Other Diffraction Methods

EM plays a crucial role in the development of thermodynamic data, especially for defective solids, multi-phase solids and solids with coexisting intergrowth structures. These microstructural details, which are essential to catalytic properties, cannot be revealed readily by other diffraction methods which tend to average structural information. The formation of anion vacancies in catalytic reactions and the resulting extended defects are described here, from which an improved understanding of the formation of CS planes and their role in catalysis can be obtained. These general results are applicable to other CS structures. 

Other diffraction methods include electron diffraction, which may be used to determine the structures of gases or of volatile liquid substances that cannot be obtained as crystals suitable for x-ray diffraction, and neutron diffraction, which has special application for crystals in which the exact location of hydrogens is desired. Hydrogen does not have sufficient scattering power for x rays to be located precisely by x-ray diffraction. 

Chapter 9, "Other Diffraction Methods," builds upon your understanding of X-ray crystallography to help you understand other methods in which diffraction provides insights into the structure of large molecules. These methods include fiber diffraction, neutron diffraction, electron diffraction, and various forms of X-ray spectroscopy. These methods often seem very obscure, but their underlying principles are similar to those of X-ray crystallography. 

Single JT molecules or clusters in a crystalline environment may be found distorted, when they are investigated by X-ray (or other) diffraction methods. In his monumental work Liehr extensively dealt with JT instabilities and static distortions for many types of molecular structures. We have previously seen (Sect. 2.1) that one of the equivalent distorted configurations could be stabilized by any weak low symmetry perturbation In order to be sure that a given distortion is JT induced and not due to lattice forces a comparison with isomorphous compounds containing non-JT ions or an analysis of analogous compounds are necessary ... 

Another characteristic of electron diffraction in comparison with other diffraction methods is multiple scattering. The interaction between the electron beam... 

In the case of the powder method, all the symmetry equivalent reflections superimpose because they all have the same spacing (Section 3.5.3). Thus, this method does not allow us to observe the Laue symmetry, only the metric of the unit cell. For the same reason, the rotation method is also poorly adapted for the determination of the symmetry. There are other diffraction methods, which are not described in this work, by which all the reflections may be individually observed without superposition from which it is also possible to determine the Laue class of a crystal and consequently the crystal system. Is it possible to obtain other information about the symmetry and, in particular, the Bravais class and the space group ... 

Many valuable new results may be expected in this respect from the wider ranging application of electron paramagnetic resonance spectroscopy, NMR spectroscopy and solution X-ray examinations, and from the introduction of neutron diffraction and other diffraction methods in the chemistry of non-aqueous solutions. New possibilities in this field, which have by no means been fully exploited, are Mossbauer and ESCA investigations of rapidly frozen solutions. 

By means of X-ray diffraction or a number of other diffraction methods, the distance between the centers of the ions in a crystal can be measured. These distances can then be used to devise an extensive and consistent system of effective ionic radii for the solid state [2,3]- The numerical values of these so-called ionic radii are slightly dependent upon the coordination number. In Table 1 (see inside back cover) are given, among other data, the effective radii according to Goldschmidt for a coordination number of six. 

One other diffraction method that has proved useful for sufficiently volatile organometallic compounds is electron diffraction. In this technique the organometallic compound is introduced into a vacuum chamber through a nozzle, and an electron beam is passed through the stream of molecules. The resulting diffraction pattern contains much less information than does an X-ray diffraction pattern, but by making simple assumptions about the structure of the molecule, valuable data can be obtained. A useful feature of the results is that they refer to the molecule in an isolated state in a vacuum, so solvation or crystal packing effects are absent. 

Other DifFraction Methods Used to Study Proton Transfer Reactions... 

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