Bloch wave degeneracies, which can be observed in high-energy electron diffraction, are summarized. Critical voltages of non-systematic reflections for 4 beam and 5 beam cases, and zone axis critical voltage effect are discussed. Bloch wave degeneracy observed in non-symmorphic space group is explained. Pseudo-critical voltage effect observed in non-centrosymmetric crystal is also discussed. The experimental measurement of the degeneracy is mentioned briefly. Examples of application of the critical voltage effect for estimation of accurate structure factors are discussed.
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