Non-anomalous high-resolution imaging of crystalline materials

Anomalous effects, which result from beams which are kinematically- and dynamically-forbidden under space-group extinction rules, are commonly seen in high-resolution images of many crystalline materials, particularly in thicker regions. Reported examples are first briefly reviewed and it is pointed out that crystal and beam tilt can both be contributory factors. Specific calculations of diffracted wave amplitudes and phases, as well as convergent beam electron diffraction patterns, for rutile (TiO₂) crystals illustrate the sources of the anomalous effects, and experimental 500 kV images from SiC (6H polytype), Mo₅O₁₄ and TiO₂ are presented to demonstrate the extreme image sensitivity to local imaging conditions and to emphasise the importance of accurate beam and crystal tilt when imaging in space-group-forbidden zones.