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Extended X-ray absorption fine structure

About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.


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TL;DR: In this paper, the dispersion of Ni ions in these samples has been studied by X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS), and Xray absorption fine structure (XAFS), showing that the atomic concentration of Ni and Mg against the depth is almost constant and close to those of bulk concentrations.
Abstract: NiO−MgO samples (NixMg1-xO:x = moles of Ni/(moles of Ni + moles of Mg)) were prepared by impregnation of MgO powder with an aqueous solution of Ni(NO3)2, followed by calcination at 773 K. The dispersion of Ni ions in these samples has been studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS). XPS of the depth profile of the samples shows that the atomic concentration of Ni and Mg against the depth is almost constant and close to those of bulk concentrations. The analyses by EXAFS and XANES reveal the absence of the preferential segregation of nickel oxide phase or magnesium oxide phase in all samples, suggesting the substitution of Ni ions for Mg ions. A detailed analysis by the curve-fitting method indicates that Ni ions dissolving into MgO are dispersed at random, regardless of the compositions of the samples. These results suggest that these samples form solid solutions over the entire composition range, even by calcination at 773 K.

85 citations

Journal ArticleDOI
TL;DR: In this paper, the local structure and effective chemical valency of Mn impurity atoms incorporated in wideband-gap (Ga,Mn)N epilayers have been investigated by using x-ray absorption fine structure techniques.
Abstract: Local structure and effective chemical valency of Mn impurity atoms incorporated in wide-band-gap (Ga,Mn)N epilayers have been investigated by using x-ray absorption fine structure techniques. The x-ray results provide direct evidence for the substitution of majority Mn atoms for the Ga sites in GaN, with an effective valency close to Mn(II), up to a rather high Mn concentration about 2 at. %. A small fraction of the impurity atoms could also form Mn clusters.

85 citations

Journal ArticleDOI
TL;DR: The ion pairing structure of Ni(Br)2 solutions (0.2 and 0.4 molal) under supercritical conditions was determined using x-ray absorption fine structure (XAFS) spectroscopy as discussed by the authors.
Abstract: The ion pairing structure of Ni(Br)2 solutions (0.2 and 0.4 molal) under supercritical conditions was determined using x-ray absorption fine structure (XAFS) spectroscopy. These first measurements of the average bulk structure show that approximately one Br− counterion is associated with each Ni2+. The Ni2+-to-Br− distance of 2.40 A is very accurately determined and the strength of this interaction, as indicated by the Debye–Waller factor (σ2=0.009 A2), shows that the bromine anion is very tightly bound to the nickel cation under these supercritical conditions. In addition to the onset of ion pairing interactions, there is also a dramatic transition in the hydration structure. Results show a loss of about 50% of the waters in the first shell upon going from ambient to a hydrothermal condition of 425 °C and 690 bar. Finally, we use molecular dynamics simulations with refined intermolecular potentials to directly calculate XAFS spectra that are shown to quantitatively reproduce the experimental results for ...

85 citations

Journal ArticleDOI
TL;DR: In this paper, the effect of large electric fields on the optical absorption coefficient of germanium at room temperature was studied and the experimental results were in fairly good agreement with the theoretical predictions of Franz, Keldysh, and others.
Abstract: By use of the carrier-depleted region of a reverse-biased $p\ensuremath{-}n$ junction, it has been possible to study the effect of large electric fields on the optical absorption coefficient of germanium at room temperature. In the energy range below the edge of the direct interband transitions, the absorption coefficient increases very strongly with field, and the experimental results are in fairly good agreement with the theoretical predictions of Franz, Keldysh, and others. In the region of the indirect transitions, the changes in the absorption coefficient with field oscillate between positive and negative values as a function of energy. This behavior can be related to the phonon-assisted processes first observed by Macfarlane et al.

85 citations

Journal ArticleDOI
TL;DR: In this work, in situ extended X-ray absorption fine structure (EXAFS) at the LIII edge of Pt was used to show that isolated Pt atoms interact with supported cobalt clusters without forming observable Pt--Pt bonds.
Abstract: The addition of platinum metal to cobalt/alumina-based Fischer-Tropsch synthesis (FTS) catalysts increases both the reduction rate and, consequently, the density of active cobalt sites. Platinum also lowers the temperature of the two-step conversion of cobalt oxide to cobalt metal observed in temperature programmed reduction (TPR) as Co3O4 to CoO and CoO to Co0. The interaction of the alumina support with cobalt oxide ultimately determines the active site density of the catalyst surface. This interaction can be controlled by varying the cobalt loading and dispersion, selecting supports with differing surface areas or pore sizes, or changing the noble metal promoter. However, the active site density is observed to depend primarily on the cluster size and extent of reduction, and there is a direct relationship between site density and FTS rate. In this work, in situ extended X-ray absorption fine structure (EXAFS) at the LIII edge of Pt was used to show that isolated Pt atoms interact with supported cobalt clusters without forming observable Pt--Pt bonds. K-edge EXAFS was also used to verify that the cobalt cluster size increases slightly for those systems with Pt promotion. X-ray absorption near-edge spectroscopy (XANES) was used to examine the remaining cobalt clusters after the first stage of TPR, and it revealed that the species were almost entirely cobalt (II) oxide. After the second stage of TPR to form cobalt metal, a residual oxide persists in the sample, and this oxide has been identified as cobalt (II) aluminate using X-ray photoelectron spectroscopy (XPS). Sequential in situ reduction of promoted and unpromoted systems was also monitored through XPS, and Pt was seen to increase the extent of cobalt reduction by a factor of two.

85 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023110
2022264
2021156
2020164
2019164
2018151