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Some aspects of the diffraction of electrons by single crystals are discussed using low-order approximations to the general theory developed in the first paper of this series (Cowley & Moodie, 1957) and some qualitative arguments concerning general results are presented. The first-order approximation involves the treatment of a crystal as a phase object for electrons. This leads to a complex form for the atomic scattering factor and the structure factor which introduces complications to, but opens up new possibilities for, the process of structure analysis from single-crystal electron diffraction patterns. It is shown that forbidden reflections cannot be generated by dynamic scattering effects in the symmetrical spot patterns obtained when the electron beam is parallel to a principal crystal axis, but lattice defects can give rise to forbidden rcflections in such pattcrns from thick crystals. (auth)

Simple expressions are derived for the diffraction intensities given by coherently diffracting regions of a crystal in which the lattice is elastically bent, on the assumption that the amount of bending within such a region is small. Evidence is provided that under normal experimental conditions the geometric coherence of the incident radiation is usually such that coherent diffraction cannot take place between atoms more than a few hundred ~ngstrSm units apart, and the assumption of small amounts of bending in the coherently diffracting region gives a good approximation even for highly bent clay-mineral crystals. The profiles of the broad hit bands observed in X-ray powder patterns of metahalloysite were calculated. The assumption that the crystallites are bent about a single axis improved the agreement with the measurements of Brindley & Robinson (1948). I t is shown that some results obtained in electron-diffraction structure analyses of single crystals may be explained on the assumption of very small amounts of bending and that such an assumption may provide an explanation for the apparent discrepancy between theoretical prediction and experimental evidence on the range of application of electron-diffraction structure-analysis methods for both polycrystalline and single-crystal samples.

The scattering of electrons by three-dimensional potential fields, and, in particular, the potential fields associated with a crystal lattice, is considered in terms of the new approach to physical optics proposed by Cowley & Moodie.