Extended X-ray Absorption Fine Structure Technique Research Articles

Analysis of Anharmonic EXAFS Spectra of Crystalline Nickel Using High-order Debye-Waller Factors

The extended X-ray absorption fine structure (EXAFS) has been developed into a powerful technique and is widely applied to determine many structural parameters and dynamic properties of materials. The EXAFS technique is now the technique of choice in many materials science investigations, and the EXAFS data analysis is being performed in many laboratories spread around the world. In this work, the anharmonic EXAFS spectra of crystalline nickel (Ni) has been analyzed based on the quantum anharmonic correlated Einstein model. The anharmonic EXAFS oscillation presented in terms of the Debye-Waller factors using the cumulant expansion approach up to the fourth-order. This calculation model has been developed from the high-order anharmonic effective potential that described the contribution of their nearest-neighbor atoms to the pair interaction potential. The analytical expressions of the anharmonic EXAFS cumulants are not only explicit forms but also satisfy all of their fundamental properties in temperature dependence. The analysis of the anharmonic EXAFS spectra was performed by evaluating the contributions of the cumulants to the amplitude reduction and the phase shift of the anharmonic EXAFS oscillation. The numerical results for Ni were in good agreement with those obtained using the other theoretical methods and experiment at various temperatures, which are useful for analyzing the experimental EXAFS data of the metal crystals.

Theoretical Study of the Thorium Structural in Solution by EXAFS Techniques

Thorium with symbol Th is radioactive metal which emits the alpha rays. This metal can be founded in different forms and can cause the negatives impacts on human health and environment. This is the reason which explains the importance to know the structure of this metal in different forms including the structure of hydrated Thorium. Structural information helps to understand and assess peculiarities this ion in different natural and technological processes taking place in aqueous solutions. The most and efficacy technique to know about distances between atoms and coordination numbers is EXAFS (extended X Ray Absorption Fine Structure) technique because it permits to know these above informations. My project research is was to determine the structure of hydrated, solvated Thorium ion in metal ions. For my research, I preferred to use the EXAFS method to study the structure of the hydrated Th ion and to determine the Th-O bond distance, and from that estimate the coordination number. Th-O distance in the case of hydrated Thorium appeared to be 2.44 Å which corresponds to 9-coordination, as also found in the solids. The Th-O bond distance in solvated plutonium is ca. 2.41 Å, which corresponds to eightcoordination. These informations have shown me that for Thorium ligand size and coordination number are correlated. In case of Thorium hydrated studies, I saw that shown don’t precipitate at the law concentration. As conclusion, when pH is in range, centrifugation is possible and thorium is in monomeric form, but for pH above 4 Thorium is restrained and centrifuging shown that Thorium is in non-ionic form.

Low-range thermal investigation of zincblende-type ZnS by combined extended X-ray absorption fine structure and X-ray diffraction techniques

Extended X-ray absorption fine structure (EXAFS) has been measured at the zinc K edge in zinc sulfide (ZnS), in the temperature range of 20 K–300 K, in order to investigate the local structure and temperature effect in a zincblende-type ZnS compound. X-ray Diffraction (XRD) data have been carried out at the temperature range of 100 K–300 K for the investigation of the crystallographic local structure of ZnS. The cumulant method and the least-square fitting procedures are used in the EXAFS technique whereas the Rietveld method is used for the refinement of the XRD experimental data. The temperature dependence and the thermal expansion of the first bond distance and the Debye-Waller factor are exhibited and discussed in terms of anisotropy. The difference between the first bond distance measured by EXAFS and XRD is discussed. The Einstein frequencies are compared to the vibrational densities of states (VDOS) due to: the transverse acoustic mode, longitudinal acoustic and/or optic phonon modes; while the force constants have been correlated with the valence force field (VFF) models.

Effect of polytypism on the long and short range crystal structure of InAs nanostructures: An EXAFS and Raman spectroscopy study

The authors present a study on growth mechanism and crystalline structure of zinc blende (ZB)/wurtzite (WZ) polytypic InAs nanostrutures grown on Si-substrate using x-ray diffraction, Raman spectroscopy, and extended x-ray absorption fine structure (EXAFS) spectroscopy. A significant change in the bond length is observed in the short range crystal structure as an effect of the interfacial strain, and the structure exhibits disorder due to twin defect/stacking fault formation at the homointerfaces of ZB/WZ segments in the polytypic crystalline structure, which is related to the ZB:WZ phase ratio. The average ratio of ZB and WZ crystal phase present in the nanostructures is estimated through EXAFS technique. This study opens up an alternate pathway for quantitative estimation of crystal phases over a large area of polytypic samples with distribution in size as well as morphology. Raman spectroscopy study uses E2H and transverse-optical phonon modes to identify the WZ and ZB structural phases of these polytypic nanostructures. It reveals that the interfacial strain shifts the transverse and longitudinal optical phonon frequency significantly and the frequency shifts are proportional to the concentration ratio of ZB and WZ crystal phases. The ZB:WZ ratio derived from EXAFS technique is correlated with Raman spectroscopy results. The long range crystalline structure of the nanostructures is also explored using x-ray diffraction technique, and the observations are corroborating with EXAFS and Raman spectroscopy results. The present study not only elucidate in-depth understanding of the local structure of polytypic InAs but also suggests that careful engineering of interfacial strain through controlled growth can further improve the performance of the polytypic ZB/WZ homointerface based optoelectronic devices.

Atomistic solvation structure of calcium ion in poly(vinyl alcohol) as studied by Molecular Dynamics simulation and X-ray Absorption Spectroscopy

Molecular Dynamics (MD) simulation and the extended X-ray absorption fine structure (EXAFS) spectroscopy were employed to obtain the atomistic solvation structure of calcium (Ca2+) ion in poly(vinyl alcohol) (PVA). From MD simulation results, it is obvious that there is only one dominant shell from oxygen atoms around calcium ion. The distance between the calcium ion and oxygen atoms in the first shell is in good agreement with experiment within 0.1Å. For the Ca K-edge EXAFS spectrum of CaCl2/PVA sample, there is no evidence for the formation of significant numbers of Ca2+–Cl− contact ion pairs. Ca2+ ion can induce the conformational changes of PVA backbone compared to those of neat PVA. The MD and EXAFS technique is a powerful method to elucidate the detailed solvation structure of the probed ion in polymer matrix at the atomistic scale.

X-ray Absorption Study of the Solvation Structure of Cu2+ in Methanol and Dimethyl Sulfoxide

The solvation structure of Cu(2+) in methanol (MeOH) and dimethyl sulfoxide (DMSO) has been determined by studying both the extended X-ray absorption fine structure (EXAFS) and the X-ray absorption near-edge structure (XANES) regions of the K-edge absorption spectra. The EXAFS technique has been found to provide a very accurate determination of the next-neighbor coordination distances, but it is inconclusive in the determination of the coordination numbers and polyhedral environment. Conversely, quantitative analysis of the XANES spectra unambiguously shows the presence of an average 5-fold coordination in both the MeOH and DMSO solution, ruling out the usually proposed octahedral Jahn-Teller distorted geometry. The EXAFS and XANES techniques provide coherent values of the Cu-O first-shell distances that are coincident in the two solvents. This investigation shows that the combined analysis of the EXAFS and XANES data allows a reliable determination of the structural properties of electrolyte solutions, which is very difficult to achieve with other experimental techniques.

Investigation of cesium adsorption on soil and sediment samples from Fukushima Prefecture by sequential extraction and EXAFS technique

Previous studies have shown that radiocesium (mainly 137 Cs) was retained at the very surface of soils in Fukushima Prefecture. Clay minerals and micas are assumed as the main sorbents for cesium (Cs) in Fukushima, but direct evidence is lacking for this hypothesis. In this study, radiocesium in the natural sample (soil and sediment) from Fukushima Prefecture was investigated through sequential extraction experiment (modified BCR method), which showed that more than 94% of 137 Cs was fixed in the residual phase. The results indicated that most of Cs occurred in the interlayer of phyllosilicate minerals. Furthermore, Cs LIII-edge extended X-ray absorption fine structure (EXAFS) showed that the Cs species adsorbed on the natural samples were very similar to those adsorbed on clay minerals and micas. This finding provided the direct evidence on the significant contribution of clay minerals or micas to Cs retention in soils from Fukushima Prefecture.

Adsorption of Eu(III) on titanate nanotubes studied by a combination of batch and EXAFS technique

The effects of pH, contact time and natural organic ligands on radionuclide Eu(III) adsorption and mechanism on titanate nanotubes (TNTs) are studied by a combination of batch and extended X-ray absorption fine structure (EXAFS) techniques. Macroscopic measurements show that the adsorption is ionic strength dependent at pH 6.0. The presence of humic acid (HA) / fulvic acid (FA) increases Eu(III) adsorption on TNTs at low pH, but reduces Eu(III) adsorption at high pH. The results of EXAFS analysis indicate that Eu(III) adsorption on TNTs is dominated by outer-sphere surface complexation at pH 6.0. At pH < 6.0, Eu(III) consists of ∼ 9 O atoms at R Eu-O ≈ 2.40 A in the first coordination sphere, and a decrease in NEu-O with increasing pH indicates the introduction of more asymmetry in the first sphere of adsorbed Eu(III). At long contact time or high pH values, the Eu(III) consists of ∼2 Eu at R Eu-Eu ≈ 3.60 A and ∼ 1 Ti at R Eu-Ti ≈ 4.40 A, indicating the formation of inner-sphere surface complexation, surface precipitation or surface polymers. Surface adsorbed HA/FA on TNTs modifies the species of adsorbed Eu(III) as well as the local atomic structures of adsorbed Eu(III) on HA/FA-TNT hybrids. Adsorbed Eu(III) on HA/FA-TNT hybrids forms both ligand-bridging ternary surface complexes (Eu-HA/FA-TNTs) as well as surface complexes in which Eu(III) remains directly bound to TNT surface hydroxyl groups (i.e., binary Eu-TNTs or Eu-bridging ternary surface complexes (HA/FA-Eu-TNTs)). The findings in this work are important to describe Eu(III) interaction with nanomaterials at molecular level and will help to improve the understanding of Eu(III) physicochemical behavior in the natural environment.

Hydration Properties of the Bromide Aqua Ion: the Interplay of First Principle and Classical Molecular Dynamics, and X-ray Absorption Spectroscopy

The hydration properties of the bromide aqua ion have been investigated using state of the art density functional theory (DFT) based molecular dynamics with dispersion-corrected atom-centered pseudopotentials for water and classical molecular dynamics simulations. The reliability of the theoretical results has been assessed by comparing the attained structural results with the extended X-ray absorption fine structure (EXAFS) experimental data. The EXAFS technique is mainly sensitive to short distances around the bromine atom, and it is a direct probe of the local solvation structure. The comparison shows that the DFT simulation delivers a good description of the EXAFS experimental signal, while classical simulation performs poorly. The main reason behind this is the neglect of polarization effects in the classical ion-water interaction potentials. By taking advantage of the reliable information on the Br(-) local hydration structure it has been possible to highlight the contribution of hydrogen atoms to the EXAFS spectra of halide aqueous systems.

Local atomic structure in tetragonal pure ZrO2nanopowders

The local atomic structures around the Zr atom of pure (undoped) ZrO2nanopowders with different average crystallite sizes, ranging from 7 to 40 nm, have been investigated. The nanopowders were synthesized by different wet-chemical routes, but all exhibit the high-temperature tetragonal phase stabilized at room temperature, as established by synchrotron radiation X-ray diffraction. The extended X-ray absorption fine structure (EXAFS) technique was applied to analyze the local structure around the Zr atoms. Several authors have studied this system using the EXAFS technique without obtaining a good agreement between crystallographic and EXAFS data. In this work, it is shown that the local structure of ZrO2nanopowders can be described by a model consisting of two oxygen subshells (4 + 4 atoms) with different Zr—O distances, in agreement with those independently determined by X-ray diffraction. However, the EXAFS study shows that the second oxygen subshell exhibits a Debye–Waller (DW) parameter much higher than that of the first oxygen subshell, a result that cannot be explained by the crystallographic model accepted for the tetragonal phase of zirconia-based materials. However, as proposed by other authors, the difference in the DW parameters between the two oxygen subshells around the Zr atoms can be explained by the existence of oxygen displacements perpendicular to thezdirection; these mainly affect the second oxygen subshell because of the directional character of the EXAFS DW parameter, in contradiction to the crystallographic value. It is also established that this model is similar to another model having three oxygen subshells, with a 4 + 2 + 2 distribution of atoms, with only one DW parameter for all oxygen subshells. Both models are in good agreement with the crystal structure determined by X-ray diffraction experiments.

Ab initio X-ray Absorption Spectroscopy Study of the Solvation Structure of Yttrium (III) in Dimethyl Sulfoxide

The solvation structure of yttrium (III) in dimethyl sulfoxide has been determined by studying both the extended X-ray absorption fine structure (EXAFS) and the X-ray absorption near edge structure (XANES) regions of the Y K-edge absorption spectra. Although the EXAFS technique provides accurate information about the next neighbors coordination distances, no unambiguous determination of the coordination polyhedron is obtained. This failure is counteracted by the study of the near-edge part of the absorption spectrum (XANES) because of its high sensitivity to the bonding geometry. We have performed an extensive and systematic ab initio computation of the Y K-edge XANES spectrum of yttrium (III) in dimethyl sulfoxide within the multiple-scattering framework. The comparison between the experimental data and the theoretical calculations has demonstrated that the solvation sphere of the yttrium cation is best modeled by eight dimethyl sulfoxide molecules each oriented to give an Y-O-S angle close to 130 degrees .

Formation of Ge−S Bonds from AOT-Coated GeO2 Nanoparticles at High Temperature: An in Situ Heating EXAFS Investigation

Cubic germanium dioxide (GeO2) nanoparticles coated by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) were synthesized with a reverse micelle technique. In situ heating extended X-ray absorption fine-structure technique was used to investigate the change of the local atomic structure and electronic structure around germanium. A direct formation of Ge−S bond was found from GeO2 at high temperature and ambient pressure. This Ge−S bond was identified to be temperature-dependent. The change of the local structure with heating is discussed. This study provides a routine process to synthesize GeS2 from GeO2 at higher temperature and ambient pressure.

Relationships between Hg(ii)–S bond distance and Hg(ii) coordination in thiolates

Extended X-ray absorption fine structure (EXAFS) spectroscopy is a sensitive structural probe of the coordination environments of Hg(II) with thiol (sulfhydryl) groups, and is equally applicable to solid and aqueous organic or inorganic matter. Information on the number and geometric arrangement of S ligands can be derived from metal-ligand distances because the distances vary with Hg stereochemistry and can be accurately measured by the EXAFS technique. To improve the reliability of determining coordination structures of Hg with thiol groups, correlations among Hg-S bond distance, Hg coordination, and S-Hg-S angle in homoleptic Hg(II)-thiolates were calculated from analysis of the structures of the 162 Hg(SR)n motifs (n = 2, 3, 4) contained in the Cambridge Structural Database v. 5.28. Graphical correlations of bond distance with coordination number and with bond angle show distinct ranges of values characteristic of specific structural configurations.

Structural characterization of the Co2FeZ (Z=Al, Si, Ga, and Ge) Heusler compounds by x-ray diffraction and extended x-ray absorption fine structure spectroscopy

This work reports on the structure of Fe containing, Co2-based Heusler compounds that are suitable for magnetoelectronic applications. The compounds Co2FeZ (where Z=Al, Si, Ga, and Ge) were investigated using the x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) techniques. Using XRD, it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure. For compounds containing Ga or Ge, the XRD technique cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique was used to elucidate the structure of these two compounds. Analysis of the EXAFS data indicated that both compounds crystallize in the L21 structure.

Strain-Induced Bond Buckling and Its Role in Insulating Properties of Cr-DopedV2O3

Structural transformations around both V and Cr atoms in (V1-xCrx)2O3 across its metal-insulator transition (MIT) at x approximately 0.01 are studied by extended x-ray absorption fine-structure technique. Our new results for Cr made possible by the use of a novel x-ray analyzer that we developed reveal the substitutional mechanism of Cr doping. We find that this system has a buckled structure with short Cr-V and long V-V bonds. This system of bonds is disordered around the average trigonal lattice ascertained by x-ray diffraction. Such local distortions can result in a long range strain field that sets in around dilute Cr atoms in microscopic regions. We suggest that such locally strained regions should be insulating even at small x. The possibility of local insulating regions within a metallic phase, first suggested by Rice and Brinkman in 1972, remains unaccounted for in modern MIT theories.

Analysis of Local Structures around Ni Atoms Doped inZnO-Based Diluted Magnetic Semiconductors by Fluorescence EXAFS

Zn1−xNixO (x = 0.001, 0.01, 0.02, 0.05 and 0.20)powders are prepared by sol-gel method. An extended x-ray absorptionfine structure technique (EXAFS) for the Ni K-edge is employed to probethe local structures around Ni atoms doped in ZnO powders byfluorescence mode. The near edge EXAFS of the samples does not changein the range of Ni concentration from x = 0.001 to 0.05, which isconsistent with the results of x-ray diffraction of the samples. Thesimulation results for the first shell EXAFS signals indicated that Niatoms are substituted in Zn sites.

Structural Anomalies of Rb and Br Ionic Nanosolutions in Hydrophobic Slit-Shaped Solid Space as Revealed by the EXAFS Technique

The hydration structure of RbBr electrolytic solutions confined in slit-shaped nanospaces of carbon was determined with the extended X-ray absorption fine structure (EXAFS) technique and related analysis. The nitrogen adsorption isotherms at 77 K were measured to evaluate the porosity of RbBr-deposited carbon samples. The EXAFS results indicate that the electrolytic solution confined in hydrophobic nanospaces has a restricted hydration structure depending on the average pore width of the nanospaces. Also the asymmetric-field effect of slit-pore geometry on the formation of unique hydration structures of ions in nanospaces was unveiled with the analysis of EXAFS spectra. A marked decrease of the hydration number around a Rb ion was observed in slit-shaped nanospaces, indicating the compressed hydration structure around a Rb ion restricted in slit-shaped nanospaces. The distorted hydration structure around a Br ion in the slit-shaped pore of 0.7 nm was formed to increase the hydration number, though partial...

EXAFS studies of the trapping site structure for molecules isolated in cryogenic matrices

We present here results concerning the first attempt of determining the trapping site structure of molecules isolated in inert matrices at low temperature by the EXAFS (Extended X-ray Absorption Fine Structure) method. The experiments have been performed at the K edge of argon, silicon, sulfur, and chlorine for pure solid argon, and for SiH4, OCS, and HCl isolated in different cryogenic matrices. The EXAFS technique is sensitive to the local environment around the absorbing atom, and the spectral features induced by the matrix material (Ar, Xe, N2, and CH4) are clearly evidenced here. The data allow a characterization of the double substitutional site for OCS in argon and xenon, while no structure can be determined for the accommodation of SiH4 in argon. A discussion of the best choice for the guest/host system to obtain a good EXAFS signal is included.

高圧下でのＸ線を用いた構造解析と格子振動解析

The single-crystal x-ray diffraction method using a diamond anvil cell is an important method for crystal structure determination under high-pressure. The x-ray absorption spectroscopy under high-pressure provides the detailed information on local structure around particular kinds of atoms, even if the crystal structure is unknown. Extended x-ray absorption fine structure (EXAFS) spectroscopy is useful as a probe of vibration dynamics under pressure. An anharmonic effective pair potential can be investigated using the EXAFS technique. The knowledge of the local structure is of primary importance for the understanding of the complicated physical properties of solid solutions. The change of the spin state of the Co4+ ion in the perovskite-type Sr (Co, Mn) O3 solid solution whose magnetic properties varies according to the compositions have been explained based on the local structure analysis.

Surface atomic structures of Fe2O3 nanoparticles coated with cetyltrimethyl ammonium bromide and sodium dodecyl benzene sulphonate: an extended x-ray absorption fine-structure study

Fe2O3 nanoparticles coated with sodium dodecyl benzene sulphonate (DBS) or cetyltrimethyl ammonium bromide (CTAB) were prepared by using a microemulsion method in the system water/toluene. The nanoparticles were characterized by means of transmission electron microscopy and average particle sizes of 5.0 nm and 6.0 nm were found for DBS-modified and CTAB-modified nanoparticles respectively. The local atomic structures of these iron(III) oxide nanoparticles were probed by using the extended x-ray absorption fine-structure technique. Fe K absorption spectra were collected at beam line 4W1B of Beijing Synchrotron Radiation Facility. A structural model was proposed for describing their atomic structures. The Fe-O bond length at the surface of DBS-coated Fe2O3 nanoparticles was found to be similar to that in bulk Fe2O3, but there was about 0.04 Å expansion for the CTAB-coated Fe2O3 nanoparticles. On the basis of the model proposed in this paper, the thicknesses of the surface layers were estimated to be 0.5 nm and 0.7 nm, respectively, for the DBS-coated and CTAB-coated Fe2O3 nanoparticles. The anharmonicity of the atomic vibration and the asymmetry of atom-pair distribution were found to be larger at the surface of the nanoparticles than in the bulk material, while the Debye-Waller factors are almost the same for the surface and the core parts of the nanoparticles. It can be concluded that the atomic structure of the nanoparticle surface is ordered, but the atom-pair distribution is asymmetric.