Zone-axis diffraction patterns (ZAP) contain obvious geometrical information about crystals.
Like SAD, CBED is most useful when the beam is oriented along a zone axis in the crystal, giving a symmetrical ZAP pattern. On the other hand, unlike SAD patterns, CBED ZAPs contain three-dimensional (3D) information, due to the enhanced HOLZ signatures. In ZAPs, the HOLZ reflections appear as a ring of reflection spots. If it is not at a zone-axis orientation, the HOLZ spots may appear as an arc or randomly distributed and are always farther away from the direct beam than the ZOLZ reflections.
Unlike diffraction patterns which blink on and off, the Kikuchi line patterns rotate when the crystal is tilted. Therefore, tilting TEM sample along Kikuchi lines is a convenient way to orientate a crystal to certain zone axis.
For indexing CBED patterns, it’s easiest to index ZAPs so that the TEM specimen should be tilted around under Kossel conditions until we find a ZAP where Kikuchi bands intersect, then a C2 aperture size is selected to form diffraction disks (K-M conditions) in the HOLZ ring.