Bloch-wave interpretations of Gj ø nnes – Moodie lines in convergent-beam electron diffraction for non-symmorphic space-group crystals

The contrast of Gj ø nnes – Moodie (GM) lines which appear in convergent-beam electron diffraction patterns for non-symmorphic space-group crystals is explained using Bloch waves. In the two-dimensional space groups p2mg and pg the Bloch waves for electron diffraction are described. In both space groups along the Δ line, Bloch waves are arranged as two different types, and it is shown that the two types of Bloch waves do not contribute to the intensity of forbidden reflections. Along the position where the forbidden reflection satisfies the Bragg condition, degeneracies of two Bloch waves are found and it is shown that the degenerated pair of Bloch waves do not contribute to the intensity. These Bloch-wave results provide a new perspective in the understanding of the contrast mechanism of GM lines previously described using scattering polynomials. They also advance the understanding of Bloch-wave behaviour in high-energy electron diffraction.

Source: Acta Crystallographica Section A - May 4, 2021 Category: Chemistry Authors: Matsuhata, H. Tags: many-beam dynamical electron diffraction Gj ø nnes – Moodie lines non-symmorphic space groups degeneracy of Bloch waves time-reversal degeneracy Herring criterion research papers Source Type: research