X-ray neutron

We have thus far discussed the diffraction patterns produced by x-rays, neutrons and electrons incident on materials of various kinds. The experimentally interesting problem is, of course, the inverse one given an observed diffraction pattern, what can we infer about the stmctirre of the object that produced it Diffraction patterns depend on the Fourier transfonn of a density distribution, but computing the inverse Fourier transfomi in order to detemiine the density distribution is difficult for two reasons. First, as can be seen from equation (B 1.8.1), the Fourier transfonn is... 

As with synchrotron x-rays, neutron diffraction facilities are available at only a few major research institutions. There are research reactors with diffraction facilities in many countries, but the major ones are in North America, Europe and Australia. The are fewer spallation sources, but there are major ones in the United States and the United Kingdom. 

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. 

L. H. Schwartz and J. B. Cohen. Diffraction from Materials. Springer-Verlag, Berlin, 1987. A recent text that includes X-ray, neutron, and electron diffiaction, but emphasizes XRD in materials science. A good introduction and highly recommended. 

The classical approach for determining the structures of crystalline materials is through diflfiaction methods, i.e.. X-ray, neutron-beam, and electron-beam techniques. Difiiaction data can be analyzed to yield the spatial arrangement of all the atoms in the crystal lattice. EXAFS provides a different approach to the analysis of atomic structure, based not on the diffraction of X rays by an array of atoms but rather upon the absorption of X rays by individual atoms in such an array. Herein lie the capabilities and limitations of EXAFS. 

What is the nature of the defects seen in the EPR spectra For alkali and alkali earth halogenides it is well known that irradiation with X-ray, neutrons, gamma-radiation, or electrons produce paramagnetic color centers (F-center) [109-111]. If these centers are created in large amounts, they can be stabilized by the formation of metal clusters as observed for MgCl2 films after prolonged electron radiation [106]. From the temperature dependence... 

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,... 

X-Ray/Neutron Diffraction Data for Hydrido-Bridged Complexes... 

The structure factor S(q as defined in Eq. (54) in terms of the Ising pseudospins Si, in the framework of the first Bom approximation describes elastic scattering of X-rays, neutrons, or electrons, from the adsorbed layer. SCq) is particularly interesting, since in the thermodynamic limit it allows to estimate both the order parameter amplitude tj/, the order parameter susceptibility X4, and correlati length since for q near the superstructure Bragg reflection q we have (k = q— q%)... 

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... 

To examine the reliability of X-ray charge densities at a time of rapid development of new methods, the Commission on Charge, Spin and Momentum Densities of the lUCr organized a project under which a single substance, a-oxalic acid dihydrate, was studied in a number of laboratories using X-ray, neutron, and theoretical methods. The report by Coppens on the study, published in 1984, established unequivocally the qualitative reproducibility of chemically significant features in deformation density maps, which had not been generally accepted. 

Because neutron beams are much weaker in intensity than X-rays, neutron diffraction requires large single crystals (Iff-lOOmm in volume as compared to the 0.1 mm crystal volume used in X-ray diffraction work). However, it is possible to obtain useful structural data by analysis of neutron-diffraction profiles from polycrystalline materials (Cheetham Taylor, 1977). 

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