The symmetry of convergent-beam electron diffraction patterns from bicrystals

Abstract

A discussion of the relationship between the symmetry of bicrystals and the resulting symmetry of convergent-beam electron diffraction (CBED) patterns is presented. For this purpose bicrystal symmetry is defined from the symmetry of the dichromatic pattern or complex formed by the interpenetrating lattices or structures of the individual crystals in a bicrystal. The interrelation between the possible coloured point groups and the diffraction groups, characterizing the symmetry of a CBED pattern, is established. These results are illustrated by a determination of the symmetry of thin twinned Au crystals from suitable CBED patterns. These experiments give information about the state of relative translation of the two crystals at the grain boundary and are consistent with the state of zero translation expected for a (111) coherent twin boundary in Au. It is pointed out that the interpretation of the CBED pattern symmetry may be complicated by a non-ideal location of the boundary plane in a bicrystal.