Bragg-diffracted beams, information obtained

Generally the Bragg eqnation is nsefnl in describing the directions 29hki of diffracted beams, bnt provides no information to help ns to understand the magnitudes of the intensities obtained. The concepts to be described in detail in Chapter 6 are mnch more informative abont the intensities of the diffracted beams. 

This Chapter is concerned with methods for obtaining the relative phase angles for each Bragg reflection so that the correct electron-density map can be calculated and, from it, the correct molecular structure determined. When scattered light is recombined by a lens, as described in Chapters 3 and 6, the relationships between the phases of the various diffracted beams are preserved. In X-ray diffraction experiments, however, only the intensities of the Bragg reflections are measured, and information on the relative phases is lost. An attempt is maxle to remedy this situation by deriving relative phases by one of the methods to be described in this Chapter. Then Equation 6.3 (Chapter 6) is used to obtain the electron-density map. Peaks in this map represent atomic positions. 

The relative positions of a sufficient number of reflections arising from microstructural periodicities enable unambiguous identification of morphology. Further information can be obtained by preparing oriented specimens, and obtaining diffraction patterns for different orientations. For example, in an oriented lamellar phase with the beam incident parallel to the layers, Bragg... 

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