Electron diffraction electronic defects

Hyde, B. G., Bevan, D. J. M., Eyring, L., Intern. Conf. Electron Diffraction Crystal Defects, Melbourne, Australia, 1965, Pergamon Press, 1966. 

The physical characterization in terms of phases, crystal structures, and molecular and electronic defects presents a different picture. The inorganic azides, simple though they may be in comparison with most energetic substances, are nonetheless molecularly complex, and the elucidation of the crystal structures has been the subject of investigation for 40 years. In the last decade important refinements to the earlier knowledge have become possible through the combined advances in chemical preparation, neutron diffraction, and computational techniques. The advances are presented in Chapter 3, and their importance to azide research is well illustrated by the frequency with which the information in that chapter is utilized in the other chapters. 

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

In this chapter shock modification of powders (their specific area, x-ray diffraction lines, and point defects) measurements via analytical electron microscopy, magnetization and Mossbauer spectroscopy shock activation of catalysis, solution, solid-state chemical reactions, sintering, and structural transformations enhanced solid-state reactivity. 

Investigations based on equation (a) are indirect. Direct structural studies using diffraction techniques (X-ray or neutron), or electron microscopy, while they cannot detect the low concentrations of defects present in NiO or CoO are indispensible to the study of grossly non-stoichiometric oxides like FeO, TiOj, WOj etc., and particularly electron microscopes with a point-to-point resolution of about 0.2 nm are widely used. The first direct observation of a point defect (actually a complex of two interstitial metal atoms, and two oxygen atoms in Nb,2029) was made" using electron microscopy. 

The diffraction pattern obtained in the detector plane when the beam scan in a STEM instrument is stopped at a chosen point of the image comes from the illuminated area of the specimen which may be as small as 3X in diameter. In order to form a probe of this diameter it is necessary to illuminate the specimen with a convergent beam. The pattern obtained is then a convergent beam electron diffraction (CBED) pattern in which the central spot and all diffraction spots from a thin crystal are large discs rather than sharp maxima. Such patterns can normally be interpreted only by comparison with patterns calculated for particular postulated distributions of atoms. This has been attempted, as yet, for only a few cases such as on the diffraction study of the planar, nitrogen-rich defects in diamonds (21). 

Another kind of disorder corresponding to an important subcase of class (i) can arise from defects in the stacking of ordered layers of macromolecules along one lattice direction. This kind of disorder produces broadening of reflections in the X-ray diffraction patterns and streaks in the electron diffraction patterns of single crystals. 

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