A comparative study of the spectra recorded at RRCAT synchrotron BL-8 dispersive EXAFS beamline with other beamlines

The aim of the present work is to make a comparative study of the EXAFS spectra recorded at the BL-8 dispersive EXAFS beamline at 2 GeV Indus-2 synchrotron source at RRCAT, Indore (India) with those recorded at other synchrotron EXAFS beamlines, viz., X-19A at NSLS, BNL (USA), EXAFS wiggler beamline 4-1 at the SSRL (USA) and beamline 11.1 at ELETTRA (Italy). For this purpose, EXAFS spectra at Cu K-edge in copper metal have been recorded at these four beamlines. Further, EXAFS spectra at Cu K-edge in a copper complex have also been recorded at BL-8 beamline and beamline 11.1 at ELETTRA (Italy). The obtained experimental μ(E) data have been background-subtracted and then normalized. The normalized data have been then converted to χ(k) data, which have been Fourier-transformed and then fitted with the theoretical model, thereby yielding different structural parameters. It has been shown that the results obtained from the EXAFS spectra recorded at the BL-8 beamline are comparable with those obtained from other synchrotron EXAFS beamlines and also with the crystallographic results reported by earlier workers. The reliability, usefulness and data quality of the BL-8 beamline have been discussed.     

The local atomic structures in Si1−x Ge x random solid solutions

The local structures of Si_1?x Ge _x solid solutions can be described with FDUC model (the five distorted unit cells) based on a phenomenological valence force field approach. Bond lengths and strain energies in these solid solutions can be predicted. The distorted feature of the local structure is well described by the five distorted unit cells. Calculated results are compared with those obtained from EXAFS, indicating the excellent agreement between theoretical predictions and EXAFS measurements with the largest divergence not larger than 0.01 Å. In addition, The relationship betweenSi _1?x Ge _x and III–V random solid solutions is also discussed.     

A study of the EXAFS associated with the K absorption discontinuity of copper in CuGa and CuGe alloys

The extended X-ray absorption fine structure (EXAFS) associated with the K absorption discontinuity of copper has been studied in the pure metal and in some copper-gallium and copper-germanium alloys, using a bent crystal X-ray spectrograph. The results obtained are discussed in the light of the recent theory of EXAFS proposed by Lytle, Sayers and Stern. It has been shown for the first time that there exists in alloys a correlation between the EXAFS parameter α₁ and e/a ratio. This correlation has been used to obtain the value of the lattice parameter “a” for Cu-27 at.% Ge alloy, which is not available in literature.     

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.     

Local atomic structure of Cu(In x Ga1−x )Se2 (x = 0.25, 0.5, and 0.75) systems for solar cells

The local atomic structure of thin surface layers of crystalline quasi-binary Cu(In _x Ga_1?x )Se₂ solid solutions was studied by SIMS and EXAFS techniques. The SIMS method showed that the elemental composition of the sample changes most significantly in thin layers at a depth of 5–10 nm; in deeper layers, the component concentrations correspond to the bulk values. The EXAFS method in the x-ray fluorescence mode showed that the results obtained are in agreement with the assumption that quaternary crystalline quasi-binary Cu(In _x Ga_1?x )Se₂ solid solutions exhibit local disorder while average long-range order is detected from x-ray diffraction data.     

Zr-induced structural changes in Hf1−x Zr x O2 high-k thin films

The local symmetries around the Hf sites in thin films of Hf_1?x Zr _x O₂/Si(100) were probed using grazing incidence extended X-ray absorption fine-structure spectroscopy (EXAFS). The effects of the Zr incorporation on the local crystal symmetries were investigated using Hf L₃ EXAFS at the Beamline X23A2 of the Brookhaven National Laboratory. The Zr ratios in the various films were set to between 0.0 and 1.0. Significant changes in the local environment were observed for x = 0.25 or greater values. For x = 0.0, the film local structure around Hf sites remain in the equilibrium monoclinic phase as referenced from our previous studies on HfO₂ thin films on Si(100). When Zr is introduced, tetragonal symmetry around the Hf atom appears and becomes dominant at x = 0.63. Using the EXAFS theoretical simulations and non-linear least-square fit results, the fractions of the monoclinic versus tetragonal phases were identified in each film.     

谐波对EXAFS振幅的影响

本文讨论了晶体单色器高次衍射形成的谐波对EXAFS振幅的影响。给出了一般计算表达式和计算实例。这种影响是使EXAFS振幅减小,它与谐波和基波强度之比、样品厚度及吸收系数等因素有关。分别测量了不同厚度的铜箔样品有谐波与无谐波时的K吸收EXAFS谱,得出的单配位层EXAFS振幅的变化测量值与计算值相符。从而提供了对谐波影响进行修正的方法。 关键词：     

EXAFS方法在金属玻璃结构研究中的误差分析

本文对EXAFS方法在金属玻璃结构研究中所得径向距离的误差进行了分析。所做的模拟计算表明当前流行的关于这个问题的误差理论不能圆满地解释这一现象。本文分析了玻璃态和晶态的相移函数,指出在相似的体系中玻璃态和晶态相移函数与k的关系采用线性函数近似时,其斜率可能是不同的,而且无序度相差越大,两者的偏差可能也越大。在EXAFS数据处理过程中,没有考虑这种偏差的影响是造成EXAFS方法结果中径向距离误差的一个原因。本文还提出了一个修正参考样品(晶态)相移函数斜率偏差的方法,并将它应用于Cu₅₅Zr₄₅金属玻璃的EXAFS数据处理中,所得结果与用X射线异常散射方法测定的结果符合得很好。 关键词：     

Local surrounding of Mn in LaMn1−xCoxO3 compounds by means of EXAFS on Mn–K

A systematic study of LaMn_1?xCo_xO₃ 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?xCo_xO₃ (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.     

A study of EXAFS associated with the Re LIII discontinuity in metal and some compounds

The authors have reanalysed the data on the EXAFS associated with the rhenium L_III absorption discontinuity in the metal and in some of its compounds, and have interpreted them in the light of the recent theory proposed by Lytle, Sayers and Stern. It has been shown that the α parameter of EXAFS is a measure of valency and that the metallic valency of rhenium is two.     

Impurity-ligand distances derived from magnetic resonance and optical parameters

Non empirical methods for obtaining information from EPR, ENDOR and optical data on the true impurity-ligand distance,R, as well as on the true variations, ΔR, induced by chemical and hydrostatic pressures, phase transitions in the host material and temperature changes are discussed through this work. Special attention is addressed to spectroscopic parameters of d-impurities whose dependence onR can reasonably be calculated theoretically for the superhyperfine (shf) tensor or the lowest optical transitions but not for fine structure terms. In the case of impurities with unpaired σ-electrons it is shown that the isotropic shf constant,A _s, is specially sensitive toR changes. The microscopic origin of this fact is discussed in detail. Determination of trueR values from experimentalA _s has been carried out for impurities like Mn²⁺ or Ni⁺ with encouraging results. In the case of Mn²⁺ in fluorides, results obtained by this method coincide with those reached through EXAFS and the analysis of the experimental 10 Dq. Finally, for some selected complexes, a view is offered on the dependence of several EPR and optical parameters upon metal-ligand distances. The main conclusion is that ΔR values of the order of 0.1 pm can be detected using EPR and optical parameters thus improving by an order of magnitude the sensitivity reached through EXAFS. In particular the use of ENDOR allows one to measure ΔR values close to 0.01 pm.     

能量分辨率对EXAFS的影响

测量EXAFS谱时,系统的能量分辨率是影响其结果的重要因素。本文提出了分辨率影响结构参数的计算方法,以Ge,Ni样品为例,测量及计算了不同分辨率下的结构参数的差异。测量结果与计算结果符合得很好,讨论了分辨率如何影响EXAFS谱及结构参数,给出了修正方法。 关键词：     

Local structure and magnetic properties of Mn substituted manganites studied by EXAFS and Dc magnetic measurements

We report extended X-ray absorption fine structure (EXAFS) measurements at the Mn K edge and magnetic measurements performed on (La_1−xCa_x)(Mn_1−yM_y)O₃ samples (M=Cr or Ni; x=0.37 and 0.75 and y=0.03 or 0.08). The Mn substitution produces important effects on both the sides of the LaMnO₃-CaMnO₃ phase diagram. For x<0.5 the ferromagnetic-metallic phase maintains its main character even after Mn substitution, but both the doping species (Ni or Cr) lower T_C and broaden the magnetic transition, and the EXAFS study evidences two Mn-O distances, suggesting the presence of zones of distorted insulating phase. For x>0.5, after the doping with Cr, the charge ordered phase persists but on a shorter scale, whereas the Jahn-Teller distortion is weakened as indicated by EXAFS measurements, and the formation of ferromagnetic clusters is evidenced by magnetic measurements.EXAFS and magnetic measurements are in mutual agreement, thus confirming the correlation between the local disorder determined by charge localization and magnetic degrees of freedom.     

X-ray K-absorption fine structure study of CuxZn1-xFe2O4 (x = 0.2, 0.4, 0.6 and 0.8)

For Cu_xZn_1-xFe₂O₄ spinel ferrites (x = 0.2, 0.4, 0.6 and 0.8), EXAFS of Cu and Fe K-absorption edges have been studied employing LSS theory. With change in copper content, in the spinel system, the basic dependence of cation distribution on degree of inversion has been reported by plotting variation of bond distance “d” with EXAFS parameter ‘α’. This is further substantiated by plotting chemical shifts (ΔE) with EXAFS parameter ‘α’. The necessity of the determination of final state wavefunction for the knowledge of distribution of charge is stressed.     

X-ray absorption investigation of some high oxidation states of six-co-ordinated iron in oxides of perovskite or K2NiF4-type structures

Synchrotron radiation has been used to investigate the X-ray absorption properties (Fe K-edge and EXAFS) of oxides containing iron with a high oxidation state (+3, +4, +5) in regular (O_h) or distorted (D_4h) octahedral co-ordination. It is shown that the usual monotonic variation of the edge energy with oxidation state depends closely on the electrical properties of the material and on the symmetry of the iron site. Moreover, the main absorption line is split when the symmetry of the iron site is lowered from O_h, to D_4h. New evidence is thus given that X-ray absorption spectroscopy can provide useful information about the absorbing atom site: it could be used in principle to point out certain types of distortions of the first shell of neighbors around the absorbing atom. A full EXAFS analysis of this shell and XANES (X-ray absorption near edge structure) data are also reported and discussed.     

The effect of self‐consistent potentials on EXAFS analysis

A theory program intended for use with extended X‐ray‐absorption fine structure (EXAFS) spectroscopy and based on the popular FEFF8 is presented. It provides an application programming interface designed to make it easy to integrate high‐quality theory into EXAFS analysis software. This new code is then used to examine the impact of self‐consistent scattering potentials on EXAFS data analysis by methodical testing of theoretical fitting standards against a curated suite of measured EXAFS data. For each data set, the results of a fit are compared using a well characterized structural model and theoretical fitting standards computed both with and without self‐consistent potentials. It is demonstrated that the use of self‐consistent potentials has scant impact on the results of the EXAFS analysis.     

X‐ray‐Raman‐scattering‐based EXAFS beyond the dipole limit

X‐ray Raman scattering (XRS) provides a bulk‐sensitive method of measuring the extended X‐ray absorption fine structure (EXAFS) of soft X‐ray absorption edges. Accurate measurements and data analysis procedures for the determination of XRS‐EXAFS of polycrystalline diamond are described. The contributions of various angular‐momentum components beyond the dipole limit to the atomic background and the EXAFS oscillations are incorporated using self‐consistent real‐space multiple‐scattering calculations. The properly extracted XRS‐EXAFS oscillations are in good agreement with calculations and earlier soft X‐ray EXAFS results. It is shown, however, that under certain conditions multiple‐scattering contributions to XRS‐EXAFS deviate from those in standard EXAFS, leading to noticeable changes in the real‐space signal at higher momentum transfers owing to non‐dipole contributions. These results pave the way for the accurate application of XRS‐EXAFS to previously inaccessible light‐element systems.     

Extended ELS fine structures above the M2,3 edges of Cu and Ni

Very detailed electron energy loss spectra of M_2,3 core levels of Cu and Ni in the reflection mode are presented. A careful analysis of the oscillations up to about 300 eV above the edges shows strict analogies to EXAFS and demonstrates that structural information can be obtained with this technique. A comparison with X-ray results above the Cu and Ni K-edges shows that very high accuracy in the radial distribution function can be obtained also with ELS spectra taken above shallow core levels.     

The effect of statistical noise on structural parameters in EXAFS data analysis

Summary We analyse the effect of the statistical noise on the physical parameters derived from an EXAFS spectrum. Two separate classes of experimental cases are discussed, namely heavy and light backscatterers. The weight of the noise in the two cases is, in fact, quite different, depending on thek range useful for the analysis of the data. We find that for heavy backscatterers the statistical noise can usually be neglected, while for light backscatterers it can be one of the main limitations in obtaining reliable results from EXAFS. Paper presented at the Workshop on ?EXAFS Data Analysis in Disordered Systems?, held in Parma, October 5–7, 1981.     

A connection between Raman intensities and EXAFS Debye-Waller factors in amorphous solids

For tetrahedrally-bonded amorphous systems a connection is derived between the Raman scattering intensity and the Debye-Waller factor in the Extended X-ray Absorption Fine Structure (EXAFS). Specifically it is argued that mechanisms contributing to the reduced Raman intensity would yield spectra proportional to the projected density of modes contributing to the EXAFS Debye-Waller factor, provided correlations in bond-bond motions are negligible. Model calculations support this conjecture. An expression for the coupling constant appearing in the Raman intensity is also obtained.