Diffraction, amorphous substances electron

The basic modem data describing the atomic stmcture of matter have been obtained by the using of diffraction methods - X-ray, neutron and electron diffraction. All three radiations are used not only for the stmcture analysis of various natural and synthetic crystals - inorganic, metallic, organic, biological crystals but also for the analysis of other condensed states of matter - quasicrystals, incommensurate phases, and partly disordered system, namely, for high-molecular polymers, liquid crystals, amorphous substances and liquids, and isolated molecules in vapours or gases. This tremendous... 

Especially methods of electron microscopy are important at study of X-ray amorphous substances and polyphase nanomixtures which are distributed very widely in the nature such as agate, bauxite, bitumen, coal, natural glasses etc., as X-ray diffraction is almost useless at analyzing such mostly disordered materials. 

An ingenious approach to the autoionization process was suggested by Tomelini and Fanfoni [32]. They examined diffraction processes in the framework of autoionization, i.e., a second-order process. More exactly, they studied only the second stage of autoionization, namely, the secondary electron emission from the intermediate state where the core hole is coherently distributed over the crystal. With such an approach it was shown that in crystals this diffraction contribution enhances fine structure as compared to the amorphous substance. However, the applicability of such a model is determined by the probability of the occurrence of the core hole distributed coherently over the crystal. More conventional... 

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

Electron diffraction, which is a somewhat more sensitive method of detecting the presence of crystalline substances, likewise fails to show any evidence of a crystalline phase in fresh silica-alumina catalyst (354). The electron-diffraction patterns do reveal amorphous bands similar to those obtained with evaporated silica films. 

Presence on diffraction patterns the evolved products of a wide maximum at small comers allowed to assume X-ray amorphous or nanocrystalline a stmcture of the received substances. By a method of scanning electronic microscopy (SEM) it was revealed, that substances have low crystallinity and nonfibrillary morphology (Figure 4.1(a)). 

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