High-resolution reflex images of defects in crystals

Abstract

Iijima (1975) obtained a series of electron microscope images of crystallographic shear planes (CSPs) in tungsten trioxide, some of which showed light and dark bands accompanying the defect having a periodicity of approximately 20 A. This oscillatory contrast was interpreted, using the weak phase approximation (Cowley and Iijima 1972), as resulting from small displacement of cations from their original positions. We have obtained electron micrographs of CSPs in magnesium-doped rutile which show oscillatory contrast similar to that observed by Iijima. A simple geometrical analysis suggests that the contrast is an artifact caused by spherical aberration in the microscope objective lens. The many-beam dynamical theory of electron diffraction and the method of periodic continuation were used to calculate the image of widely-spaced, unstrained CSPs. The computer-simulated images showed light and dark bands adjacent to the CSP and a similar variation in spacing for changing objective lens defocus as did the experimental images. The agreement between experimental and calculated images without the necessity to introduce periodic strain fields confirms that the contrast is an electron optical effect, and cannot be interpreted intuitively. Similar reflex images will occur in general for any type of defect, e.g. dislocations, precipitates, etc., imaged by high-resolution phase contrast electron microscopy.