Lift‐off protocols for thin films for use in EXAFS experiments

Lift‐off protocols for thin films for improved extended X‐ray absorption fine structure (EXAFS) measurements are presented. Using wet chemical etching of the substrate or the interlayer between the thin film and the substrate, stand‐alone high‐quality micrometer‐thin films are obtained. Protocols for the single‐crystalline semiconductors GeSi, InGaAs, InGaP, InP and GaAs, the amorphous semiconductors GaAs, GeSi and InP and the dielectric materials SiO₂ and Si₃N₄ are presented. The removal of the substrate and the ability to stack the thin films yield benefits for EXAFS experiments in transmission as well as in fluorescence mode. Several cases are presented where this improved sample preparation procedure results in higher‐quality EXAFS data compared with conventional sample preparation methods. This lift‐off procedure can also be advantageous for other experimental techniques (e.g. small‐angle X‐ray scattering) that benefit from removing undesired contributions from the substrate.     

熔融Rb2ZnCl4的分子动力学模拟研究

采用在RbCl模拟和ZnCl₂模拟中选用的两体有效势，进行了晶态和熔融态Rb₂ZnCl₄的分子动力学模拟.模拟给出了Rb₂ZnCl₄ 6种径向分布函数，Zn-Cl和Rb-Cl间径向分布函数与新近的广延X射线吸收精细结构实验结果很好相符.在模拟产生的瞬态构型基础上进行了键序参数分析，研究了晶态和熔融态中的局部结构.模拟结果表明，Zn原子与其近邻Cl原子构成相当稳定的正四面体结构，并且熔融Rb_关键词：     

Theoretical and analytical Radial Distribution Function for non-polar mixtures

The theoretical and analytical Radial Distribution Function (RDF) for non-polar mixtures is presented, and the estimation methods of model parameters (such as the minimum molecular diameter d_ii, mean calibration factor of volume β_V and mean free path of relative motion λ_r,ij) are discussed. The calculated thermodynamic properties for H_e-H₂ fluid mixtures by this RDF fit well with that of MC simulations and classical results from the Yukawa-type EOS, in which the Double Yukawa (DY) potential was used and the quantum effect was considered. Compared with the semi-empirical RDF expressions in references, the advantage of this theoretical and analytical RDF is that it can be used for all kinds of potential functions of non-polar molecules, and it covers a very wide range of temperatures and densities. The extensive practical application of this RDF for chemistry, physics and technology needs further 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.     

用X射线异常色散效应测定二元非晶合金Fe82B18偏径向分布函数方法的简化

本文采用Mo,Cu,Co三种不同波长的X射线辐射,测定了Fe₈₂B₁₈二元非晶合金的径向分布函数。比较了不同波长辐射对测定径向分布函数的影响。应用X射线异常色散方法计算了Fe₈₂B₁₈二元非晶合金的偏径向分布函数。在计算时略去了B-B原子对对径向分布函数的贡献,因而简化了计算过程,并相对提高了计算精度。所得结果与国际上使用同类方法或其它方法的结果相近。 关键词：     

X-ray study of metal oxide based anodes for Li-ion batteries

Cells were made from Co₃O₄ and Co₂SnO₄ and lithium metal and tested to determine reversible lithium capacity. Li is reversibly inserted into Co₃O₄, as was observed by electrochemistry, coupled with changes of cobalt oxidation state as observed by Co K-edge EXAFS. On lithium insertion the Co₃O₄ is reduced to yield only metallic cobalt species, and then on lithium removal an oxide of Co is formed. The EXAFS data further showed that the initial reduction was to Co(II) and then to metallic Co, and that both the metallic and oxide phases were disordered, having low co-ordination numbers and large shell spacings. The electrochemical behaviour of the Co₂SnO₄ cells was closer to that of SnO₂ than Co₃O₄, but did exhibit changes obviously caused by the cobalt. EXAFS on Co₂SnO₄ cells revealed that the Co is reduced to metallic cobalt on the initial discharge, but does not convert back to an oxide on cycling. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

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.     

Structural study of highly dispersed cerium dioxides

Cerium dioxide samples of different dispersivity, obtained by different methods, have been investigated by X-ray diffraction (XRD) analysis, full-profile analysis of diffraction patterns, calculation of radial distribution function (RDF) of electron density, and extended X-ray absorption fine-structure (EXAFS) spectroscopy. According to the XRD data, the samples studied can be identified in the fluorite structure with conventional unit-cell parameters. However, the local structure of highly dispersed samples, determined by the RDF and EXAFS methods, changes: the coordination spheres for the Ce-O distances are represented by two distances.     

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.     

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.     

Measurement of two-dimensional distribution profile of lead in a flame by planar laser-induced fluorescence spectroscopy

Abstract  

Producing refuse-derived fuel (RDF) is one of the most effective measures of refuse treatment. However, RDF often consists of high level of lead. To reduce lead emission during combustion, understanding of lead behaviors in flames is required. In this study, we have applied planar laser-induced fluorescence spectroscopy to detect lead not only in a non-luminous methane–air flame, but also in a luminous RDF flame. In a methane–air flame, the number density of Pb atoms does not depend on the flame temperature, but also on the combustion environments. In RDF flames, because of acceleration of oxidizing process of Pb, Pb fluorescence profile obtained at 25% O₂ became weaker than that at 20% O₂.     

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.     

X-ray absorption spectroscopy in YBa2Cu3O7−x superconductors with variable oxygen content

We report the results of XANES and EXAFS measurements performed on YBa₂Cu₃O_7–x compounds withx ranging from 0 to 1. Data for Cu and YK-edges are presented and definite trends in the behaviour of the main near edge structures are singled out, which allow to address the question of the Cu and Y oxidation states and to elucidate the role of the chemical environment in determining the shape of the absorption lines. The inter-atomic distances as obtained by EXAFS generally agree with the results of X-ray diffraction data with the exception of the Y–O distances. Possible sources of this discrepancy are indicated.Sadly, Prof. G.M. Antonini died before the appearance of this paper     

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.     

Surface EXAFS via differential electron yield

Surface‐sensitive analysis via extended X‐ray absorption fine‐structure (EXAFS) spectroscopy is demonstrated using a thickness‐defined SiO₂ (12.4 nm)/Si sample. The proposed method exploits the differential electron yield (DEY) method wherein Auger electrons escaping from a sample surface are detected by an electron analyzer. The DEY method removes local intensity changes in the EXAFS spectra caused by photoelectrons crossing the Auger peak during X‐ray energy sweeps, enabling EXAFS analysis through Fourier transformation of wide‐energy‐range spectral oscillations. The Si K‐edge DEY X‐ray absorption near‐edge structure (XANES) spectrum appears to comprise high amounts of SiO₂ and low Si content, suggesting an analysis depth, as expressed using the inelastic mean free path of electrons in general electron spectroscopy, of approximately 4.2 nm. The first nearest neighbor (Si—O) distance derived from the Fourier transform of the Si K‐edge DEY‐EXAFS oscillation is 1.63 Å. This value is within the reported values of bulk SiO₂, showing that DEY can be used to detect a surface layer of 12.4 nm thickness with an analysis depth of approximately 4.2 nm and enable `surface EXAFS' analysis using Fourier transformation.     

The pH dependence of Am(III) complexation with acetate: an EXAFS study

The complexation of acetate with Am(III) is studied as a function of the pH (1–6) by extended X‐ray absorption fine‐structure (EXAFS) spectroscopy. The molecular structure of the Am(III)–acetate complexes (coordination numbers, oxygen and carbon distances) is determined from the raw k³‐weighted Am L_III‐edge EXAFS spectra. The results show a continuous shift of Am(III) speciation with increasing pH value towards the complexed species. Furthermore, it is verified that acetate coordinates in a bidentate coordination mode to Am(III) (Am—C distance: 2.82 ± 0.03 Å). The EXAFS data are analyzed by iterative transformation factor analysis to further verify the chemical speciation, which is calculated on the basis of thermodynamic constants, and the used structural model. The experimental results are in very good agreement with the thermodynamic modelling.     

A SAFT model for associating Lennard-Jones chain mixtures

The recently proposed equation of state of statistical associated fluid theory (SAFT) is extended to associating Lennard-Jones (LJ) chain mixtures. In this extension, a new radial distribution function (RDF) for LJ mixtures is derived around the LJ potential size (σ _ij ). The RDF expression is completely analytical and real. Comparisons with computer simulation data under various conditions indicate that the RDF is very accurate up to its first peak. The new RDF, together with a previously established equation of state for LJ mixtures, is employed to study LJ chain mixtures by combining with Wertheim's first-order perturbation theory. The resulting equation of state is tested satisfactorily against computer simulation data for both non-associating and associating LJ chain mixtures, with a performance similar to its predecessors for pure LJ chains and LJ mixtures. The SAFT model is uniquely featured by being totally mixing-rule free and by being adjustable at both chain bonding and association sites. Moreover, the SAFT model is formulated very generally, so that it is applicable to both homonuclear and heteronuclear chain mixtures.     

Temperature dependence of EXAFS in amorphous arsenic

EXAFS measurements at different temperatures on amorphous arsenic are reported. The low values of the EXAFS mean square relative displacement σ² for the first coordination shell are consistent with a distribution of As₄ pyramidal units throughout the amorphous network, giving a locally very ordered first shell, whose disorder is essentially dynamic in character. The temperature dependence of the vibrational contribution to the EXAFS Debye-Waller factor is well described by an Einstein oscillator model.     

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.     

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.