CONTRIBUTION OF EXAFS-LIKE TECHNIQUES TO THE DETERMINATION OF LOCAL ORDER: APPLICATION TO TRIBOLOGY: The determination of local order at atomic scale becomes more and more fundamental for understanding superficial phenomena such as tribology and catalysis. Among a few techniques available in this field, we are only concerned with the EXAFS-like ones (EXAFS, SEXAFS, EXELFS, SEELFS, etc.). The difficulties appearing all along the numerical treatment of such spectra are carefully analysed and a methodology is proposed in view to obtain reliable radial distribution functions. Some applications of these techniques are presented in two fields connected with tribology: friction induced amorphisation in high vacuum and characterization of interfacial solid bodies created during a wear process including an antiwear additive.
X-ray diffraction topography is a widely used method to study crystal lattice defects by visualization. The properties of synchrotron radiation relevant to topography methods extend the possibilities of investigations. These properties are the following: a high intensity, a broad spectral range, a natural collimation, a linear polarization in the horizontal plane, and a pulsed time structure. The application of synchrotron radiation to X-ray topographic studies is described and some recent examples of experiments are presented.
The first half of this paper explains how X-ray diffraction can be sensitive to surface structure and morphology, even though X-rays interact only weakly with matter and hence penetrate deeply into the bulk medium. The basis of the crystal truncation rod construction is given, which demonstrates this sensitivity in a formal way. This is then illustrated with details of two problems of current interest which have been studied with synchrotron radiation at the National Synchrotron Light Source in New York. The structure of the Si(111)7×7 reconstructed surface, as determined in the vertical direction by X-ray reflectivity, is presented as a straightforward application of crystal truncation rods. Then we discuss alkali adsorption on Ag(110) surfaces which induces a "missing row" reconstruction. We measured the trends in the induced structural parameters as a function of Cs coverage on Ag(110), but found the greatest changes were associated with the location of the Cs instead. At high coverage this is ordered only in a one-dimensional sense, but as the coverage is reduced it becomes partially registered in the second direction, and, surprisingly, occupies a site higher above the surface.
In this article, we review the application of synchrotron X-ray topography to the study of the defect structure of materials. Following a discussion of source and detector requirements, in situ dynamic, stroboscopic, and ultra-high strain sensitivity experiments are discussed in the context of the new, third generation, synchrotron radiation sources. The intensity and continuous spectrum of synchrotron radiation is particularly important but further use of the time structure and polarization is timely. The future potential of the technique is discussed in the context of recent results at the European Synchrotron Radiation Facility in Grenoble.
Powder diffraction covers a variety of applications ranging from high resolution structure determination to time-resolved experiments. This wide range can be covered if the properties of synchrotron radiation, especially the continuous spectrum and the high collimation, are properly taken advantage of. The latter property favours parallel beam geometry. The instrumental function for such a setup is aberration-free and can be derived using only the angular characteristics of its optical components, namely the divergence of the incoming beam and the reflection or transmission curves of the optical elements. The continuous spectrum allows either anomalous dispersion experiments or the use of a broad energy band for experiments on a short time scale. To achieve high flexibility in combination with the use of specialised setups for the different types of experiments a special diffractometer has been built at HASYLAB.
The paper gives recent applications of EXAFS to the structural study of microcrystalline bimetallic particles. Special attention has been paid to the case of systems composed of neighbouring elements. Also it is shown how the use of X-ray diffraction techniques and surface science methods like ion scattering can be indispensable to interpret the EXAFS data correctly. In catalysis and surface science, experiments performed in total reflection mode can provide a detailed description of the adsorption site of atoms on a flat surface.
Extended X-ray absorption fine structure (EXAFS) spectroscopy offers additional experimental evidence to the solution of site occupancy problem in ternary NiAlX alloys. A study of local order in the stoichiometric Ni_{3}Al_{1-x}Fe_{x} ternary alloys (x = 0.02, 0.05, 0.10, 0.15, and 0.25) by EXAFS at the K-edge of Fe in the energy range from 7000 eV to 7600 eV are presented. Three models of substitutional behavior are considered - the preferential substitution of Fe atoms in: 1) Ni sites, 2) Al sites, 3) both sites. Data analysis was performed with theoretical and experimentally determined scattered photoelectron phases and amplitudes. The results of the EXAFS analysis are consistent with the picture in which most of the Fe atoms substitute Al sites and less than 25% of Fe atoms substitute Ni sites. Alloys with more than 10 at% of Fe showed higher tendency for ordering than those containing less than 10 at% Fe.
Applications of extended X-ray absorption fine structure and X-ray edge structure to catalysis are described. Special attention is paid to the study of supported mono- and bimetallic catalysts. The method used to collect in situ data, under reaction conditions are also considered.
Synthetic alunite was obtained by two different synthesis methods. Powder data obtained with a standard Bragg-Brentano geometry of two analyzed samples are reported for a broad angular range. The chemical analysis as well as the unit-cell dimensions indicate that the analyzed alunite is K and Al deficient and contains excess water. A need for high resolution diffraction experiments is expressed.
By X-ray powder diffraction and magnetometric methods we investigated the effect of phase hardening during γ-α-γ transitions. In this case austenite becomes more stable to subsequent γ-α-γ transitions, the dislocation density increases, the grain reduce in fragments and the hardness increases, too. In addition the γ-solid solution in some steels is unstable during ageing. The phase hardening was found to stabilize the γ-solid solution of Fe-Ni alloy to subsequent thermal treatment. This is consistent with an increase in carbon mobility after γ-α-γ transitions. That is why decarbonizations of austenite take place during increase in extent of phase hardening.
Progress in the instrumentation and, in particular, in the photon sources makes it possible to implement a number of established X-ray spectroscopies in a high-lateral-resolution mode. We discuss the general trends of this field, and then we present a detailed analysis of a particular and very interesting branch: photoemission spectromicroscopy. The results include a recent world record in lateral and energy resolution, obtained by the MAXIMUM system at Wisconsin, and microimages of materials science systems as well as of neuron networks.
The X-ray standing wave technique has demonstrated in these last years to be a powerful method in the study of interfaces. In this paper the fundamentals of the technique will be given, together with examples of applications in the field of metal-semiconductor, of buried semiconductor-semiconductor interfaces and in structural studies of Langmuir-Blodgett films.
The principles of small-angle X-ray scattering are briefly reviewed with emphasis on the particular advantages of the use of synchrotron radiation. The application of the technique is illustrated by several examples concerning precipitates in alloys, internal oxidation, porous materials, as well as the structure of bone.
Sintered AlN ceramics were implanted by Ti, Fe and Cu ions up to 1.9 × 10^{17} atoms/cm^{2} at mean energies of 70-110 keV in order to investigate the role of the chemical properties of the implanted species. on the phase formed during the implantation process. X-ray absorption studies were performed at room and at liquid nitrogen temperature to give information on the resulting systems and local environments of the Ti, Fe and Cu atoms. We observe the formation of TiN even for as-implanted samples, while the Cu ions aggregate to clusters. The Fe implanted samples show an intermediate behavior with both nitride formation and Fe clustering. In conclusion, the heat of formation is found to be a key parameter for the final system.
Local atomic arrangement in evaporated amorphous Cd-As and Zn-P films was investigated by extended X-ray absorption fine structure. Results of modelling indicate that bonds between atoms of the same kind occur for both (Cd,Zn)-rich and (As,P)-rich compositional regions. This specific chemical ordering, much alike to that in the corresponding crystalline polymorphs, appears to play a predominant role in determining of the local structures in these materials.
The polycrystalline samples of copper and vanadium doped as well as undoped (ZrO)_{0.8}(Y_{2}O_{3})_{0.2} were synthesized and studied using X-ray diffraction, scanning electron microscopy and electron paramagnetic resonance (EPR) techniques in order to investigate their structural and magnetic behaviour. The effect of exposure to lithium vapours on the host lattice of these ceramic materials is also studied using EPR technique. It has been observed from X-ray diffraction study that the samples consist of mainly two phases monoclinic and tetragonal. The monoclinic phase is in dominant proportion. Scanning electron microscope study showed that the morphology of the samples is similar to each other. EPR studies showed interesting changes on diffusion of lithium into the lattice of the host materials.
X-ray absorption experiments were performed at the Kedge of Co and Zn in Zn_{1-x}Co_{x}S and Zn_{1-n}Co_{x}Se compounds with x = 0.25, 0.16, 0.10, 0.05, 0.00 in the sulphides samples and x = 0.07, 0.02, 0.00 in the selenide ones. Analysis of the extended X-ray absorption fine structure oscillations using phase and amplitudes either from McKale code or from standard samples, gave the distance, number of atoms and the Debye-Waller factors for nearest neighbours. We found that the cation-anion distances Zn-S(Se) or Co-S(Se) are systematically shorter in ZnS than in ZnSe matrix, the Zn-anion distance is always larger than the Co-anion one without any significant Co content dependence. This allowed us to estimate the covalent radius of Co in the studied matrices to be 0.025 Å smaller than the Zn covalent radius and stated that covalent radius of Co is independent of the type of surrounding atoms. In addition the Debye-Waller factors indicated a better ordering in ternary compounds than in the binary standard ones in agreement with rocking curve measurements.
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