EXAFS spectra discussion

Fe(II)C20 2 H O is the only bivalent metallic oxalate whose XRD structure has been solved It is nevertheless isostructural with the Ni(II) and with the Zn(II) analogs. In the nickel derivative for example, the square planar NiO site within the planar ribbons is completed with two axial water ligands. The shortest Ni—Ni intermolecular contribution, computed from X-ray powder diffraction data of Ref. corresponds to 4 Ni at 5.20 A. The room temperature EXAFS spectrum (Fig. 14i, j) present 3 peaks. Peaks 1 and 2 are unambiguously identified as Ni—O and Ni—C contributions. The third peak will be discussed later on. Upon lowering the temperature towards 30 K, these first peaks are almost invariant while new contributions appear at larger distances. Therefore, the three first peaks are certainly due to intramolecular contributions while the temperature dependent contributions probably corresponds to intermolecular Ni—Ni distances. 

The other cofactor often neglected in discussions of the OEC is chloride. Again, EXAFS data have been obtained to address whether or not chloride is bound to the manganese cluster (33, 358, 359). These experiments are technically very difficult. There is the possibility that chloride (or bromide in bromide-substituted samples) may be observed in the EXAFS spectrum. However, we believe that it is unlikely that one can resolve this issue, because the signal-to-noise ratio may not be sufficient (156) to observe only one chloride per four manganese ions. Very recently, chloride EXAFS of manganese model complexes has been reported (360). This new application of EXAFS may be able to resolve this issue. 

EXAFS is part of the field of X-ray absorption spectroscopy (XAS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternatively called the near-edge X-ray absorption fine structure, NEXAFS). The combination of XANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article following this one for a discussion of SEXAFS and NEXAFS. 

In the first section will be presented XAS from the physical principles to data analysis and measurements. Then section 2 will be devoted to a discussion of a few examples to illustrate the power and limitations of XAS for gaining structural information. Examples are focused on EXAFS studies on nanocrystalline materials. Detailed reviews for applications on other fields of materials science or for presenting the complementary information available by the study of the X-ray Absorption Near Edge Structure (XANES) part of the X-ray absorption spectrum can be found in a number of books [3-5], A brief overview of the recent development of the technique regarding the use of X-ray microbeams available on the third generation light sources will be finally presented in the last section. 

In theory, to describe a XANES spectrum is not an easy task. The equations discussed earlier in this chapter for EXAFS are not valid at low k-values (i.e., energies close to that of the edge), and instead the X-ray absorption will have to be calculated from first principles, which is a specialism in itself. Fortunately, XANES spectra can usually be very well interpreted with the help of reference spectra of known compounds, and constructing linear combinations of references to fit the spectrum of the catalyst often works well to obtain quantitative information on composition. 

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