Crystallographic techniques diffraction pattern

As illustrated by Eig. 4.13, an electron microscope offers additional possibilities for analyzing the sample. Diffraction patterns (spots from a single-crystal particle and rings from a collection of randomly oriented particles) enable one to identify crystallographic phases as in XRD. Emitted X-rays are characteristic for an element and allow for a determination of the chemical composition of a selected part of the sample. This technique is referred to as energy-dispersive X-ray analysis (EDX). 

X-ray crystallographic techniques when extended to polymeric solids some interesting features of the internal structure of these substances. It was found that good majority of polymers diffract X-rays like any crystalline substance but many behave like amorphous materials giving very broad and diffuse X-ray diffraction patterns. This is seen in following figure. 

For symmetry determinations, the choice of the pertinent technique among the available techniques greatly depends on the inferred crystallographic feature. A diffraction pattern is a 2D finite figure. Therefore, the symmetry elements displayed on such a pattern are the mirrors m, the 2, 3, 4 and 6 fold rotation axes and the combinations of these symmetry elements. The notations given here are those of the International Tables for Crystallography [1]. 

The crystallographic data for these phases, including tables of calculated d-spacings and intensities for X-ray powder diffraction patterns, have been collected in Chapter 13. Characterization by electron microscopy, a particularly important technique because of the nature of the materials, is reviewed in Chapters 14 and IS. 

Although crystallographical protein structures are firmly based on experimental data, it must be borne in mind that for resolutions of around 2 A and worse, the electron density maps are not sufficiently detailed to resolve individual atoms. In order to circumvent this problem, molecular modeling techniques are often applied and usually yield reliable structural models that best represent the measured diffraction patterns (25). However, the likelihood of... 

Crystallographic investigation requires only one single crystal, of adequate size and quality for this purpose, yield is unimportant. Thus, different techniques must sometimes be used to improve the quality and increase the size of crystals. The sample for study must be a single crystal (unless powder diffraction is to be used) this means one that gives a clean diffraction pattern capable of... 

In order to exploit the heavy atom method with crystals of conventional molecules, or to utilize the isomorphous replacement method or anomalous dispersion technique for macro-molecular structure determination, it is necessary to identify the positions, the x, y, z coordinates of the heavy atoms, or anomalously scattering substituents in the crystallographic unit cell. Only in this way can their contribution to the diffraction pattern of the crystal be calculated and employed to generate phase information. Heavy atom coordinates cannot be obtained by biochemical or physical means, but they can be deduced by a rather enigmatic procedure from the observed structure amplitudes, from differences between native and derivative structure amplitudes, or in the case of anomalous scattering, from differences between Friedel mates. 

Crystallographic study was conducted using X-ray diffraction (XRD) technique. XRD patterns of powdered samples were obtained on a diffractometer (40 kV/40 mA, Mac Science, MISXHF") using monochromated Cu Ka radiation and a scintillation detector, at a scanning rate of 0.02 oo/s. 

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