Umweganregung

Two or more diffractions occur (dynamical diffraction) to excite a forbidden reflection due to a screw axis or a glide plane (symmetry element of a space group). This phenomenon is termed "Umweganregung." In electron diffraction, kinematically forbidden reflections become allowed reflections due to Umweganregung. Thus, to examine the existence or absence of a screw axis or a glide plane, it is necessary to select a specimen orientation without Umweganregung paths. It should be noted that CBED which takes dynamical diffraction into account enables us to unambiguously determine the existence or absence of the screw axis or the glide plane.

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