Extended X-ray Absorption Fine 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.

EXAFS of titanium LIII edge using a compact laboratory system based on a laser-plasma soft X-ray source

We present results of applying a compact laboratory system based on a laser-plasma soft X-ray (SXR) source to the extended X-ray absorption fine structure (EXAFS) spectroscopy of titanium in the vicinity of the LIII absorption edge. The source, operating with a plasma produced as a result of irradiation a krypton/helium double-stream gas-puff target, was optimized for an intense emission in the spectral range between 200 and 700 eV. A broad SXR spectrum and high photon flux are indispensable in the EXAFS studies. The experimental setup assured the simultaneous acquisition of both the reference and the absorption spectra. A grazing-incidence flat-field spectrometer was used to record the spectra. The absorption spectra of a thin (200-nm thick) titanium sample revealed the features in the EXAFS region allowing for quite accurate determination of the radial distances between the atoms. Results are in good agreement with the output of numerical modeling based on photoelectron wave function scattering and with the data reported for using a synchrotron source. This confirms the suitability of the source for using it in the standard EXAFS method.

Chemical-state-selective X-ray absorption spectroscopy by detecting bond-specific Auger electrons for SiO2/SiC interface

Chemical-state-selective Si K-edge extended X-ray absorption fine structure (EXAFS) measurements of SiO2 and SiC are demonstrated by detecting bond-specific Auger electrons in SiC coated with a very thin SiO2 film. Differential-electron-yield (DEY) mode is used for the measurements. Each EXAFS spectrum may be subject to the following two spectrally overlapping influences: (i) the background spectrum formed by energy-losing SiC Auger electrons overlaps the SiO2 Auger peak, and (ii) the resonant SiO2 Auger peak overlaps the SiC Auger peak. The SiO2- and SiC-selective DEY–EXAFS spectra differ from each other and are similar to the spectra of bulk SiO2 and SiC, respectively, in the EXAFS regions, indicating that the two influences are negligible, and that this method can be considered valid for selection of chemical states.

Accurate predictions of iron redox state in silicate glasses: A multivariate approach using X-ray absorption spectroscopy

Pre-edge features in the K absorption edge of X-ray absorption spectra are commonly used to predict Fe3+ valence state in silicate glasses. However, this study shows that using the entire spectral region from the pre-edge into the extended X-ray absorption fine-structure region provides more accurate results when combined with multivariate analysis techniques. The least absolute shrinkage and selection operator (lasso) regression technique yields %Fe3+ values that are accurate to ±3.6% absolute when the full spectral region is employed. This method can be used across a broad range of glass compositions, is easily automated, and is demonstrated to yield accurate results from different synchrotrons. It will enable future studies involving X-ray mapping of redox gradients on standard thin sections at 1 × 1 μm pixel sizes.

Theoretical calculations of X-ray absorption spectra of a copper mixed ligand complex using computer code FEFF9

The terms X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) refer, respectively, to the structure in the X-ray absorption spectrum at low and high energies relative to the absorption edge. Routine analysis of EXAFS experiments generally makes use of simplified models and several many-body parameters, e.g. mean free paths, many-body amplitude factors, and Debye-Waller factors, as incorporated in EXAFS analysis software packages like IFEFFIT which includes Artemis. Similar considerations apply to XANES, where the agreement between theory and experiment is often less satisfactory. The recently available computer code FEFF9 uses the real-space Green's function (RSGF) approach to calculate dielectric response over a broad spectrum including the dominant low-energy region. This code includes improved treatments of many-body effects such as inelastic losses, core-hole effects, vibrational amplitudes, and the extension to full spectrum calculations of optical constants including solid state effects. In the present work, using FEFF9, we have calculated the X-ray absorption spectrum at the K-edge of copper in a complex, viz., aqua (diethylenetriamine) (isonicotinato) copper(II), the crystal structure of which is unknown. The theoretical spectrum has been compared with the experimental spectrum, recorded by us at the XAFS beamline 11.1 at ELETTRA synchrotron source, Italy, in both XANES and EXAFS regions.

Low kinetic energy photoelectron diffractions for C 1s and O 1s electrons of free CO molecules in the EXAFS region

We have measured molecular-frame photoelectron angular distributions from carbon 1s and oxygen 1s levels of CO molecules up to a photoelectron kinetic energy (KE) of ∼150 eV. The backward-scattering intensities exhibited a strong modulation as a function of the kinetic energy of the photoelectrons, whereas the intensities for the forward-scattering gradually increased and then became nearly constant over KE ∼100 eV. Multiple scattering calculations with a muffin-tin potential qualitatively reproduced the experimental results. The present results may be considered as the observation of low-energy photoelectron diffraction patterns for gaseous free CO molecules, which are involved in modulations in extended X-ray absorption fine structure (EXAFS) spectra.

Arsenic Speciation in Biological Samples Using XAS and Mixed Oxidation State Calibration Standards of Inorganic Arsenic

The speciation of elements without pre-edge features preformed with X-ray absorption near edge structure (XANES) can lead to problems when the energy difference between two species is small. The speciation of arsenic (As) in plant samples was investigated using the mixtures As2S3/As2O5, As2S3/As2O3, or As2O3/As2O5. The data showed that the energy separation (eV) between As2O5 and As2S3 was 5.8, between As2O3 and As2O5 was 3.6, and between As2S3 and As2O3 was 2.1. From the intensity of the white-line feature and the concentration of As species, calibration curves showing a limit of detection of approximately 10% were generated. In addition, an error of +/-10% was determined for the linear combination-XANES (LC-XANES) fitting technique. The difference between the LC-XANES fittings and calculations from the calibration curves was <10%. The data also showed that the speciation of As in a sample can be determined using EXAFS (extended X-ray absorption fine structure). Finally, it was also shown that both EXAFS and XANES of the sample should be examined to determine the true speciation of an element. Even though there is a difference of 2 eV between As(III) bound to O and As(III) bound to S, in the EXAFS region the As(III)-S and As(III)-O ligands are clearly visible. However, distinction between the As(III)-O and As(V)-O ligands in the EXAFS spectra was not clearly visible in this study.

Local surrounding of Mn in LaMn 1−xCo xO 3 compounds by means of EXAFS on Mn–K

A systematic study of LaMn 1− x Co x O 3 perovskite series by means of X-ray absorption spectroscopy in the extended X-ray absorption fine structure (EXAFS) range of the K-absorption edge of Mn is reported. The Mn–K edge absorption measurements in the EXAFS region were performed to study the local surrounding of Mn ions. Polycrystalline powder samples of LaMn 1− x Co x O 3 ( x=0, 0.02; 0.2; 0.4; 0.5; 0.6; 0.8) prepared by solid-state reaction were used. The EXAFS spectra were analyzed with the FEFF8 computer program. The Mn–O distances of Mn to the nearest oxygen surroundings were evaluated for the samples in the series and compared with the Co–O distances obtained by EXAFS in V. Procházka et al., JMMM 310 (2007) 197 and with results of X-ray powder diffraction in C. Autret, J. Phys. Condens. Matter 17 (2005) 1601.

Reduction of the Yb valence inYbAl3nanoparticles

Measurements of specific heat, dc magnetic susceptibility, and Yb ${L}_{\text{II}}$ and ${L}_{\text{III}}$ x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) on ${\text{YbAl}}_{3}$ milled alloys are reported. X-ray diffraction patterns are consistent with a reduction in particle size down to 10 nm and an increase in the lattice strain up to 0.4% for 120 h of milling time. A decrease in the mean valence from 2.86 for the unmilled alloy to 2.70 for 120 h milled ${\text{YbAl}}_{3}$ is obtained from the analysis of XANES spectra. From the analysis of spectra in the EXAFS region, an increase in the mean-square disorder of neighbor distance with milling time is detected in good agreement with the results of x-ray diffraction. Size effects strongly influence the magnetic and thermal properties. The value for the maximum of the magnetic susceptibility decreases around 30% for 120 h milled alloy and an excess specific heat, with a peak around 40 K in the milled samples, is derived. These changes in the physical properties along the milled ${\text{YbAl}}_{3}$ alloys are associated with the reduction in particle size. Such a reduction leads to the existence of a large number of ${\text{Yb}}^{2+}$ atoms at the surface with respect to the bulk affecting the overall electronic state.

A Five-coordinate Metal Center in Co(II)-substituted VanX

In an effort to structurally probe the metal binding site in VanX, electronic absorption, EPR, and extended x-ray absorption fine structure (EXAFS) spectroscopic studies were conducted on Co(II)-substituted VanX. Electronic spectroscopy revealed the presence of Co(II) ligand field transitions that had molar absorptivities of approximately 100 m(-1) cm(-1), which suggests that Co(II) is five-coordinate in Co(II)-substituted VanX. Low temperature EPR spectra of Co(II)-substituted VanX were simulated using spin Hamiltonian parameters of M(S) = |+/-1/2), E/D = 0.14, g(real(x,y)) = 2.37, and g(real(z)) = 2.03. These parameters lead to the prediction that Co(II) in the enzyme is five-coordinate and that there may be at least one solvent-derived ligand. Single scattering fits of EXAFS data indicate that the metal ions in both native Zn(II)-containing and Co(II)-substituted VanX have the same coordination number and that the metal ions are coordinated by 5 nitrogen/oxygen ligands at approximately 2.0 angstroms. These data demonstrate that Co(II) (and Zn(II) from EXAFS studies) is five-coordinate in VanX in contrast to previous crystallographic studies (Bussiere, D. E., Pratt, S. D., Katz, L., Severin, J. M., Holzman, T., and Park, C. H. (1998) Mol. Cell 2, 75-84). These spectroscopic studies also demonstrate that the metal ion in Co(II)-substituted VanX when complexed with a phosphinate analog of substrate D-Ala-D-Ala is also five-coordinate.

Oxygen and phosphorus coordination around iron in crystalline ferric ferrous pyrophosphate and iron-phosphate glasses with UO2 or Na2O

Fe K-edge x-ray absorption fine-structure (XAFS) measurements were performed on glass samples of (Fe3O4)0.3(P2O5)0.7 with various amounts of Na2O or UO2. Near-edge and extended XAFS regions are studied and comparisons are made to several reference compounds. We find that iron in the base glass is ∼25% divalent and that the Fe2+ coordination is predominantly octahedral, while Fe3+ sites are roughly split between tetrahedral and octahedral coordinations. Also, we measure roughly one Fe–O–P link per iron. Substitution of Na2O or UO2 up to 15 mol% primarily affects the first Fe–O shell. The results are compared to data from the related material Fe3(P2O7)2.

X-Ray Photoacoustic Spectroscopy of ZnO Thin Film

The hard X-ray photoacoustic spectrum of ZnO thin film has been obtained in the EXAFS (extended X-ray absorption fine structure) region using synchrotron radiation. It is shown that information on EXAFS is included in the X-ray photoacoustic signal intensity and phase spectra which reflect the heat production processes in ZnO thin film. However, the results showed that the increases and changes in the photoacoustic signal intensity were different from those of the photoacoustic signal phase in the EXAFS region.

Electronic and x-ray-absorption structure in compressed copper.

Using electronic band-structure techniques, we calculate the K- and L-edge absorption spectra for fcc copper over a wide range of compressions. As copper is compressed the x-ray-absorption structure greatly increases and copper deviates more and more from its normal-density free-electron-like behavior. We explain this additional structure in terms of an increased scattering amplitude with pressure and relate it to both extended x-ray-absorption fine-structure (EXAFS) theory and a simple Friedel-model calculation. We show that specific band symmetry points can be well correlated with the x-ray-absorption near-edge structure features, but that this procedure is useless at higher energies above the edge (the EXAFS region). We suggest that the energy dependence of the peaks in the spectra can be used as a high-pressure volume diagnostic. We also discuss general complications and limitations in our knowledge about high-pressure x-ray-absorption spectra as well as overall changes in the electronic structure of copper under compression. To correlate our results with applied pressure, we also present zero-temperature equation-of-state calculations (pressure versus volume).

Synchrotron-radiation studies of the transition of hydrogenated amorphous carbon to graphitic carbon

Using the synchrotron-radiation facility at Brookhaven National Laboratory, we have studied the electronic structure of thin hydrogenated amorphous carbon films produced by dc glow discharges of hydrocarbons. Valence-band photoemission measurements on these films yielded information on the occupied valence-band density of states, while near-edge absorption at the carbon $K$ edge elucidated features of the unoccupied conduction band. The as-deposited, unannealed films are essentially amorphous, showing no extended x-ray absorption fine-structure (EXAFS) features. Valence-band photoemission results show that these films are insulating, suggesting tetrahedral (diamondlike) coordination. The near-edge-absorption results, however, show some unsaturated carbon-carbon bonds as evidenced by a $1s\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*}$ feature. Upon annealing, the films become more graphitic as evidenced by both the photoemission and near-edge-absorption measurements showing fine structure in agreement with that observed for crystalline or microcrystalline graphite. They also become conductive. Annealing is accompanied by evolution of hydrogen from the films, and this appears correlated with the development of certain features of the band structure as the film orders. The annealed films also show the beginnings of features in the EXAFS region.

Electronic relaxation effects on x-ray spectra of titanium and transition-metal carbides and nitrides

High-resolution titanium $K$ x-ray absorption spectra from pure metal, carbide, and nitride are measured using synchrotron radiation. It is shown that the local partial density of states at the metal site is adequate to describe the near-edge features observed. The magnitude of the electron screening to a suddenly switched-on core-hole potential is investigated and found to be correlated to the density of occupied electron states at the Fermi surface. The dynamical aspects of the screening process show up in the extended x-ray absorption fine-structure region of the spectra and influence the phase and amplitude of the photoelectron wave function. Several structural properties of these compounds are also considered.