New insight on the local structure of Cu2+ion in the solution of CuBr2by EXAFS studies
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CuBr2 solutions at different concentrations were studied by extended X-ray absorption fine structure (EXAFS) at the Cu K edge. In the saturated solution Cu2+ ions have chemical bonds with 3.0 oxygen atoms and 0.9 Br ion at about 1.96 Å and 2.42 Å, respectively. It indicates that the CuBr4-2 configuration exists with a ratio of 25% under this condition. In the dilute solutions no evidence of Br ions contributions in the first shell around Cu2+ ions occurs. The almost identical X-ray absorption near edge structure (XANES) and EXAFS characters address similar local environments around Cu2+ in agreement with results of the EXAFS fit taking into account only the contributions of Cu-O bonds.Keywords:
XANES
Local structure
K-edge
XANES
K-edge
Absorption edge
Coordination number
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XANES
X-Ray Spectroscopy
Coordination number
X-ray absorption spectroscopy
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We use x-ray absorption spectroscopy to investigate the local structure and electronic properties of bulk La$_{1-y}$Sr$_{y}$Co$_{1-x}$Nb$_x$O$_3$ ($y=$ 2$x$ as LSCNO and $y=$ 0 as LCNO) samples. The x-ray absorption near-edge spectra (XANES) of LSCNO at Co K-edge affirm the valence state of Co in 3+. However, in the case of the LCNO, a subtle variation in the valence state of Co ions from 3+ to 2+ is evident with Nb substitution. The detailed analysis of the Fourier transform (FT) of the extended x-ray absorption fine structure (EXAFS) for the LSCNO samples exhibit the two groups of the bond-lengths owing to the Jahn-Teller (JT) distortion in the CoO$_6$ octahedra, which manifest that the Co$^{3+}$ ions exist in the intermediate spin-state (t$^5_{2g}$e$^1_g$) at room temperature. However, we find that the JT distortion is not present in LCNO samples for $x>$0.025 due to an increase in the high-spin Co$^{2+}$/Co$^{3+}$ ions accompanied by the Nb--induced structural transformation. Intriguingly, the La L$_3$-edge spectra for the LSCNO samples exhibit that the La ions exist in the trivalent state and the local disorder around La atoms decreases with Sr and Nb substitution. Interestingly, the simulated FT of the EXAFS spectra at the La L$_3$-edge demonstrates the three groups of La--O bond lengths, which exhibit a monotonous change with Sr and Nb substitution. Moreover, the XANES measured at Sr and Nb K-edges confirm their oxidation state to be in 2+ and 5+, respectively.
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K-edge
X-ray absorption spectroscopy
Absorption edge
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K-edge
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XAFS (X-ray absorption fine-structure) spectra were measured near K-absorption edges of Ce (40.5 keV), Dy (53.8 keV), Ta (67.4 keV) and Pt (78.4 keV). The blunt K-edge jump due to the finite lifetime of the core hole was observed. This makes it difficult to extract EXAFS (extended X-ray absorption fine-structure) functions at low k values. Local structure parameters can be evaluated from the EXAFS spectra above K-absorption edges in the high-energy region as well as from those above L(III)-edges. It was found that the finite-lifetime effect of the core hole is effectively taken into the photoelectron mean-free-path term, as predicted theoretically.
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K-edge
Absorption edge
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This review gives a brief description of the theory and application of X-ray absorption spectroscopy, both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), especially, pertaining to photosynthesis. The advantages and limitations of the methods are discussed. Recent advances in extended EXAFS and polarized EXAFS using oriented membranes and single crystals are explained. Developments in theory in understanding the XANES spectra are described. The application of X-ray absorption spectroscopy to the study of the Mn4Ca cluster in Photosystem II is presented.
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Abstract X‐ray absorption spectroscopy (XAS), often referred to as X‐ray absorption fine structure (XAFS) spectroscopy, is a powerful technique available for determining the local coordination site structure and electronic properties of specific atoms in molecules. XAS can be performed on molecules in the gaseous, solution, or solid states, and generally every element is spectroscopically active. This technique utilizes high intensity X rays supplied by synchrotron radiation sources to selectively excite core electron transitions for a specific element of choice. XAS spectra are divided into two spectral regions, the X‐ray absorption near edge structure (XANES) region and the extended X‐ray absorption fine structure (EXAFS) region. XANES spectra can be utilized to provide: (1) qualitative information regarding the ligands coordinated to the absorbing atom, (2) semi‐quantitative information regarding the redox state of the absorbing atom, and (3) metal‐ligand coordination geometry. Simulations of the EXAFS data can provide information regarding absorber‐scatterer bond length values to extremely high accuracy (ca. ±0.02 Å), as well as coordination numbers and ligand identity at reduced accuracies. The high accuracy in bond lengths achieved from XAS makes this technique extremely sensitive for measuring subtle structural perturbations within molecules under perturbed or turnover conditions. When utilized with other techniques, XAS can supply the metrical parameters required to understand the chemical properties of specific elements in a molecule.
X-ray absorption spectroscopy
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A combined X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) methodology is here presented on a series of partially and fully reduced Au(III) samples. This allows monitoring the relative fraction of Au(III) and Au(0) in the studied samples, displaying a consistent and independent outcome. The strategy followed is based, for the first time, on two structural models that can be fitted simultaneously, and it evaluates the correlation among strongly correlated parameters such as coordination number and the Debye-Waller factor. The results of the present EXAFS and XANES approach can be extended to studies based on X-ray absorption spectroscopy experiments for the in situ monitoring of the formation of gold nanoclusters.
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Nanoclusters
Coordination number
K-edge
X-ray absorption spectroscopy
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CuBr2 solutions at different concentrations were studied by extended X-ray absorption fine structure (EXAFS) at the Cu K edge. In the saturated solution Cu2+ ions have chemical bonds with 3.0 oxygen atoms and 0.9 Br ion at about 1.96 Å and 2.42 Å, respectively. It indicates that the CuBr4-2 configuration exists with a ratio of 25% under this condition. In the dilute solutions no evidence of Br ions contributions in the first shell around Cu2+ ions occurs. The almost identical X-ray absorption near edge structure (XANES) and EXAFS characters address similar local environments around Cu2+ in agreement with results of the EXAFS fit taking into account only the contributions of Cu-O bonds.
XANES
Local structure
K-edge
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XANES
Chemisorption
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