Diffraction methods features

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. 

It is well known that crystal and electronic structures are interdependent and define the reactivity of chemical substances. In Section 1.4.2, it was noted that copper-porphyrin complex gives cation-radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation-radical, which belongs to n-type. The literature sources note, however, some differences in the reactivity of individual positions. A frequently observed feature in these n-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of porphyrin consistent with a localized structure rather than the delocalized structure usually ascribed to cation-radical. A pseudo Jahn-Teller distortion has been named as a possible cause of this alternation, and it was revealed by X-ray diffraction method (Scheidt 2001). 

For optimal use of the electron-diffraction method in large amplitude-motion studies, it is important to take advantage of the knowledge concerning potential functions as obtained by spectroscopical methods. Some features of the spectroscopically obtained findings are given in the next section. 

Intermediate Alkali Feldspars. The alkali feldspars with intermediate composition vary in their TL signal when seen in detail. All of them differ significantly as a group from the end members. However, several interesting common features throw light on their overall behaviour. These features suggest a new means for identifying structure, which differs from the traditional optical and X-ray diffraction methods. 

The study of crystals, mainly through their interaction with electromagnetic radiation has developed into the modern science of crystallography. Four distinct variations of crystallography have developed in the hands of mathematicians, physicists, mineralogists and chemists. The four branches have the fundamentals in common, but each has developed its own flavour and applications. Chemical crystallography focusses on molecular structure as revealed by diffraction methods. It has many features in common with theoretical methods for the study of molecular structure. 

Thousands of crystal structures have been analyzed by diffraction methods. Whenever covalence is the dominant chemical interaction, well-defined molecular units, held together by secondary forces such as van der Waals and/or hydrogen bonds, can be identified as the regular building blocks of the crystals. The geometrical features of such molecular units define the chemist s notion of structure. Still, there is no theory that defines molecular structure or electron density from first principles. 

In many cases, high-temperature modifications of sulfidic compounds cannot be quenched for room temperature examination. Inversion twinnings, crystal morphology, or other crystallographic features may indicate the appearance of polymorphism. Under these circumstances differential thermal analysis (DTA) can be suitable for the determination of the exact phase transition temperatures. DTA determinations are practically valuable if used in conjunction with high-temperature X-ray diffraction methods. DTA apparatus can operate up to 1100 °C and can be specially designed for sulfides2-4) individual experimental techniques are included in these references. 

While it is to some extent arbitrary, a classification of this kind provides a means of discussing some of the general features now emerging from studies of metallic oxides. We have stressed the evidence that a nonstoichiometric phase is disordered, but may be related to chemically similar phases of fixed composition where an anomaly of structure is ordered and identifiable by x-ray diffraction methods. Where such ordered phases are found, it is possible that features of them are retained as blocks or domains with short range order in the related berthollide. Efforts should be directed towards order-disorder effects, with a view to reconsidering the status of the nonstoichiometric compound with a very wide composition range. 

In terms of the structural features that are probed with various analytical methods, solid state nuclear magnetic resonance (SSNMR) may be looked upon as representing a middle ground between IR spectroscopy and X-ray powder diffraction methods. The former provides a measure of essentially molecular parameters, mainly the strengths of bonds as represented by characteristic frequencies, while the latter reflect the periodic nature of the structure of the solid. For polymorphs differences in molecular environment and/or molecular conformation may be reflected in changes in the IR spectrum. The differences in crystal structure that define a polymorphic system are clearly reflected in changes in the X-ray powder diffraction. Details on changes in molecular conformation or in molecular environment can only be determined from full crystal structure analyses as discussed in Section 4.4. 

One feature of NMR can sometimes be misleading. Compared with most other techniques it samples molecules over a relatively long time-scale (typically 0.01 to 0.1 seconds). Some molecules are fluxional with atoms exchanging rapidly between different positions, and when this happens NMR may see these positions as equivalent. For example IR and diffraction methods show clearly that the PF5 molecule has a trigonal bipyramidal structure with two different F positions (equatorial and... 

An interesting feature of XANES for structure determination in condensed matter is that it can be used to study the local structure both in crystalline and disordered materials. Therefore once a local structure has been solved for a crystalline material also the similar structure in amorphous, liquid or complex materials, where diffraction methods cannot be applied, can be solved. 

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