Friedel's Law in Electron Diffraction as Applied to the Study of Domain Structures in Non‐Centrosymmetrical Crystals

Abstract

AbstractIt is shown that a strong contrast may arise in dark‐field electron microscope images between two crystal parts related by an inversion operation, as a result of the violation of Friedel's law. Friedel's law is violated only in the diffracted beam and only in cases where a multiple‐beam situation prevails. Moreover the excited reflections must belong to B zone which reveals the non‐centrosymmetrical character of the crystal. The interfaces between two domains related by an inversion operation, which are proposed to call inversion boundaries are imaged by α‐like fringes which nevertheless have properties which differ somewhat from those of pure α‐fringes. These considerations are applied to the domain structure observed in the γ‐phase. In this case the inversion boundaries separate regions which have suffered displacements as well. It is shown that the displacement vector of such boundaries can be given an unambiguous meaning, although strictly speaking the structures in the two parts cannot be derived one from the other by means of a parallel displacement. It is demonstrated in a concrete case that the observed contrast can be accounted for by a four. beam calculation.