The critical voltage effect in transmission electron microscopy. V. The case of non‐centrosymmetric crystals

The critical voltage effect is discussed for the case of noncentrosymmetric crystals (systematic reflections only). An appropriate measure of the noncentrosymmetry is found in terms of the phase angles θ_g and θ_2g of the lowest order extinction distances i.e. Δ = θ_2g–2θ_g. When |Δ| ≦ 3° a critical voltage effect is still expected. For larger |Δ|‐values usually no critical voltage effect will occur. As an application the case is considered of interstitial alloys such as V₂H, Nb₂H, and Nb₂O. For these alloys the critical voltage effect may be used to determine whether the H or O are in octahedral or tetrahedral interstitial positions.