The scattering of electrons by atoms and crystals. III. Single-crystal diffraction patterns

Some aspects of the diffraction of electrons by single crystals are discussed ush~g 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 struetttre 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 reflections in such patterns from thick crystals.