EXAFS interatomic distances

EXAFS Extended X-ray absorption fine structure spectroscopy. A spectroscopic technique which can determine interatomic distances very precisely. 

EXAFS Extended x-ray absorption fine structure [177, 178] Variation of x-ray absorption as a function of x-ray energy beyond an absorption edge the probability is affected by backscattering of the emitted electron from adjacent atoms Number and interatomic distance of surface atoms... 

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

The Fourier transform of the EXAFS of Figure 5 is shown in Figure 6 as the solid curve It has two large peaks at 2.38 and 2.78 A as well as two small ones at 4.04 and 4.77 A. In this example, each peak is due to Mo—Mo backscattering. The peak positions are in excellent correspondence with the crystallographically determined radial distribution for molybdenum metal foil (bcc)— with Mo—Mo interatomic distances of2.725, 3.147, 4.450, and 5.218 A, respectively. The Fourier transform peaks are phase shifted by -0.39 A from the true distances. 

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

From results on interatomic distances derived from analysis of EXAFS data, one can draw some conclusions about the structure of platinum-iridium clusters (13,17). If the clusters were truly homogeneous, the interatomic distance characteristic of the platinum EXAFS should be identical to that characteristic of the iridium EXAFS. When we analyze EXAFS data on the clusters, however, we do not find this simple result. We find in general that the distances are not equal. The data indicate that the clusters are not homogeneous in other words,the environments about the platinum and iridium are different. We conclude that the platinum concentrates at the surface or boundary of the clusters. In the case of very highly dispersed platinum-iridium clusters on alumina, the clusters may well have "raft-like" two dimensional structures, with platinum... 

Recently we reported EXAFS results on bimetallic clusters of iridium and rhodium, supported on silica and on alumina (15). The components of this system both possess the fee structure in Efie metallic state, as do the components of the platinum-iridium system. The nearest neighbor interatomic distances in metallic iridium and rhodium are not very different (2.714A vs. 2.690A). From the results of the EXAFS measurements, we concluded that the interatomic distances corresponding to the various atomic pairs (i.e., iridium-iridium, rhodium-rhodium, and iridium-rhodium) in the clusters supported on either silica or alumina were equal within experimental error. Since the Interatomic distances of the pure metals differ by only 0.024A, the conclusion is not surprising. 

Table 8.53 shows the main features of XAS. The advantages of EXAFS over diffraction methods are that the technique does not depend on long-range order, hence it can always be used to study local environments in amorphous (and crystalline) solids and liquids it is atom specific and can be sensitive to low concentrations of the target atom (about 100 ppm). XAS provides information on interatomic distances, coordination numbers, atom types and structural disorder and oxidation state by inference. Accuracy is 1-2% for interatomic distances, and 10-25 % for coordination numbers. 

The basic intent behind any EXAFS data analysis is to be able to extract information related to interatomic distances, numbers, and types of backscattering neighbors. In order to accomplish this, there are a number of steps involved in the data analysis, and these include ... 

Such a function exhibits peaks (Fig. 9C) that correspond to interatomic distances but are shifted to smaller values (recall the distance correction mentioned above). This finding was a major breakthrough in the analysis of EXAFS data since it allowed ready visualization. However, because of the shift to shorter distances and the effects of truncation, such an approach is generally not employed for accurate distance determination. This approach, however, allows for the use of Fourier filtering techniques which make possible the isolation of individual coordination shells (the dashed line in Fig. 9C represents a Fourier filtering window that isolates the first coordination shell). After Fourier filtering, the data is back-transformed to k space (Fig. 9D), where it is fitted for amplitude and phase. The basic principle behind the curve-fitting analysis is to employ a parameterized function that will model the... 

Coordination Number (N) and Interatomic Distance (R) of Co-O and Co-Co Shells Calculated from Co K-edge EXAFS Spectra of Calcined Catalysts, and Polycrystalline Co304 and a-Co2Si04 (Figure 6.10)... 

Information concerning the orientation of the molecule can be inferred from the polarization dependence. NEXAFS is sensitive to bond angles, whereas EXAFS is sensitive to the interatomic distances. Linearly polarized X-rays are best suited for molecules possessing directional bonds. This is best exemplified for flat 7T-conjugated molecules lying flat on surfaces. When the electric field vector E is aligned along the surface normal, features due to the out-of-plane tt orbitals... 

In Sect. 2, the structure of AU55 is discussed, utilizing the interatomic distances obtained from EXAFS [39,40,41]. DSC results [57] give further evidence that Au, is a stoichiometric molecular compound, and recent SIMS results [35, 36], confirm the molecular mass, the structure, and the stoichiometry of this compound, as originally postulated by Schmid [16]. 

EXAFS measurements on AU55 prepared by Schmid [39,41], as well as on independently prepared samples [40], have shown clearly that there is only a single interatomic distance between the gold atoms in this material, a distance about 4% shorter than that in bulk gold. 

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