K-edge Extended X-ray Absorption Fine Structure Research Articles

X-ray source improvements for EXAFS measurement on SGIII prototype facility

A proper x-ray source is crucial to extended x-ray absorption fine structure (EXAFS) measurement as parameter diagnosis for compressed materials on large laser facilities. In this article, modification of the CH capsule as an x-ray source has been done through simulations and experiments for the SGIII prototype laser facility. On the one hand, simulation results show that the radiation intensity of the CH capsule increases largely while wall thickness decreases. On the other hand, chlorine atoms whose atomic number is higher than that of carbon were brought into the capsule by doping on the inner side wall to enhance x-ray radiation intensity. CH capsules were designed, and experiments were carried out on the SGIII prototype laser facility. The x-ray source spectrum, size, radiation intensity, radiation pulse, and Ti K-edge EXAFS spectrum under ambient conditions were compared and discussed. Experimental results show that, with the doping of chlorine atoms and decreasing the wall thickness from 9 µm to 6 µm, the x-ray radiation intensity increases by about 63.5 times, energy efficiency is improved a lot, and other properties are also improved. The chlorine atom doped CH capsule with 6 µm wall thickness is proved to be the best x-ray source in the experiments in this article. Its size is 57 ± 0.5 µm, pulse width is 240 ps, and radiation intensity is about 9.1 times the undoped one with the same wall thickness. The methods can be extended to other laser facilities for the EXAFS measurement of other materials.

The effect of orbital-lattice coupling on the electrical resistivity of YBaCuFeO5 investigated by X-ray absorption

Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques were used to investigate the valence state, preferred orbital and local atomic structure that significantly affect the electrical and magnetic properties of a single crystal of YBaCuFeO5 (YBCFO). An onset of increase of resistivity at ~180 K, followed by a rapid increase at/below 125 K, is observed. An antiferromagnetic (AFM)-like transition is close to the temperature at which the resistivity starts to increase in the ab-plane and is also observed with strong anisotropy between the ab-plane and the c-axis. The XLD spectra at the Fe L3,2-edge revealed a change in Fe 3d eg holes from the preferential {bf{3}}{{boldsymbol{d}}}_{{{bf{x}}}^{{bf{2}}}{boldsymbol{-}}{{bf{y}}}^{{bf{2}}}} orbital at high temperature (300–150 K) to the {bf{3}}{{boldsymbol{d}}}_{{{bf{3}}{bf{z}}}^{{bf{2}}}{boldsymbol{-}}{{bf{r}}}^{{bf{2}}}} orbital at/below 125 K. The analysis of the Fe K-edge EXAFS data of YBCFO further revealed an unusual increase in the Debye-Waller factor of the nearest-neighbor Fe-O bond length at/below 125 K, suggesting phonon-softening behavior, resulting in the breaking of lattice symmetry, particularly in the ab-plane of Fe-related square pyramids. These findings demonstrate a close correlation between electrical resistivity and coupling of the preferred Fe 3d orbital with lattice distortion of a single crystal of YBCFO.

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.

Structural characterization of Zr-doped ZnO films deposited on quartz substrates by reactive radio frequency magnetron co-sputtering

Zr-doped ZnO (Zn1−xZrxO, 0 ≤ x ≤ 0.067; ZZO) thin films were grown at room temperature by reactive radio frequency co-sputtering on quartz substrates The ZZO films have a strong preferred orientation toward the c-axis but this orientation weakened as x increased. The size of crystallites of the ZZO films decreased and the surfaces of the ZZO films smoothened as x increased. The Zn atom were divalent and slightly affected only in the c-axis. Zn K-edge X-ray absorption near edge structure (XANES) spectra of the ZZO films show that the Zn atom were divalent and slightly affected only in the c-axis. The Zr atoms were tetravalent and their K-edge XANES spectra were slight different from that of ZrO2. Polarization-dependent Zn K-edge extended x-ray absorption fine-structure (EXAFS) spectra revealed that the local structural variation was stronger along the c-axis than along other axes. The Zn atom site was well replaced by Zr atom regardless of x. Average optical transmittance in the visible region was ≥90%, and as x increased, optical band gap increased and blue-shifted.

Synchrotron X-ray absorption spectroscopy study of disordered cation distribution of nanosized zinc ferrite powders

The zinc ferrite (ZnFe2O4) powders of various nanoparticle sizes were synthesised at different milling times (0 to 24 h) of the as-combusted powders. The non-equilibrium site occupancy of zinc (Zn2+) and ferric (Fe3+) ions was investigated through Zn and Fe K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. The XRD and SEM strongly confirm the particle size of these ferrites decreasing with the increasing milling time. Compared with the bulk specimen of zinc ferrite, both XANES and EXAFS spectra of zinc ferrite powders clearly exhibit the large translocation of Zn2+ ions from the tetrahedral (A) sites to the octahedral (B) sites and the opposite translocation of some of Fe3+ ions without affecting the long-range structural order. Moreover, the curve-fitting analysis of Zn and Fe K-edge EXAFS spectra indicates that the degree of inversion increases as the particle size decreases resulting in significant differences in the magnetic behaviours.

Investigation of the local environment of iodate in hydroxyapatite by combination of X-ray absorption spectroscopy and DFT modeling

Iodate-substituted hydroxyapatites are novel phases of potential interest for the immobilisation of radioactive iodine-129. However, the local environment of the iodine in these phases is still unclear. Here, a combined experimental–computational strategy has been used to investigate the mode of incorporation of iodates (IO3−) in Ca–hydroxyapatite and Sr–hydroxyapatite lattices. On one hand, I K- and L3-edge XANES (X-ray Absorption Near Edge Structure) spectra are presented, showing that while the local structure around the iodate is similar in the two substituted hydroxyapatite lattices, it differs significantly from the one observed in a series of model compounds (NaIO3, KIO3, Ca(IO3)2·H2O). I K-edge EXAFS (Extended X-ray Absorption Fine Structure) spectra were then analysed, revealing the lack of order around the iodate, and also the absence of local clustering of the iodates along the hydroxyl columns of the apatite. Further insight into the local environment of iodates in apatites was then obtained by DFT (Density Functional Theory) computational modeling of iodate-substituted Ca–hydroxyapatite lattices.

Metal–insulator transition in V1−xWxO2: structural and electronic origin

The driving mechanism of the metal-insulator transition (MIT) in VO(2) has always attracted attention, in particular with regards to understanding if and how the doping mechanism may tune the MIT transition temperature. However, due to the lack of detailed local structural information, in this oxide the underlying MIT mechanism is still matter of debate. In this contribution on the V(1-x)W(x)O(2) system, we attempt to clarify the origin of the MIT induced by tungsten doping. Combining W L(3)-edge and V K-edge extended X-ray absorption fine-structure (EXAFS) spectroscopy, the local structures around both V and W have been obtained. The data point out the occurrence of internal stress along the V-V chains induced by doping. It reaches a critical value that remains constant during the transition. The main effect of the internal stress on the vanadium local structure has also been identified. Actually, upon increasing the dopant concentration, the tilt of the V-V pairs towards the apex oxygen atoms in the VO(6) octahedron decreases while the V-V bond lengths remain unchanged. The electronic structure has also been investigated by O K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Actually, at high doping concentrations the interaction of O(2p) and the V d(∥) state increases, while the hybridization of O(2p) and V π* decreases. The O(2p)-V(3d) hybridization is therefore an essential parameter correlated with the decreasing transition temperature in the V(1-x)W(x)O(2) system.

Effects of concentration and freeze-thaw on the first hydration shell structure of Zn2+ ions

To investigate the effects of salt concentration and freeze-thaw (FT) on the first hydration shell of Zn2+ ions in Zn(NO3)2 aqueous solutions, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to examine Zn K-edge EXAFS spectra of Zn(NO3)2 aqueous solutions with various concentrations before and after FT treatment. The influences of salt concentration and freeze-thaw on the structural parameters of the first hydration shell of Zn2+ ions, including hydration number, Zn-O distance and thermal disorder, were analyzed. The results show that Zn2+ ions have 3.2–6.8 nearest oxygen neighbors with the Zn-O distance being 0.202–0.207 nm. In highly concentrated solutions, Zn2+ ions are hydrated with four water molecules in a tetrahedral form. The dilution of Zn(NO3)2 aqueous solutions increases the number of water molecules in the first hydration shell of Zn2+ ions to six with their octahedral arrangement. Both the hydration number in the first hydration shell of Zn2+ ions and the degree of thermal disorder increase when the FT treatment is operated in Zn(NO3)2 aqueous solutions.

CONTINUOUS-WAVE PHOTOLUMINESCENCE AND NANOSTRUCTURAL PROPERTIES OF POROUS SILICON

The photoluminescence (PL) emitted by porous silicon has been investigated by using the continuous tuneable UV Synchrotron Radiation Source. One sample was investigated for orange PL emission wavelength at temperatures 77–295 K. The PL peak is found to shift to higher frequency with decreasing temperature. Information about the nanostructure of porous silicon has been determined from PL and Extended X-ray Absorption Fine Structure (EXAFS), as well as from electron microscopy. In particular, the optical properties of silicon-based nanostructured materials, obtained from PL and photoluminescence excitation measurements, have been correlated with structural information from Si –K-edge EXAFS. Electron microscopy was used to study the relation between the nanostructure and PL of porous Si , and to investigate porous Si structure. Platelet Si and Si crystallites in porous Si layers were observed. The size of crystallites ranged from 4 to 6.5 nm. Diffraction patterns show these porous Si samples have a crystalline structure.

Local structural changes in paramagnetic and charge-ordered phases ofSm0.2Pr0.3Sr0.5MnO3: an EXAFS study

Sm0.5−xPrxSr0.5MnO3 exhibits a varietyof ground states as x is varied from 0 to 0.5. At an intermediate doping ofx = 0.3 a charge-ordered CE-type antiferromagnetic insulating (AFI) ground state is seen. Thetransition to this ground state is from a paramagnetic-insulating (PMI) phase through aferromagnetic-metallic phase (FMM). Local structures in PMI and AFI phases of thex = 0.3 sample have been investigated using Pr K-edge and Sm K-edge extended x-rayabsorption fine structure (EXAFS). It can be seen that the tilting and rotation of theMnO6 octahedraabout the b-axis are responsible for the charge-ordered CE-type antiferromagnetic ground state at lowtemperatures. In addition a shift in the position of the rare-earth ion along thec-axis has to be considered to account for observed distribution of bond distances aroundthe rare-earth ion.

Understanding Degradation of Doped Electroluminescent ZnS Phosphors

ABSTRACTWe have shown recently that the degradation of AC electroluminescence (EL) in ZnS:Cu,Cl and similar phosphors can be significantly reversed by a short anneal near 200°C. To better understand the degradation/rejuvenation processes, we investigated EL degradation and rejuvenation under different conditions. To probe for changes in the local atomic structure about the Cu sites, we collected Extended X-ray Absorption Fine Structure (EXAFS) data at the Cu K-edge EXAFS data for several as-made, thermally degraded (240°C anneal), and EL degraded powders.

Localization of excitation in InGaN epilayers

Energy scalability of the excitation–emission spectra of InGaN epilayers, quantum wells and light-emitting diodes provided indirect evidence for a fundamental common cause of the remarkable optical properties of this commercially important semiconductor alloy. Phase segregation on the nanoscale (accidental quantum dot formation) has generally been accepted as the mechanism of the spectral energy scaling (K.P. O’Donnell, R.W. Martin and P.G. Middleton, Phys. Rev. Lett. 82 237 (1999)). Recently, however, the downsizing of the InN bandgap, from 2 to about 1 eV, has prompted a re-examination of the observations. Here, we present new structural evidence of InGaN nanostructure, obtained from a comparative analysis of Ga and In K-edge EXAFS (extended X-ray absorption fine structure) of a wide range of In x Ga1− x N epilayer samples. The mean In–Ga and Ga–In next-nearest-neighbour (NNN) separations are found to be unequal in length for InN-poor (0.1 < x < 0.4) samples. The degree of inequality increases with decreasing InN fraction, x, and therefore correlates with luminescence efficiency in this range of alloy composition. We propose that the breakdown of In/Ga randomicity in InGaN alloys is associated with efficient excitation–emission in blue-green light-emitting devices. Although non-randomicity may lead to a weak quasi-localization of excitation, through the suppression of energy back-transfer, the issue of strong exciton localization in InGaN is not directly addressed by these results.

EXAFS study on dynamic structural property of porous morph-genetic SiC

Novel porous morph-genetic silicon carbide has been fabricated through sintering treatment, after infiltrating the methyl organic silicone resin to the bio-template. Its dynamic transition of structure during sintering process is investigated by extended X-ray absorption fine structure (EXAFS) for the first time. By analyzing Si K-edge EXAFS, it is found that the coordination number of the nearest C shell remains almost unchanged while that of the nearest Si shell dramatically changes when the structure is transformed from amorphous into crystalline state.

Arsenic speciation in synthetic jarosite

Iron sulfate minerals may incorporate arsenic released from oxidizing arsenian pyrite and other sulfide minerals. For example, in the southern Mother Lode gold district of California, As is concentrated in jarosite, ideally KFe 3(SO 4) 2(OH) 6, that formed by the oxidation of sulfides in mineralized outcrops and mine tailings containing ∼100 to ∼2000 ppm As. These weathering crusts, which are vulnerable to storm erosion, are of concern as a source for the transport of As into watersheds. Both arsenatian jarosite and scorodite [FeAsO 4·2H 2O] provide only temporary storage mechanisms for As in the environment because of the limited pH and redox conditions over which they are stable. To evaluate the extent of incorporation of As into jarosite, a series of jarosites in the system K 2O–Fe 2O 3–As 2O 5–H 2SO 4–H 2O was synthesized at 95 °C and 1 bar. Potassium arsenate was included at relative molar proportions of 1–50% As T,aq (defined as 100*As/(As+S)) in the starting solutions. Arsenic was incorporated into all samples at higher proportions relative to S than in the starting solutions. Synchrotron X-ray diffraction results indicated that jarosite was formed from starting solutions that contained up to 20% As T,aq. An X-ray amorphous phase also formed in samples containing As and, at 25% As T,aq in the starting fluid, only an amorphous phase was obtained. Starting fluids with 33% and 50% As T,aq produced poorly crystalline scorodite. Scanning electron microscope (SEM) images show textural differences among the jarosite samples, from relatively uniform anhedral to subhedral particles 0.5–1 μm across in low-As jarosite samples, to smaller grains (<0.2 μm) dispersed in a groundmass in the samples richer in As. Results from SEM, SXRD and Raman spectroscopy suggest that most As in the synthetic samples enters the amorphous phase, but quantitative analytical electron microprobe (AEM) analyses showed that the grains of jarosite contain up to ∼30% As T,s. Iron K-edge and As K-edge extended X-ray absorption fine-structure (EXAFS) spectra of the arsenatian jarosite showed As and Fe environments consistent with substitution of As for S. SXRD indicated unit-cell expansion with increased substitution of As in jarosite. The extent to which As is accommodated in the jarosite structure may be limited by the charge-balance mechanism and deficiency of Fe in octahedral sites. The X-ray amorphous material produced in the syntheses exhibits short-range order and is similar to scorodite over small domains, on the order of 10–14 Å. Raman spectroscopy showed several broad peaks in the 750–950-cm −1 Raman shift spectral region, associated with As–O vibrations. The relative intensities of bands related to As–OFe and As–O vibrations are also consistent with substitution of As for S in jarosite, and with small, scorodite-like domains in the amorphous material. Collectively, these data show that the strong association between Fe and arsenate during nucleation of the solid materials leads to As retention in the oxidized Fe minerals produced during weathering of arsenian pyrite and other As-rich sulfide minerals.

Compressed octahedral coordination in chain compounds containing divalent copper: structure and magnetic properties of CuFAsF6 and CsCuAlF6.

Crystal structures and magnetic investigations of CuFAsF6 and CsCuAlF6 are reported. Together with KCuAlF6, these appear to be the only examples of Jahn-Teller pure Cu(II) compounds containing only one type of ligand that exhibits a compressed octahedral coordination geometry. The Rietveld method has been used for refining the CsCuAlF6 structure based on neutron powder diffraction data at 4 K. The compound crystallizes in space group Pnma (no. 62) with a=7.055(1), b=7.112(1), c=10.153(1) A and Z=4 at 4 K. The structure is built from infinite [CuF5]n(3n-) chains of [CuF6]4- octahedra running along the [1 0 0] direction and (AlF6)3- octahedra connected by corners in the trans position, thus giving rise to chains oriented along the [0 1 0] direction. Single crystals of CuFAsF6 were prepared under solvothermal conditions in AsF5 above its critical temperature. The structure was determined from single-crystal data. CuFAsF6 crystallises in the orthorhombic space group Imma (No. 74) with a=10.732(5), b=6.941(3), c=6.814(3) A and Z=4 at 200 K. The structure can also be described in terms of one-dimensional infinite [CuF5]n(3n-) chains of tilted [CuF6](4-) octahedra linked by trans-vertices running along the b axis. The [CuF5]n(3n-) chains are connected through [AsF6]- units sharing joint vertices. The compressed octahedral coordination of CuII atoms in CuFAsF6 and CsCuAlF6 compounds at room temperature is confirmed by Cu K-edge EXAFS (extended x-ray absorption fine structure) analysis. For both compounds strong antiferromagnetic interactions within the [CuF5]n(3n-) chains were observed (theta(p)=-290+/-10 K and theta(p)=-390+/-10 K for CuFAsF6 and CsCuAlF6, respectively). The peculiar magnetic behaviour of chain compounds containing divalent copper at low temperature could be related to uncompensated magnetic moments in the one-dimensional network.

Fischer-Tropsch synthesis: study of the promotion of Pt on the reduction property of Co/Al2O3 catalysts by in situ EXAFS of Co K and Pt LIII edges and XPS.

The addition of platinum metal to cobalt/alumina-based Fischer-Tropsch synthesis (FTS) catalysts increases both the reduction rate and, consequently, the density of active cobalt sites. Platinum also lowers the temperature of the two-step conversion of cobalt oxide to cobalt metal observed in temperature programmed reduction (TPR) as Co3O4 to CoO and CoO to Co0. The interaction of the alumina support with cobalt oxide ultimately determines the active site density of the catalyst surface. This interaction can be controlled by varying the cobalt loading and dispersion, selecting supports with differing surface areas or pore sizes, or changing the noble metal promoter. However, the active site density is observed to depend primarily on the cluster size and extent of reduction, and there is a direct relationship between site density and FTS rate. In this work, in situ extended X-ray absorption fine structure (EXAFS) at the LIII edge of Pt was used to show that isolated Pt atoms interact with supported cobalt clusters without forming observable Pt--Pt bonds. K-edge EXAFS was also used to verify that the cobalt cluster size increases slightly for those systems with Pt promotion. X-ray absorption near-edge spectroscopy (XANES) was used to examine the remaining cobalt clusters after the first stage of TPR, and it revealed that the species were almost entirely cobalt (II) oxide. After the second stage of TPR to form cobalt metal, a residual oxide persists in the sample, and this oxide has been identified as cobalt (II) aluminate using X-ray photoelectron spectroscopy (XPS). Sequential in situ reduction of promoted and unpromoted systems was also monitored through XPS, and Pt was seen to increase the extent of cobalt reduction by a factor of two.

Local structure of amorphous ice as revealed by O K-edge EXAFS.

The oxygen K-edge extended X-ray absorption fine structure (EXAFS) spectrum of an ice film prepared by deposition of water vapor on a substrate at 100 K was measured in the surface-sensitive Auger yield mode. Five distinct peaks are revealed in the Fourier transform spectrum of the EXAFS data. The peaks are attributed to O-H bonds (with overlapping contributions from intramolecular covalent and intermolecular hydrogen bonds) as well as to intermolecular O...O scattering paths in the distance range of 1-7 A. The pattern of the longer O...O distances resembles that of a high-pressure crystalline modification of ice (ice II).

X-ray Excited Optical Luminescence Studies of InGaN and Rare-Earth Doped GaN Epilayers

ABSTRACTA successful attempt to use X-ray Excited Optical Luminescence (XEOL) for the detection of Extended X-ray Absorption Fine Structure (EXAFS) in III-nitrides is reported. The samples studied were InGaN and rare-earth (RE) doped GaN epilayers. For the first time Ga K-edge EXAFS oscillations were measured by monitoring the well-known “yellow” emission of GaN at 560 nm. The analysis of Optically Detected (OD) EXAFS data confirmed the expected local structure for Ga in GaN. The intensity oscillation of the “yellow” band when X-ray energy was scanned across the Ga K-edge indicates that core excitation of Ga atom has a high probability of transfer to defects responsible for “yellow” emission.

A study of the Nb3Ge system by Ge K-edge extended x-ray absorption finestructure and x-ray absorption near-edge structure spectroscopy

The local structure of Nb3Ge intermetallic superconductor has been studiedby Ge K-edge absorption spectroscopy. Extended x-ray absorption finestructure (EXAFS) experiments show two Ge–Nb distances. In additionto the crystallographic distance of ∼2.87 Å, there exists a second Ge–Nbdistance, shorter than the first by ∼0.2 Å, assigned to a phase with short-rangesymmetry related to local displacements in the Nb–Nb chains. The x-rayabsorption near-edge structure (XANES) spectrum has been simulated by fullmultiple-scattering calculations considering the local displacements determined bythe EXAFS analysis. The XANES spectrum has been well reproducedby considering a cluster of 99 atoms within a radius of about 7 Å fromthe central Ge atom and introducing determined local displacements.

EFFECT OF THE ANNEALING TEMPERATURE ON THE ELECTRONIC AND ATOMIC STRUCTURES OF EXCHANGE-BIASED NiFe–FeMn BILAYERS

In this study we measured the Fe, Mn, and Ni L2,3-edge X-ray absorption near-edge structure (XANES) and K-edge extended X-ray absorption fine structure (EXAFS) of the ferromagnetic (FM) NiFe and antiferromagnetic (AFM) FeMn bilayer films prepared with various annealing temperatures. The branching ratios of the white-line intensities in the Fe, Mn, and Ni L2,3-edges XANES spectra and consequently the magnetic properties of these exchange-biased FM NiFe - AFM FeMn bilayers are found to depend strongly on the annealing temperature. We find that the first peak in the Fe, Mn, and Ni K-edge EXAFS Fourier transform spectra are very similar, which suggests that the nearest-neighbor bond lengths among Fe, Mn, and Ni atoms are essentially the same in the NiFe–FeMn bilayers. However, the peaks at distances greater than ~ 3 Å appear to be sensitive to the annealing temperature especially for the Fe and Mn K-edge spectra, which suggests that annealing alters the atomic structures of the next-nearest-neighbor and more distant shells surrounding the Fe and Mn atoms in the NiFe–FeMn bilayers.