have studied the microscopic solvation structure of Co(2+) in liquid water by means of density functional theory (DFT)-based Car-Parrinello molecular dynamics (CPMD) simulations and extended X-ray absorption fine structure (EXAFS) data analysis. The effect of the number of explicit water molecules in the simulation box on the first and second hydration shell structures has been considered. Classical molecular dynamics simulations, using an effective two-body potential for Co(2+)-water interactions, were also performed to show box size effects in a larger range. We have found that the number of explicit solvent molecules has a marginal role on the first solvation shell structural parameters, whereas larger boxes may be necessary to provide a better description of the second solvation shell. Car-Parrinello simulations were determined to provide a reliable description of structural and dynamical properties of Co(2+) in liquid water. In particular, they seem to describe both the first and second hydration shells correctly. The EXAFS signal was reconstructed from Car-Parrinello simulations. Good agreement between the theoretical and experimental signals was observed, thus strengthening the microscopic picture of the Co(2+) solvation properties obtained using first-principle simulations.

X-ray absorption spectroscopy of strongly disordered glasses: Local structure around Ag ions ing−Ag2O∙nB2O3

The local structure around Ag ions in silver borate glasses $g\text{\ensuremath{-}}{\mathrm{Ag}}_{2}\mathrm{O}∙n{\mathrm{B}}_{2}{\mathrm{O}}_{3}$ $(n=2,4)$ was studied by x-ray absorption spectroscopy at the Ag $K$ edge for temperatures from $77\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}450\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Extended x-ray absorption fine structure (EXAFS) analysis based on cumulant expansion or multishell Gaussian model fails for these systems. Therefore, the radial distribution functions (RDFs) around Ag ions were reconstructed using a method based on the direct inversion of the EXAFS expression. The RDFs consist of about eight atoms (oxygens and borons), exhibit a relatively weak temperature dependence, and indicate the presence of strong static disorder. Two main components can be identified in RDFs, located at about $2.3--2.4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $2.5--3.4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, respectively. The chemical types of atoms contributing to the RDF were determined via a simulation of configurationally averaged x-ray absorption near-edge structure (XANES) and EXAFS signals. The immediate neighborhood of Ag contains mostly oxygens while borons dominate at larger distances. The combination of EXAFS and XANES techniques allowed us to determine a more complete structural model than would be possible by relying solely on either EXAFS or XANES alone.

Chemical stability of Ni-enriched nanodomains in alkali borate glasses

The stability of Ni nanodomains observed in low alkali borate glasses (10 mol% alkali oxide) has been investigated at higher alkali contents using Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. EXAFS data indicate that the Ni nanodomains disappear progressively when adding alkali oxide, whatever the nature of the alkali. This medium range reorganization around Ni is associated with a change of the local site from 6- to 5-fold coordination at 27–30 mol% alkali oxide and 4-fold coordination at higher alkali oxide content. Ni–Ni connections are still present for [5]Ni but no second neighbors are observed for [4]Ni in the EXAFS signal. The changes at both the local and medium range around Ni are likely associated to the disappearance of the large, rigid borate groups with the addition of alkali oxide.

Crystal chemistry of uranium (V) and plutonium (IV) in a titanate ceramic for disposition of surplus fissile material

We report X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra for the plutonium LIII and uranium LIII edges in titanate pyrochlore ceramic. The titanate ceramics studied are of the type proposed to serve as a matrix for the immobilization of surplus fissile materials. The samples studied contain approximately 10 wt% fissile plutonium and 20 wt% natural uranium, and are representative of material within the planned production envelope. Based upon natural analogue models, it had been previously assumed that both uranium and plutonium would occupy the calcium site in the pyrochlore crystal structure. While the XANES and EXAFS signals from the plutonium LIII are consistent with this substitution into the calcium site within pyrochlore, the uranium XANES is characteristic of pentavalent uranium. Furthermore, the EXAFS signal from the uranium has a distinct oxygen coordination shell at 2.07 Å and a total oxygen coordination of about 6, which is inconsistent with the calcium site. These combined EXAFS and XANES results provide the first evidence of substantial pentavalent uranium in an octahedral site in pyrochlore. This may also explain the copious nucleation of rutile (TiO 2) precipitates commonly observed in these materials as uranium displaces titanium from the octahedral sites.

Local order around of germanium atoms in Ga 10Ge 25S 65 glass by EXAFS

Irreversible photoexpansion effect has been observed in amorphous Ga 10Ge 25S 65 glasses when its surface was exposed to light with energy greater than the band gap, 3.52 eV. A volume change of about 5% was reached in bulk samples by controlling illumination time and the laser power density. To understand the atomic scale processes of the photoexpansion effect, extended X-ray absorption fine structure (EXAFS) spectroscopy has been used as a local probe of the germanium environment in the glass samples before and after illumination. Modifications are observed in the average coordination shell around Ge atoms in the illuminated sample compared to the non-illuminated one. For the non-illuminated sample, the Ge coordination shell is described by a distorted tetrahedron of sulfur atoms at around 2.20 Å. After illumination, the EXAFS signal can be explained by introducing an additional contribution to this average environment. Based on an analysis of the EXAFS data we proposed a two-shell model of 0.5 oxygen atoms at 2.01 Å and 3.6 sulfur atoms at a 2.20 Å. The existence of Ge–O bonds in the glass after illumination was confirmed by infrared measurements.

Exafs study of the local atomic structure of an Al 5Ti 2Cu alloy

The aim of the present paper is to investigate the local atomic arrangement of an Al{sub 5}Ti{sub 2}Cu alloy, in order to determine if it is consistent with a cubic or tetragonal symmetry. From this point of view, the Extended X-ray Absorption Fine Structure (EXAFS) seems the most suited method, as it allows one to determine the local chemical environment of atoms. The analysis of the local order in the Al{sub 5}Ti{sub 2}Cu alloy is consistent with a random distribution f copper on the Al sites rather than with a preferential ordering on these sites. This means that the structure of this alloy should exhibit a cubic symmetry. However, the EXAFS signals do contain a very large amount of structural information. In the future, it would be interesting to extend the k range analysis, principally at the Cu-K edge by acquiring the data at liquid nitrogen temperature and to include multiple scattering effects in the analysis of the spectra at the Ti-K edge.

EXAFS study in organic superconductors

Abstract Cu K-edge extended X-ray absorption fine structure (EXAFS) measurements have been made in the organic superconductors, ϰ-(BEDT-TTF)2Cu(NCS)2 and ϰ-(BEDT-TTF)2Cu[N(CN)2]Br from 300 K down to 20 K. Some anomalies of the local structure were found from the temperature dependence of EXAFS signal, in particular nearest neighbors distance (R). The anomalies can be ascribed to the hydrogen bonds between cations and anions.

X‐ray absorption of As low‐energy ion implanted into Si(100) grown by molecular‐beam epitaxy

Information about the local environment of dopants in semiconductor materials is required to understand and help optimize the electrical activity in highly doped samples. The near‐edge (XANES) and extended x‐ray absorption fine structure (EXAFS) components of x‐ray absorption spectra provide detailed information on local geometric and electronic structure about selected atoms. Electron yield and fluorescence detection of As K EXAFS and XANES, with sample rotation to eliminate substrate diffraction, have been used to study the As sites in Si(As) epilayers grown by molecular‐beam epitaxy (MBE) and doped in situ by low‐energy As implantation. Marked differences are found in both the near edge and EXAFS signals between samples prepared under low temperature (460 °C) growth/implantation conditions and those prepared under the preferred, higher temperature (700 °C) growth conditions. The EXAFS analysis indicates that As is located primarily in the substitutional site in the optimal (700 °C) sample but that a la...

Effect of thermal treatment on cation local structure in manganese-neutralized sulfonated polystyrene ionomers

Two sets of manganese-neutralized sulfonated polystyrenes, at 2.6 and 7.6 mol % sulfonation, were subjected to three sample treatments: solvent casting from tetrahydrofuran/water, solvent casting followed by annealing, and compression molding. Both as-cast samples were void of ionic aggregates, as evidenced by the absence of a peak in their small-angle X-ray scattering (SAXS) patterns. Upon annealing, the SAXS peak appeared, indicating the formation of ionic aggregates. The compression-molded 7.6 mol % material also exhibited the SAXS peak, while the compression-molded 2.6 mol % material did not, possibly due to a slower rate of aggregation in the lower ion content material. The local coordination structure about the Mn2+ cation was examined by extended X-ray absorption fine structure (EXAFS) spectroscopy and found to be nearly identical for all six samples despite the differences in the state of aggregation. That the local structure is unaffected by the microdomain structure reflects the difference in size scale probed by SAXS and EXAFS. When a nonionic monomer is copolymerized with a small amount of ionic comonomer, profound changes in prop- erties are observed relative to the nonionic parent polymer. The ionic copolymers, termed ionomers, typically contain less than 10 mol % of a sulfonic or carboxylic acid-bearing monomer and are neutralized with metal cations. The marked increases in, for example, modulus and tear and abrasion resistance and the dramatic alterations in ion- transport propertie~l-~ that occur in these materials are now accepted as being due to aggregation of the ionic groups4i5 into microdomains. Because of the small size of these ionic aggregates, diffraction techniques can reveal little regarding their in- ternal structure. On the other hand, extended X-ray ab- sorption fine structure (EXAFS) spectroscopy is a powerful technique for determining the local atomic environment about a specific atom and has been profitably applied to numerous ionomers.6-17 The EXAFS signal is a modula- tion of the X-ray absorption coefficient on the high-energy side of an element's absorption edge, and it contains in- formation on the coordination shells surrounding that atom. The type and number of atoms in a shell, as well as the distance to and disorder within a shell, are reflected in the EXAFS data. As such, EXAFS can probe the en- vironment around ions that reside in ionic aggregates, as well as dispersed ionic groups and even ions in solution.1° One ionomer that has been studied in detail by nu- merous in~estigators~~~ ~~'~~~~~~

Standing-wave-assisted extended x-ray absorption fine-structure study of a Ni-Ti multilayer

Extended x-ray absorption fine structure (EXAFS) measurements have been made while simultaneously exciting x-ray standing waves in a multilayer structure. This allows the EXAFS signal from selected regions within the multilayer unit cell to be enhanced. The technique is applied to a Ni-Ti multilayer in which it is verified that strong standing-wave fields can be excited in the presence of substantial interfacial roughness. The measurements reveal little Ni-Ti intermixing and a strong low-Z impurity signal in the Ti layers. The standing-wave analysis indicates that the low-Z impurity is distributed throughout the Ti layers.

Extended x-ray-absorption fine-structure studies of diamond and graphite.

We have recorded the extended x-ray-absorption fine structure (EXAFS) above the carbon K edge for diamond and highly oriented crystalline graphite using 250--800-eV synchrotron radiation. The spectra are used to test phase and amplitude transferability of the C-C EXAFS signal between the two bonding forms of carbon. We find excellent phaseshift transferability with errors in distances less than 0.01 A\r{}. Amplitude transferability is worse but better than 20%. Implications of our results for structural determinations of C-C bonds in molecules and solids are discussed.

Combined extended x-ray-absorption fine structure and diffraction study of Kr adsorbed on graphite

We report the results of a combined extended x-ray-absorption fine structure (EXAFS) and x-ray diffraction study of Kr adsorbed on a ZYX graphite substrate. The data are consistent with simple models for atomic fluctuations, which are found to be surprisingly large. Thermal fluctuations weaken the Kr-Kr EXAFS signal, so that the average number of Kr neighbors and the EXAFS Debye-Waller factor cannot be determined independently. The disordered phase at the commensurate-incommensurate transition appears to have substantial local order in the Kr-C signal, consistent with the domain-wall description of that phase.

Extended x-ray absorption fine structure: Studies of zinc-neutralized sulfonated polystyrene ionomers

Extended x-ray absorption fine structure (EXAFS) measurements were performed on a series of zinc-neutralized sulfonated polystyrene ionomers and the local structure around the zinc atom was determined. An interference effect in the EXAFS signal between sulfur and oxygen atoms was found to be significant in these materials. A model for the local structure in the zinc-neutralized sulfonated polystyrene ionomers is proposed which suggests a highly ordered tetrahedral coordination of oxygen around the zinc atoms at a distance of 1.97±0.02 Å. In addition there are four sulfur atoms and four oxygen atoms at a distance of 3.15±0.05 Å. No zinc-zinc coordination within 5 Å was detected in this study.

Double flourescence detection of XANES and EXAFS signals from dilute species in competing fluorescent matrices

A double flourescence scheme for detecting XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure) signals from a dilute species in a bulk competing flourescent matrix is reported. The new scheme has been demonstrated for the case of a Western Australian mineral specimen containing ∼ 900 ppm of Au, and is particularly suitable for chemical and structural investigation of low concentration species in a matrix of similar, but slightly lower atomic number(s). The technique has the advantage of largely eliminating unwanted background arising from the bulk matrix.