Determination of crystal polarity in the Electron Microscope

Abstract

A few methods were proposed for the determination of crystal polarity using electron diffraction. The method by Taftø and Spence is based on the coupling between ZOLZ and FOLZ reflections, requiring exact tilting to excite weak high- and odd-indexed reflections. The method by Spellward and James relies on the comparison of experimentally observed and calculated CBED patterns of fairly thick crystals, where the features in the CBED discs are compared. In this paper we present a method for polarity determination readily evident from ZOLZ reflections in zone-axis microdiffraction patterns. The method is based on zero-layer interactions from thin crystals with thickness t smaller than the extinction length of the primary beam. Then the diffraction discs appear homogeneous, and the breakdown of Friedel's law is noticable in a difference of the intensities of the reflections g and -g which are not related to a mirror plane (Bijvoet-related reflections).The method can be applied using any transmission electron microscope which is designed for obtaining convergent-beam microdiffraction patterns and in principle, specimen-cooling is not necessary. The crystals have to be oriented to a low-indexed zone axis, where the diffraction plane contains a polar crystal axis.