The authors review the development of extended x-ray absorption fine structure (EXAFS) within the last decade. Advances in experimental techniques have been largely stimulated by the availability of synchrotron radiation. The theory of EXAFS has also matured to the point where quantitative comparison with experiments can be made. The authors review in some detail the analysis of EXAFS data, starting from the treatment of raw data to the extraction of distances and amplitude information, and they also discuss selected examples of applications of EXAFS chosen to illustrate both the strength and limitations of EXAFS as a structural tool.

Extended x-ray absorption fine structure—its strengths and limitations as a structural tool

The atomic configuration of metallic glasses is a long-standing issue important to the understanding of their properties Nanobeam electron diffraction experiments now enable a direct determination of the local atomic order in a metallic glass The determination of the atomic configuration of metallic glasses is a long-standing problem in materials science and solid-state physics1,2 So far, only average structural information derived from diffraction and spectroscopic methods has been obtained Although various atomic models have been proposed in the past fifty years3,4,5,6,7,8, a direct observation of the local atomic structure in disordered materials has not been achieved Here we report local atomic configurations of a metallic glass investigated by nanobeam electron diffraction combined with ab initio molecular dynamics simulation Distinct diffraction patterns from individual atomic clusters and their assemblies, which have been theoretically predicted as short- and medium-range order6,7,8, can be experimentally observed This study provides compelling evidence of the local atomic order in the disordered material and has important implications in understanding the atomic mechanisms of metallic-glass formation and properties

Direct observation of local atomic order in a metallic glass

: Fourier inversion of EXAFS data is a powerful new technique for structural investigation of non-crystalline materials with the unique capability to determine the near neighbor environment of each type of atom in a complex material. The radial structure function so obtained contains information on the number, distance to, and distribution of atoms surrounding the absorbing atom. The data on Ge and Ge02 generally agree with previous findings but with added detail. The environment of both Ge and Se in amorphous GeSe show a significant change from the crystalline material with evidence for local satisfaction of the directed bonding tendencies of each atomic species. (Author)

The Structure of Non-Crystalline Materials

Precipitation behavior of an Al–Cu alloy during isothermal aging at low temperatures

Publisher Summary This chapter describes extended x-ray absorption fine structure spectroscopy (EXAFS) spectroscopy with a breadth sufficient to communicate to the non-expert potential of this technique. It discusses the theoretical underpinnings and experimental state of the art with depth sufficient to delineate clearly to a practitioner the capabilities and limitations of this spectroscopy. A especially useful expression is derived for the EXAFS from basic considerations, delineating explicitly the approximations required at each step. The more significant of these approximations are examined in the context of both theory and experiment. The calculation of EXAFS spectra is also discussed. The structural information content of the EXAFS is set forth explicitly and compared with the information available from a diffraction measurement. Finally, a survey is presented of the wide and growing variety of the applications of this technique, with particular emphasis on those in condensed matter physics.

Extended X-Ray Absorption Fine Structure Spectroscopy

EXAFS spectra of three forms of Al-Cu alloys, namely solid solutions, dispersed G.P. zones and the θ' phase, have been obtained using synchrotron radiation. The well-known structure of the θ' phase has been used to check the validity of the theoretical parameters which enter into the analysis. These experiments lead to an understanding of the core effect around the copper atom in solid solution, the chemical composition of the G.P. zone and the correlated local deformation of the surrounding matrix. In the solid solution the average distance between Cu and its Al nearest neighbours is 2.725 ± 0.02 A, compared to 2.86 A for the Al-Al distance in the matrix. G.P. zones are shown to be monolayers with an average 50-50 composition and the spacing between the two (200) planes is measured to be 1.68 A, compared to 2.0225 A for the d200 in the matrix.

EXAFS studies on Al-Cu alloys

2014 Since a few years, EXAFS has become popular as a new tool for structural studies of local order in disordered systems, as well as in dilute ones. The first reason for such a boom is that it is a very simple absorption measurement experiment, which is easily performed as soon as one gets the high photon fluxes emitted in the Xray range by the synchrotrons and electron storage rings. It has also unique capabilities in comparison to the scattering of X-rays or neutrons : 2014 it is a very local probe which is limited to the study of the first or second coordination shell around a given

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Effet Mössbauer et techniques complémentaires : III. — L'EXAFS appliqué aux déterminations structurales de milieux désordonnés

The local atomic structure of an amorphous NiZr 2 alloy was investigated using the anomalous wide-angle x-ray scattering (AWAXS), differential anomalous scattering, and reverse Monte Carlo (RMC) simulations techniques. The AWAXS measurements were performed at eight different incident photon energies, including some close to the Ni and Zr K edges. From the measurements eight total structure factors S(K,E) were derived. Using the AWAXS data four differential structure factors DSF i (K,E m ,E n ) were derived, two about the Ni and Zr edges. The partial structure factors S N i - N i (K), S N i - Z r (K), and S Z r - Z r (K) were estimated by using two different methods. First, the S(K,E) and DSF i (K,E m ,E n ) factors were combined and used in a matrix inversion process. Second, three S(K,E) factors were used as input data in the RMC technique. The coordination numbers and interatomic distances for the first neighbors extracted from the partial structure factors obtained by these two methods show a good agreement. By using the three-dimensional structure derived from the RMC simulations, the bond-angle distributions were calculated and they suggest the presence of distorted triangular-faced polyhedral units in the amorphous NiZr 2 structure. We have used the Warren chemical short-range order parameter to evaluate the chemical short-range order for the amorphous NiZr 2 alloy and for the NiZr 2 compound. The calculated values show that the chemical short-range order found in these two materials is similar to that found in a solid solution.

https://arxiv.org/pdf/cond-mat/0212168.pdf

Structural study of an amorphous NiZr 2 alloy by anomalous wide-angle x-ray scattering and reverse Monte Carlo simulations

The extended x-ray absorption fine structure (EXAFS) spectra of Al-3 at.% Mg and Al-7 at.% Mg have been measured on the magnesium $K$ edge (1303 eV) using the soft-x-ray beam delivered by the ACO (Anneau de Collision d'Orsay) storage ring, as well as the EXAFS of elemental aluminium (1556 eV) and magnesium. With the use of quasiexperimental phase shifts determined from aluminum and magnesium data, the first-shell radius around a magnesium impurity in an Al-Mg alloy has been determined within \ifmmode\pm\else\textpm\fi{}0.015 \AA{}, yielding a direct measurement of the elastic core effect which turns out to be large \ensuremath{\sim}0.08 \AA{}. It is discussed in the framework of the elastic theory of continuous media and of the lattice static method, both of which fail to give the correct magnitude of the local relaxation.

Elastic core effect in Al-Mg alloys by extended x-ray absorption fine structure in the soft-x-ray range

The X-ray anomalous scattering technique has been used to study the local structure of the Ni 2 Zr amorphous metallic alloy. Differential anomalous scattering has been utilized to determine the environment of each species. It has also been possible to evaluate the three partial structure factors using the method of least square analysis and to calculate the partial radial atomic distribution functions.

D. Raoux

Papers

EXAFS studies on Al-Cu alloys

Effet Mössbauer et techniques complémentaires : III. — L'EXAFS appliqué aux déterminations structurales de milieux désordonnés

Structural study of an amorphous NiZr 2 alloy by anomalous wide-angle x-ray scattering and reverse Monte Carlo simulations

Elastic core effect in Al-Mg alloys by extended x-ray absorption fine structure in the soft-x-ray range

Anomalous wide angle X-ray scattering of amorphous Ni2Zr alloy