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Screw-axes and mirror planes can be deduced from CBED investigation by observing Gjönnes-Moodie (G-M) lines (or called G-M extinctions) and by determining whole-pattern (WP) symmetries, respectively.

G-M extinction or line [1] is a type of dynamical extinctions in some electron diffraction patterns. This extinction may occur, for instance, in the presence of a screw-axis or a glide planes because the multiple scattering contributions cancel each other even in the dynamical scattering regime due to destructive interference between contributions with opposite phase. Therefore, the occurrence of these dynamical absences in the kinetically forbidden reflections can be applied to confirm the presence of screw-axes and glide planes in the crystalline structure. [2] Here, the dynamical absences in the kinetically forbidden reflections are referred to G-M lines and appear as dark bands or crosses in some kinematically forbidden reflections.

For instance, in Figure 3559, the dark bands indicate that the electron beam is either parallel to a glide plane or vertical to a screw axis in the crystal, and the black crosses refer to forbidden reflections.

(a)	(b)

Figure 3559. Schematic illustration of G-M lines: (a) A screw axis perpendicular to the electron beam; and (b) Black crosses indicating forbidden reflections. The green disc denotes the transmission beam, the blue discs denote forbidden reflections, and the yellow discs denote the diffraction beams.

Table 3559. Examples of G-M extinction/line applications.

[1] Gjönnes, J., Moodie, A.F., 1965. Extinction conditions in the dynamical theory of electron diffraction. Acta crystographica 19, 65–67.
[2] J.W. Steeds and R. Vincent: J. Appl. Cryst., 1983, vol. 16, pp. 317-24.