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Physical Methods for Characterizing Solids
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
EXAFS is used for the determination of short-range order in non-crystalline materials, such as gases, liquids, amorphous powders, and nanocrystalline materials, where conventional diffraction techniques are of little use. In high energy accelerators, electrons are injected into an electron storage ring, captured, and accelerated around this circular path by a series of magnets. When the electrons are accelerated to kinetic energies above the MeV range, they are travelling very close to the speed of light and they emit synchrotron radiation (Figure 2.26). There are just over fifty synchrotron sources in the world of which only four, so-called ‘third-generation’, are the most powerful: ESRF at Grenoble, France, and PETRA-III, Hamburg, Germany operate at 6 GeV; APS, at Argonne, IL, 7 GeV; SPring-8 in Japan at 8 GeV; these huge storage rings have circumferences of up to 2300 m respectively. The UK facility DIAMOND Light Source has a ring of 561.6 m circumference and operates at 3 GeV. The X-radiation generated ranges from soft X-rays to hard X-rays (~300 eV–300 keV, 40 Å–0.04 Å). Unlike X-radiation from a conventional generator, synchrotron radiation is of uniform intensity across a broad band of wavelengths and several orders of magnitude (104–106) higher in intensity (Figure 2.27). The shortest X-ray wavelengths emerge as almost fully collimated, polarized beams.
Detector Characterization
Published in Alan Owens, Semiconductor Radiation Detectors, 2019
X-ray Absorption Fine Structure measurements (XAFS) are routinely carried out at synchrotron facilities to probe both short- and long-range order in materials. XAFS is a generic term and can be broken down into structure originating far from an absorption edge and structure originating close to the edge, which arise from different processes. Extended X-ray Absorption Fine Structure (EXAFS) is a diagnostic of short-range order by means of which details in the local geometry (atom types, bond lengths and bond angles) around the photo-absorbing atom can be extracted from far-edge spectra. X-ray Absorption Near Edge Structure (XANES), on the other hand, is a diagnostic of long-range order through which details of atom types and how they are structured collectively (the coordination environment) can be extracted from near-edge spectra. XAFS measurements, both EXAFS and XANES, can also be applied to detector metrology. For example, several authors have noted the wide spread in the radiation detection properties of Cd1-xZnxTe crystals and have attempted to use the structural information embedded in XAFS to find a link between performance and structural perfection.
exafs
Published in G B Stringfellow, Gallium Arsenide and Related Compounds 1991, 2020
Yoshikazu Takeda, Hirotaka Yamaguchi, Hiroyuki Oyanagi
EXAFS is a useful tool to determine the local structure such as the bond length, structural disorder and chemical disorder. In order to discuss the origin of the band gap anomaly from the microscopic point of view, we performed EXAFS measurements at the Ga K-edge on ordered and disordered Ga0.49In0.51P alloys lattice-matched to GaAs and on disordered alloy powders.
Structure and stability of δ-UZr2 phase in U-50 wt% Zr alloy
Published in Philosophical Magazine, 2022
EXAFS is an X-ray-based characterisation technique that gives information about the average local structure in terms of inter-atomic distance and the surrounding atomic species around a given species of the atom. The analytical regions of Figure 2 (marked by the arrows) were subjected to the background subtraction and fitting. The fitted results are further subjected to Fourier transformation followed by an additional fitting which finally gives rise to Figure 3(a) and (b) corresponding to the uranium and zirconium atom, respectively. From the fitting parameters of Figure 3(a) and (b), the near-neighbour information could be obtained. It is important to note that Figure 3(a) and (b) represents phase uncorrected fitting, which means the peak position does not indicate the near-neighbour distance in absolute terms. Finding peak positions from the graph also leads to errors due to the peak overlapping. Therefore, the inter-atomic distances can only be found out from the fitting parameters alone. The derived near-neighbour species and distances around U and Zr atoms are given in Table 1.
Investigation of temperature and pressure effects on thermodynamic parameters of intermetallic alloy in EXAFS
Published in Cogent Engineering, 2020
The extended X-ray absorption fine structure (EXAFS) spectroscopy is one of the powerful techniques for investigating structures of crystalline. The formalism for including anharmonic effects in EXAFS is often written through cumulant expansion approach. There are many methods that have been developed to study the temperature dependence of EXAFS cumulants. However, no theoretical calculations have been done to predict the dependence of cumulants and thermodynamic parameters on temperature with the effects of pressure in EXAFS spectra. It requires the more accurate interatomic interaction form for metallic systems such as the many-body embedded-atom potentials. The purpose of this work is to investigate the dependence of cumulants and thermodynamic parameters on temperature with effects of pressure in EXAFS spectra of crystals. Anharmonic correlated Einstein model has been applied for compound of copper and silver with 72% ratio through the interatomic potential which has been derived by the Morse effective potential.
Correlated Debye model for atomic motions in metal nanocrystals
Published in Philosophical Magazine, 2018
Interatomic forces are responsible for correlated atomic motions which constrain the vibrational modes and amplitudes in crystals and influence the thermal properties. Several models have been proposed to study this effect in near-neighbour coordination shells, like the classical Born–von Karman force model [1] and, in more recent times, atomistic simulations. Molecular Dynamics (MD), for example, can simulate relatively large (millions of atoms) portions of simple inorganic materials as well as complex biological systems [2–5]. In the context of polycrystalline inorganic materials, EXAFS is the reference technique to study the local atomic motion, providing detailed information on the Debye–Waller (DW) coefficients parallel and perpendicular to the bond axis, for the innermost coordination shells [6]. Besides X-ray absorption, elastic scattering can also give information on correlated motions by the Total Scattering approach (Pair Distribution Function (PDF) analysis [1,7,8] or pattern modelling by the Debye Scattering Equation (DSE) [9]), although the relevance of the information is more statistical, averaged over all coordination shells, while EXAFS has a strong point in resolving the local atomic environment of the inner shells.