Electron and Neutron Diffraction

Vitreous siUca is considered the model glass-forming material and as a result has been the subject of a large number of x-ray, neutron, and electron diffraction studies (12—16). These iavestigations provide a detailed picture of the short-range stmcture ia vitreous siUca, but questioas about the longer-range stmcture remain. 

The physical data index summarizes the quantitative data given for specific compounds in the text, tables and figures in Volumes 1-7. It does not give any actual data but includes references both to the appropriate text page and to the original literature. The structural and spectroscopic methods covered include UV, IR, Raman, microwave, MS, PES, NMR, ORD, CD, X-ray, neutron and electron diffraction, together with such quantities as dipole moment, pX a, rate constant and activation energy, and equilibrium constant. 

X-ray, neutron, and electron diffraction techniques are used to determine crystal structures and can thus be used for molecular structure determinations. Because of its high resolution and applicability to small and often weakly diffracting samples, x-ray crystallography and powder diffraction are by far the methods of choice for most structure determinations on crystalline compounds,... 

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. 

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... 

Like X-ray diffraction patterns, neutron and electron diffraction patterns provide averaged information about the structure of a compound. Details of these techniques are given in works by Hirsch et al. (1965) and West (1988). Neutron diffraction involves interaction of neutrons with the nuclei of the atoms. As the neutrons are scattered relatively evenly by all the atoms in the compound, they serve to indicate the positions of the protons in an oxide hydroxide. This technique has been applied to elucidation of the structure and/or magnetic properties of goethite (Szytula et al., 1968 Forsyth et al., 1968), akaganeite (Szytula et al., 1970), lepidocrocite (Oles et al., 1970 Christensen Norlund-Christensen, 1978), hematite (Samuelson Shirane, 1970 Fernet et al., 1984) and wiistite (Roth, 1960 Cheetham et al., 1971 Battle Cheetham, 1979). A neutron diffractogram of a 6-line ferrihydrite was recently produced by Jansen et al. (2002) and has helped to refine its structure (see chap. 2). 

Methods covered include X-ray, neutron and electron diffraction, microwave spectroscopy, and their results in terms of molecular dimensions. NMR spectroscopy is treated in some detail as befits its importance, not only proton, but particularly 13C, 15N, and, where appropriate, other nuclei. The section on mass spectrometry briefly covers fragmentation patterns. UV/Fluorescence, IR/Raman, photoelectron spectroscopy, ESR, and dipole moments are covered as appropriate. 

Clearly it is Equation (2.3) which contains all the interesting information about the lattice and the scattering process. Using this expression one can develop many of the standard formulae relating to the diffrac-tion of X-rays from a crystal and closely analogous expressions apply to neutron and electron diffraction. 

The pinwheel structure is not only observed for chiral adsorbates. An early example was reported for small molecules at low temperatures on graphite in UHV (Fig. 19). Neutron and electron diffraction experiments as well as... 

A multitude of concepts such as X-ray, neutron and electron diffraction, X-ray crystallography, low-angle scattering, powder diffraction, scattering by noncrystalline and amorphous solids, all refer to the same physical phenomenon. Whereas X-rays and electrons are scattered by extranuclear charge clouds, more massive particles like neutrons and a-particles are scattered on atomic nuclei. In principle, all of these processes are of the same type, as described for X-rays below. 

The first volume in this annual series covers X-ray. neutron, and electron diffraction over the period January 1971 to March 1972. 830pp 15.00... 

Now that the range of likely shapes has been defined by experiments on related molecules and by energy calculations, we focus on the details of specific structures that have been observed for real, crystalline cellulose molecules, primarily by x-ray, neutron, and electron diffraction studies. A number of landmark concepts have been established with electron microscopy, as well. Infrared (IR), Raman, and nuclear magnetic resonance (NMR) spectroscopy have all also been important in the quest for understanding cellulose structure. Such data, while so far not able to provide complete definitive structures themselves, constitutes additional criteria that any proposed structure must be able to explain. In addition, unlike crystallography, the resolution of spectroscopic methods is not directly affected by the dimensions of the... 

The current excitement in powder diffraction is in quantitative analysis of the data. The book is laid out in a way that facilitates understanding the information content of the data, as well as best practices for collecting and analyzing data for quantitative analysis. After a very brief overview of the basic theory of diffraction from crystals and powders, data collection strategies are described, including X-ray, neutron and electron diffraction setups using modern-day apparatus including synchrotron sources. Data corrections that are essential... 

The structure of pure liquids and liquid solutions is conveniently studied using diffraction techniques [5]. The most common of these is X-ray diffraction. Two other useful techniques are neutron and electron diffraction. In these experiments, radiation, which is usually monoenergetic, penetrates the liquid sample and is scattered through an angle 0 (see fig. 2.6). The analytical information is obtained by studying the intensity of the scattered radiation as a function of this angle. In... 

Crystal structures are mainly determined using the diffraction of X-rays, supplemented by neutron and electron diffraction, which give information that X-ray diffraction cannot supply. The topic falls into three easily separated sections. Initially one can use the positions of the diffracted beams to give information about the size of the unit cell of the material. A second stage is to calculate the intensities of the diffracted beams and to relate these data to the crystal structure. 

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