X-ray Absorption Near-edge Structure Region Research Articles

The bonding environment of chlorine in silicate melts

The local bonding environment of chlorine in silicate melts has a profound influence over the thermodynamic properties and structure of a melt, affecting the viscosity, rheology, and volatile degassing potential. To constrain the bonding environment of Cl in natural silicate melts, we have determined Cl K-edge X-ray absorption fine structure (XAFS) spectra for 44 experimentally produced silicate glasses in both the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. In the pre-edge region, the presence of a pre-edge peak indicates covalent bonding of chlorine with silicon. Addition of divalent cations suppresses this pre-edge feature, and its centroid shifts to higher energy, indicating a change to increasingly more ionic bonding. In the XANES region the main absorption edge energy, E0, and the energy of maximum intensity, EMax, are also compositionally dependent. SiO2- rich glasses have relatively low values of E0 and EMax while the addition of 2+ ions increases both to values close to those found in the end-member chlorides CaCl2, MgCl2, and FeCl2. In two Na-rich glasses, E0 and EMax are close to corresponding energies in NaCl. It appears, therefore that bonding in the glasses is closely related to that found in the simple chlorides. This may be due to clustering which generates CaCl, MgCl, FeCl and NaCl linkages either in the melts themselves or in the glasses due to rearrangements during quenching.The EXAFS parts of the glass spectra confirm the conclusions derived from the XANES region. These show that, as expected from the XANES region, addition of Ca and Fe2+ leads to R-space peaks which are closely related to those found in anhydrous CaCl2 and FeCl2 respectively.In order to determine if the spectra depend on pressure, temperature or chlorine fugacity of synthesis, 9 experiments were conducted using a single starting composition (Fe-free haplobasalt, An50Di28Fo22) across a range of temperatures (1300–1400 °C), pressures (5–20 kbar), chlorine fugacities (f(Cl2)) (1.38E−03 to 1.66E−06), and water contents (expected 0–8 wt% H2O). The results show that there is almost no change in the spectra across the XANES and EXAFS regions, indicating either that chlorine bonding is independent of the intensive parameters of the experiment or that all melts quench to glasses with the same local structure around the Cl atoms.

X-ray absorption spectroscopy study of the electronic structure and local coordination of 1st row transition metal-promoted Chevrel-phase sulfides

The electronic structures of S and Mo as well as the local coordination of Mo are investigated as a function of metal promotion Chevrel-phase (CP) sulfides. We observe the effect of metal promoter-induced electron donation into the stoichiometric range MxMo6S8 (M = Fe, Ni, Cu; x = 0–2) through analysis of X-ray absorption near-edge structure regions. We further observe the effect of this promotion on the bonding environment of Mo6 metal centers through extended X-ray absorption fine structure analysis. We monitor expansion and contraction of Mo6 octahedra with and without metal promotion, as has been predicted by Hückel molecular orbital theory. We further observe a marked tunability in the electronic structure of sulfur upon charge transfer between promoting species and Mo6S8 units. Average Mo6 octahedron Mo–Mo bond contraction from 2.76 Å to as short as 2.69 Å was observed upon incorporation of metal promoters, while intercluster separation displays a pronounced increase for promoter-host lattices compared to un-promoted Mo6S8. To corroborate spectroscopically observed phenomena, we performed computational analyses of spin-polarized densities of state for the CP materials investigated herein, where a detectable increase in sulfur-based frontier orbital population is observed in accordance with experimentally validated orbital filling.

Time-resolved XAFS measurement using quick-scanning techniques at BSRF.

A new quick-scanning X-ray absorption fine-structure (QXAFS) system has been established on beamline 1W1B at the Beijing Synchrotron Radiation Facility. As an independent device, the QXAFS system can be employed by other beamlines equipped with a double-crystal monochromator to carry out quick energy scans and data acquisition. Both continuous-scan and trapezoidal-scan modes are available in this system to satisfy the time scale from subsecond (in the X-ray absorption near-edge structure region) to 1 min. Here, the trapezoidal-scan method is presented as being complementary to the continuous-scan method, in order to maintain high energy resolution and good signal-to-noise ratio. The system is demonstrated to be very reliable and has been combined with in situ cells to carry out time-resolved XAFS studies.

Quantitative determination of cerium oxidation states in alkali-aluminosilicate glasses using M4,5-edge XANES

A calibration method for quantitative determination of Ce4+/Ce3+ in alkali-aluminosilicate glasses using Ce M4,5-edge X-ray absorption near edge structure (XANES) spectroscopy is presented. Samples of varying alkali oxide content synthesized over a range of temperatures and oxygen fugacities (fO2) were analyzed using both synchrotron-based XANES and traditional wet chemical techniques. Based on comparison to potentiometric determinations of Ce4+/Ce3+ in digested portions of our samples, Ce4+ was found to have a greater fluorescence yield (FLY) response to the incident radiation in the M4,5-edge XANES region than Ce3+. This results in an overestimation of the Ce4+/Ce3+ ratio when directly comparing the contributions of the two oxidation states to the total M-edge area, as has been used for crystalline materials. We offer a correction to M-edge determined Ce4+/Ce3+ ratios as follows:Ce4+/ΣCe=578.7Ce4+/ΣCe±5%XANESA±0.0040<Ce4+/ΣCe<0.4where the exponent A is a function of the number of non-bridging oxygens per tetrahedrally-coordinated cation (NBO/T) for a given glass composition, [Ce4+/ΣCe]XANES is the contribution from Ce4+ to the M4,5-edge area over the total area of the M4,5-edge, and Ce4+/ΣCe is the actual fraction of Ce4+ in the sample. The FLY response of Ce4+ was found to systematically increase with the degree of polymerization of the glass, suggesting that in addition to the local electronic environment, the FLY response of Ce is also dependent on longer range interactions.

EDS, HRTEM/STEM, and X-ray Absorption Spectroscopy Studies of Co-Substituted Maghemite Nanoparticles

A detailed study of the composition and structure of Co-doped maghemite nanoparticles with systematically varying composition has been carried out by transmission electron microscopy (TEM) techniques, such as high-resolution TEM, scanning TEM, and energy-dispersive X-ray spectrometry, and by X-ray absorption spectroscopy at the Fe and Co K-edges, analyzing both the extended X-ray absorption fine structure and the X-ray absorption near-edge structure regions. The latter techniques, in particular, allow us to determine the degree of inversion of divalent and trivalent metal ions among the octahedral and tetrahedral sites in the spinel structure of the nanoparticles and give detailed information on atomic distances. The samples consist of single-crystal nanoparticles with a composition corresponding to the Fe/Co ratio used in the synthesis. The degree of inversion is quite similar for all samples and close to the value found in a pure cobalt ferrite bulk sample.

XANES analysis of spectral properties and structures of arsenate adsorption on TiO2surfaces

X-ray absorption near-edge structure (XANES) of arsenate adsorption on TiO(2) surfaces was calculated using self-consistent multiple-scattering methods, allowing a structural analysis of experimental spectra. A quantitative analysis of the effect of disorder revealed that the broadening and weakening of the characteristic absorption in experimental XANES was due to the structural disorder of the arsenate-TiO(2) adsorption system. The success with calculating the scattering amplitude of a specific set of paths using the path expansion approach enables the scattering contributions of different coordination shells to the XANES to be sorted out. The results showed that the scattering resonances from high-level shells inherently overlapped onto the first-shell scattering amplitudes, and formed the fine structures in the XANES region. A variation in one oscillatory feature could be due to several structural changes affecting specific single/multiple-scattering amplitudes. Therefore, direct assignments of spectral features with structural elements should be based on adequate theoretical analysis.

Extended X-ray Absorption Fine Structure of Copper(II) Complexes at the Air-Water Interface by a Polarized Total-Reflection X-ray Absorption Technique

Copper(II) complexes spread on an aqueous solution surface were studied by a polarized total-reflection X-ray absorption fine structure (TR-XAFS) technique. The polarized TR-XAFS spectra at the Cu-K edge for copper(II) porphyrins and copper(II) chlorophyllin in a monolayer were measured in situ at the air-water interface. The polarization dependences of X-ray absorption near-edge structure (XANES) involving a 1s-->4p(z) transition allowed us to estimate the molecular orientation and the local coordination structure around the copper(II) atom in the polarization plane selectively. The extended X-ray absorption fine structure (EXAFS) region of the polarized TR-XAFS spectra for the metal complexes present at the air-water interface was successfully analyzed for the first time. The relative coordination number for the copper center evaluated from the EXAFS analysis indicated larger values in the vertical polarization than in the horizontal one, in agreement with the standing-up molecular orientation at the air-water interface estimated from the XANES region.

X-ray spectra and electronic band structure of nitrogen in Al x Ga1−x N solid solid solutions

The electronic energy structure of the valence band and the x-ray absorption near edge structure (XANES) region of nitrogen in AlxGa1−xN solid solutions and binary crystals of gallium nitride GaN and aluminum nitride AlN are calculated using the local coherent potential method and the cluster version of the muffin-tin approximation within the framework of the multiple scattering theory. It is demonstrated that the calculated electron densities of states correlate with the nitrogen K x-ray emission and nitrogen K x-ray absorption spectra. The electronic energy structure of the top of the valence band and the XANES region in AlxGa1−xN solid solutions are compared with those in the binary crystals of the GaN and AlN nitrides, and an interpretation of their specific features is proposed. An analogy is drawn between the evolution of the electronic energy structure of the valence band and the XANES region in the alloys under investigation and the evolution of the electronic band structure in the AlxB1−xN and BxGa1−xN alloys. General trends in the transformation of the structure and variations in properties of these alloys are discussed.

Speciation of Sulfate in Size-Fractionated Aerosol Particles Using Sulfur K-Edge X-ray Absorption Near-Edge Structure

X-ray absorption near-edge structure (XANES) at sulfur K-edge was used to determine the sulfate species present in size-fractionated aerosol particles based on the postedge structure after the main absorption peak in the XANES region. A comparison of the XANES spectra of reference sulfate materials and aerosol samples collected in Tsukuba in Japan clearly showed that (NH4)2SO4 was the main sulfur species in particles with a smaller diameter and CaSO4 x 2H2O (gypsum) was the main sulfur species in particles with a larger diameter. A simulation of the XANES spectra by reference materials allows us to obtain the quantitative mixing ratios of the different sulfate species present in the aerosol samples. The presence of minor sulfur species other than (NH4)2SO4 and gypsum at the surface of mineral aerosols is suggested in our simulations and by a surface-sensitive conversion electron/He-ion yield XANES. In the absence of a contribution from a large dust event, the mole concentration of gypsum in the mineral aerosol fraction (particle diameter > 1 microm) determined by XANES is similar to that of Ca which is determined independently using ion chromatography. This shows that the Ca and sulfate in the mineral aerosols are present only as gypsum. Considering that calcite is the main Ca mineral in the original material arising from an arid and semiarid area in China, it is strongly suggested that gypsum is formed in aerosol during its long-range transportation by a reaction between calcite and sulfate ions.

Synchrotron X-Ray Absorption Study of LiFePO4 Electrodes

was investigated in practical Li-ion intercalation electrodes used for Li-ion batteries by applying X-ray absorption spectroscopy (XAS). A special in situ XAS cell was constructed to charge and discharge the electrodes at current densities typically used in Li-ion batteries. Ex situ XAS measurements were made with technically relevant electrodes developed by Hydro-Québec power research using as an electrolyte and ethylene carbonate/dimethyl carbonate as a solvent. The X-ray absorption spectra were measured at the initial state [containing mainly Fe(II)], which corresponds to the uncharged state of the battery and at the delithiated state Fe(III)], which corresponds to the battery’s charged state. The X-ray absorption near-edge structure region of the X-ray absorption spectra showed a substantial Fe K-edge shift after the electrode was electrochemically oxidized. The pre-edge features of the X-ray absorption spectra revealed high spin configuration for the two oxidation states [Fe(II), and Fe(III), Reasonable ligand field stabilization energies could be extracted from the pre-edge features. Using the data analysis program EXAFSPAK with the FEFF8 code provided in this program, the EXAFS spectrum and its Fourier transformed radial structure function could be simulated satisfactorily, where only single scattering paths were used for this simulation. © 2004 The Electrochemical Society. All rights reserved.

Coordination chemistry of Ti(IV) in silicate glasses and melts: I. XAFS study of titanium coordination in oxide model compounds

The coordination environment of Ti(IV) in a variety of Ti-bearing crystalline oxide and silicate model compounds has been studied using Ti K-edge x-ray absorption fine structure (XAFS) spectroscopy at ambient temperature and pressure in order to provide a quantitative basis for interpreting Ti K-edge XAFS spectra of silicate glasses and melts (Parts II, III, and IV) Farges and Brown, 1996; Farges et al., 1996a,b). Pre-edge features of Ti K-edge XAFS spectra can be used to derive accurate information on the local coordination environment of Ti only if both pre-edge position and heights are considered. Using these features, it is also possible to distinguish between one coordination environment vs. a mixture of several others (e.g., [5]Ti vs. [4]Ti + [6]Ti). Quantitative analysis of the Ti x-ray absorption near edge structure (XANES) spectra, based on ab-initio multiple-scattering calculations for a variety of Ti-containing clusters and anharmonic analysis of the normalized XAFS oscillations, show that O first neighbors and alkali/alkaline-earth second neighbors around Ti contribute to the XAFS spectra. However, second neighbors are more prominent in the XANES region because the effects of disorder associated with these contributions are less important in this region than in the extended XAFS (EXAFS) region. Therefore, XANES spectra can be used to probe the degree of disorder in the medium-range structural environment around Ti in crystalline and amorphous materials, including Ti-bearing aperiodic structures, such as metamict, glassy, and molten compounds.

Investigations of P4O6Sn (n = 1−4) by X-ray Absorption Spectroscopy at the P and S K-Edges

X-ray absorption spectra at the P and S K-edges are reported for the molecules P4O6Sn (n = 1−4) in the gas phase. The X-ray absorption near edge structure (XANES) region is shown to be very sensitive to the valency of the excited phosphorus atom. P4O6 and (C6H5O)3PS are used as reference compounds corresponding to the different chemical environments of phosphorus in the investigated phosphorus oxide sulfides. The XANES spectra of the phosphorus oxide sulfides can be represented by a superposition of the P4O6 and (C6H5O)3PS spectra. Using XANES as a fingerprint method, the single lines in the XANES region are assigned to molecular fragments. The results are compared with MS Xα calculations of the PO3S cluster, which is assumed to contain all the interactions that are relevant for the P(V) portion in the molecules.

X-ray absorption spectroscopy in chemistry: II. X-ray absorption near edge structure

The edges in X-ray absorption spectra exhibit fine structures designated as XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure). The information content of EXAFS with regard to interatomic distances, coordination number, and the determination of degrees of order was presented in the first part of this article [Trends Anal. Chem., 11 (1992) 218]. The second part deals with the fine structures near to the edge, the XANES region, which contains information about the electronic and the geometric structure of the absorbing atom.

X-Ray Photoacoustic Spectroscopy of CuCl, CuBr, and CuI

The hard X-ray absorption spectra of CuCl, CuBr, and CuI have been measured at X-ray absorption near-edge structure (XANES) regions with a photoacoustic detector using synchrotron radiation. These spectra were compared with optical density of X-ray spectra. It is shown that the information derived from XANES is also included in the X-ray photoacoustic spectrum which reflects the heat production processes in those Cu compounds. However, the results showed that the increases and changes of the photoacoustic signal were different from those of the X-ray absorption coefficient at XANES regions.

Hard X-Ray Absorption Spectroscopy of Copper Alloys and Compounds with a Photoacoustic Detector

The hard X-ray spectra of brass, phosphor bronze, CuInSe2 and CuO have been measured at XANES (X-ray absorption near-edge structure) regions with a photoacoustic detector using synchrotron radiation. These spectra were compared with optical density of X-ray spectra. It is shown that the information derived from XANES is also included in the X-ray photoacoustic spectrum which reflects the heat production processes in those alloys and compounds. However, the results showed that the increases and changes of the photoacoustic signal were different from those of the X-ray absorption coefficient at XANES regions.

Investigation of the ‘‘join’’ between the near edge and extended x-ray absorption fine structure

The L1 and L3 x-ray absorption fine structures from the same element in the same material were compared to illustrate the ‘‘join’’ between the x-ray absorption near edge structure (XANES) and the extended x-ray absorption fine structure (EXAFS). In the XANES region the two spectra differed because of the different selection rules; however, the EXAFS spectra were very similar when adjusted for the phase shift. For the oxide samples investigated the L1,3 spectra were similar for E&amp;gt;20 eV which identifies the demarcation between XANES and EXAFS.

X-ray photoacoustic absorption spectra of CuInSe 2

The X-ray photoacoustic absorption spectrum (Cu K-edge) of CuInSe 2 , which has a chalcopyrite structure and applications to solar cells, has been measured together with that of pure Cu metal as a reference, using synchrotron radiation. It is shown that the information derived from X-ray absorption near edge structure (XANES) is also included in the photoacoustic X-ray absorption spectrum which reflects heat generation processes in material. The first energy peak value in the XANES region in CuInSe 2 shifts to lower energy than that of pure Cu metal, indicating a chemical shift in CuInSe 2 .

ＸＡＮＥＳおよびＥＸＡＦＳの原理と応用

In this review X-ray Absorption Near Edge Structure (XANES) is compared with EXAFS from the theoretical view point. In the XANES region, the full multiple scattering becomes important, but in the EXAFS region it is not important except the special case. The information about the geometric and the electronic structures obtained from the XANES is illustrated in some recent results.