Mirror-symmetric and non-mirror-symmetric glide planes in CuAsSe 1−xS x: I. A convergent beam electron diffraction study

Abstract

The question of a breakdown of Friedel's law occuring in electron diffraction from parallel-plate crystals purely due to the symmetry-breaking effect of translation glide-plane vectors has recently been the source of some debate in the literature. Convergent beam diffraction patterns from cleavage crystals of the compounds CuAsSe 0.8S 0.2 and CuAsSe, having the space group Pbna, are used to illustrate the point that the symmetry-breaking effect described is amplified in certain crystals settings close to the principal zone and can be used to distinguish b- and n-glides in orthorhombic structures in a practical way. Calculations show a similar effect at the exact zone setting which, however, would be less accessible experimentally. These results lend support to the long-held belief that CBED pattern symmetries are the transformed symmetries of a parallel-sided crystal slab, in accordance with standard N-beam transmission theory, and that this result is of positive value in space group and structure determination.