X-ray fine structure study and bonding in some ferrites

Extended X-ray absorption fine structure (EXAFS) of the iron K edges in copper, cobalt, zinc and manganese ferrites is studied and the EXAFS parameter α of the Lytle, Sayers and Stern's theory is shown to be a measure of fractional covalency. The linear relationship between metal to oxygen (iron to oxygen) distance and the parameter α leads us to conclude that the copper ferrite is the most covalent among the four ferrites studied in the present investigation.     

Extended X-ray-absorption fine structure of mixed-coordination systems

An analysis method of the extended x-ray absorption fine structure (EXAFS) for mixed-coordination systems (MCS) is provided to determine structure parameters of the same type of atoms located at different coordination sites and proportion of them. It's demonstrated that if the effects of different sites on EXAFS amplitudes are ignored, the coordination numbers gotten from EXAFS analyzing must be wrong mostly. The method has been well tested by analyzing a known structure Gd₃Ga₅O₁₂ (GGG) crystal in which the Ga atoms have two different coordination sites. Li₂O-ZnO-GeO₂ glass system is also analyzed by means of the EXAFS method for MCS. Two different coordination sites for Ge atoms in the glasses. are found and the occupancies of them and the structure parameters are determined.     

Negative thermal expansion in crystals with the zincblende structure: an EXAFS study of CdTe

The extended x-ray absorption fine structure (EXAFS) has been measured at both the K edges of cadmium and tellurium in CdTe, from liquid helium to room temperature, in order to investigate the local thermodynamic behaviour. The temperature dependences of the structural parameters obtained from the separate analysis of the two edges are perfectly consistent. The positive contribution to the thermal expansion due to the bond stretching and the negative contribution due to the tension effects are disentangled and quantified in terms of the bond thermal expansion and the perpendicular mean square relative displacement. The comparison with previous EXAFS results for Ge and CuCl shows that relevant correlations can be established between a number of local parameters measured by means of EXAFS and the properties of the lattice negative thermal expansion of tetrahedrally bonded semiconductors. The effective force constants derived from the EXAFS are compared with the force constants of a valence force field model.     

Study of metglas 2605 CO by extended and near edge X-ray absorption fine structure

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) studies of the transition metal sites in Metglas 2605 CO (Fe₆₇Co₁₈B₁₄Si₁) illustrate differences in the two sites. The average transition metal distance, obtained by standard EXAFS analysis, is larger for iron than for cobalt sites in this material. The XANES for Co and Fe are nearly identical except for a shift of the Co fine structure to high energy relative to that of Fe; the sign and magnitude of this shift is commensurate with the EXAFS results. Finally, there was no detected polarization dependence of the EXAFS, indicating that the structural manifestations of the magnetic anisotropy in this material are smaller than the detection limits of these experiments.     

Extended X-Ray absorption fine structure from hydrogen atoms in water

We report the first quantitative measurement of extended x-ray absorption fine structure (EXAFS) from hydrogen atoms. A single oscillation is observed from gaseous water consistent with the location of the covalently bonded hydrogen in H 2O. The experimental phase and amplitude of the oscillation are in excellent agreement with curved wave multiple scattering calculations for isolated water molecules. With this determination of the O-H scattering phase shift we have quantified the covalent hydrogen bond distance (0.95+/-0.03 A) in liquid water, thus demonstrating that hydrogen EXAFS can become a valuable complement to existing structural methods in chemistry and biology.     

神光III原型上扩展X射线吸收精细结构谱实验研究

介绍了在大型激光装置上进行 扩展X射线吸收精细结构(EXAFS)谱压缩物态参数测量的基本原理,以及为获得高质量EXAFS谱在神光III原型装置上进行的实验研究。实验采用玻璃靶球、CH靶丸和金球作为EXAFS谱测量的背光源,通过多发次叠加、光子数累积的方法获得了信噪比良好的金属Ti在常温常压下的EXAFS谱,数据处理结果表明,实验测得的金属Ti EXAFS谱拟合结果与同步辐射实验拟合结果相吻合,表明实验设计的正确性与可靠性。对实验结果的分析表明,影响EXAFS谱质量的因素主要是光子计数、测量系统谱分辨率、噪声以及实验器件上的瑕疵。     

Size-selective extended X-ray absorption fine structure spectroscopy of free selenium clusters

In a recent paper [M. Yao et al., J. Synchrotron Radiat. 8, 542 (2001)]], we proposed a new method for the size-selective EXAFS (extended x-ray absorption fine structure) of neutral-free clusters, in which not only the x-ray absorption process but also the deexcitation processes are utilized as the structural information. In order to verify this method experimentally, we have developed the synchronous measurements of EXAFS and photoelectron photoion coincidence and carried them out for a Se cluster beam by utilizing the third-generation intense x-ray source. The EXAFS spectra for Se small clusters have been obtained and compared critically with theoretical predictions.     

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

ABSTRACT

In 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.     

Tetragonal structure of thin nickel films on Cu(001)

The crystallographic structure of thin Ni films deposited on Cu(001) has been studied using Surface Extended X-ray Absorption Fine Structure (SEXAFS). Taking advantage of the linear polarization of the synchrotron radiation, we have shown that Ni adopts the Cu lattice parameter parallel to the interface. This lateral expansion induces a longitudinal compression of the unit cell, leading to a face centered tetragonal structure of the Ni films from 3 to 10 monolayers. The temperature dependence of the EXAFS oscillations has allowed to measure strain inside the Ni layers. Received 22 December 1998     

Extended X-ray absorption fine structure measurements of laser-shocked V and Ti and crystal phase transformation in Ti

Extended x-ray absorption fine structure (EXAFS), using a laser-imploded target as a source, can yield the properties of laser-shocked metals on a nanosecond time scale. EXAFS measurements of vanadium shocked to approximately 0.4 Mbar yield the compression and temperature in good agreement with hydrodynamic simulations and shock-speed measurements. In laser-shocked titanium at the same pressure, the EXAPS modulation damping is much higher than is warranted by the predicted temperature increase. This is shown to be due to the alpha-Ti to omega-Ti crystal phase transformation, known to occur below approximately 0.1 Mbar for slower shock waves.     

Many-particle-effects in the theory of the extended X-ray absorption fine structure

The Lee-Beni-procedure for the calculation of the extended X-ray absorption fine structure (EXAFS) is extended so as to include the effects of the electronic charge density outside the localized muffin-tin potentials. In our scheme EXAFS is caused by back-scattering of an elementary excitation of a homogeneous electron gas by localized energy dependent many-particle muffin-tin potentials. The difference between the two schemes is negligible at large k's, as expected from physical grounds. However, at small and intermediate k-values the difference is quite large. The effect of the outer electrons as compared to the Lee-Beni-model is twofold. First, they renormalize the scattered electron in the usual way. Second, they are missing within the scattering muffin-tins. Hence, we avoid to count some of the electrons twice. Results are presented for Cu as an example.     

Local structure and magnetism of Fe, Co and Ni doped Cr3Si

The lattice site occupation of 3d-type impurities in Cr₃Si doped with Fe, Co and Ni were studied using the Extended X-ray Absorption Fine Structure (EXAFS) technique, X-rays and magnetic susceptibility measurements. The EXAFS measurements were performed particularly carefully on the K-edge of chromium and cobalt. EXAFS data strengthened by simulations of the spectra show unambiguously that the impurities are occupying mostly Cr-sites. The magnetic state of the alloys has been studied theoretically using the Wien2k code. It is observed that if calculations are not carried out on an appropriately fine mesh of points in k-space, one can get the result indicating ferromagnetism although the overall magnetic moment per formula unit is weak.     

Icosahedral order in Cu-Zr amorphous alloys studied by means of X-ray absorption fine structure and molecular dynamics simulations

We have used the X-ray absorption fine structure method and molecular dynamics (MD) simulations to characterize atomic order in Cu-Zr metallic glasses (MGs). The microstructure of these MGs is described in terms of interconnected icosahedral-like clusters (superclusters) which are basic building units reproducing the stoichiometry of the system. The equilibrium MD configurations are used as an input for ab initio calculations of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra. The theoretical EXAFS and XANES spectra are compared with those measured for rapidly quenched glassy Cu-Zr alloys. We demonstrate that the experimental results are well reproduced by EXAFS modeling of the population of the superclusters derived from the MD configuration. The average local structural motif can be approximated by Cu-centered icosahedral-like cluster satisfying the condition of maximal local packing efficiency and approximating the system stoichiometry. The simulated XANES exhibits good agreement with experiment, indicating that the atomic order of the MD configuration is consistent with that of the real alloy structure over distances of about 1?nm.     

X-ray absorption spectroscopy under high pressures in diamond anvil cells

Abstract

Although potentially extremely important for understanding the high-pressure microscopic behaviour of materials, over the years the area of high-pressure EXAFS in particular using diamond anvil cells has proved to be technically difficult. This has significantly hampered its development. The interference of X-ray dimaction from the diamonds in the diamond anvil cell with the absorption signal has proved to be a challenging problem to tackle, restricting the use of high-pressure EXAFS to energies below about 11 key Below 11 keV however the technique is also limited due to absorption of incident X-rays by the diamonds making it virtually impossible to conduct X-ray absorption experiments below about 9keV In this paper we describe in detail the methodology for scanriirig high-pressure EXAFS in diamond anvil cells and examine the origins of the associated problems and ways of dealing with them. We also demonstrate that it is possible to extend the useful range of studied absorption edges from 7keV up to at least 30keV This brings about new opportunities for high pressure EXAFS using diamond anvil cells.     

Local structure of NiAl compounds investigated by extended X‐ray absorption fine‐structure spectroscopy

The local structures of pure NiAl and Ti‐, Co‐doped NiAl compounds have been obtained utilizing extended X‐ray absorption fine‐structure (EXAFS) spectroscopy. The results provide experimental evidence that Ni antisite defects exist in the Ni‐rich NiAl compounds. The site preference of Ti and Co has been confirmed. Ti occupies the Al sublattice, while Co occupies the Ni sublattice. The structure parameters obtained by EXAFS were consistent with the X‐ray diffraction results. Owing to the precipitation of α‐Cr, the local structure of NiAl‐Cr has not been obtained, making the site preference of Cr unclear.     

Development of pressure-modulated EXAFS method

Many modulation techniques have been widely applied to improve the quality of conventional spectra. Here a pressure-modulated EXAFS method is proposed to detect the small changes of local structure induced by the modulation of high pressure. In the experiment a dynamic diamond anvil cell was used to put a periodic load on the sample and lock-in amplifier to measure the modulated EXAFS signals. We have applied this technique to ZnSe and revealed a sensitivity to atomic displacement of 0.1 pm that is about ten times better than that of traditional EXAFS.     

Titanium local structure in tektite probed by X‐ray absorption fine structure spectroscopy

The local structure of titanium in tektites from six strewn fields was studied by Ti K‐edge X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) in order to provide quantitative data on Ti—O distance and Ti coordination number. The titanium in tektites possessed different coordination environment types. XANES spectra patterns revealed resemblance to high‐temperature TiO₂–SiO₂ glass and TiO₂ anatase. All samples showed that the valence of Ti is 4+. Based on the Ti—O distances, coordination numbers and radial distribution function determined by EXAFS analyses, the tektites were classified into three types: type I, Ti occupies a four‐coordinated tetrahedral site with Ti—O distances of 1.84–1.79 Å; type II, Ti occupies a five‐coordinated trigonal bipyramidal or tetragonal pyramidal site with Ti—O distances of 1.92–1.89 Å; type III, Ti occupies a six‐coordinated octahedral site with Ti—O distances of 2.00–1.96 Å. Although Ti occupies the TiO₆ octahedral site in most titanium minerals under ambient conditions, some tektites have four‐ and five‐coordinated Ti. This study indicated that the local structure of Ti might change in impact events and the following stages.     

Local order in light emitting porous silicon studied by XEOL and TEY

The local structure of porous silicon has been studied exciting its optical luminescence by X-rays (XEOL). The photoluminescence yield and the total electron yield (TEY), recorded simultaneously as a function of the X-ray energy at the Si K edge, give rise to the extended X-ray absorption fine structures (EXAFS). Analysis of EXAFS data confirms that the optical luminescence of porous Si originates from the nanocrystalline cores and shows that XEOL–EXAFS and TEY–EXAFS are sensitive to different Si local environment. It can be assumed that XEOL–EXAFS is related only to the light emitting sites while TEY–EXAFS is sampling both luminescent and non-luminescent Si sites.     

Data analysis method to achieve sub‐10 pm spatial resolution using extended X‐ray absorption fine‐structure spectroscopy

Obtaining sub‐10 pm spatial resolution by extended X‐ray absorption fine structure (EXAFS) spectroscopy is required in many important fields of research, such as lattice distortion studies in colossal magnetic resistance materials, high‐temperature superconductivity materials etc. However, based on the existing EXAFS data analysis methods, EXAFS has a spatial resolution limit of π/2Δk which is larger than 0.1 Å. In this paper a new data analysis method which can easily achieve sub‐10 pm resolution is introduced. Theoretically, the resolution limit of the method is three times better than that normally available. The method is examined by numerical simulation and experimental data. As a demonstration, the LaFe_1–xCr_xO₃ system (x = 0, 1/3, 2/3) is studied and the structural information of FeO₆ octahedral distortion as a function of Cr doping is resolved directly from EXAFS, where a resolution better than 0.074 Å is achieved.     

Use of the variational principle with the aim of determining oscillations in the extended X-ray absorption fine structure

A method of determining the smooth and oscillating components of extended X-ray absorption fine structure spectra is developed. The form of a functional that is minimized by the desired smooth function is suggested, and a series of EXAFS spectra is analyzed. A way of solution optimization is proposed. The results of applying the method to experimental spectra are reported.