Convergent beam electron diffraction

In convergent-beam electron diffraction (CBED) a highly convergent electron beam is focussed on to a small (⩽50 nm) area of the sample. Instead of the diffraction spots that are obtained in the back focal plane of the objective lens with parallel illumination in conventional selected-area electron diffraction, CBED produces discs of intensity. The point group can be determined uniquely from the symmetry within the individual discs and the overall pattern. In order to determine the point group, it is usually necessary to record a number of CBED patterns with the electron beam aligned along different zone axes, but sometimes only one, high-symmetry pattern is required. The positions of reflections in higher-order Laue zones can be used to identify the crystal system and lattice type and to detect the presence of certain glide planes. The repeat along the zone axis that is parallel to the beam can be calculated from the diameters of the Laue zones. Hence the presence ofpolymorphs can be detected. Doubly-diffracted discs in CBED often contain a ‘line of dynamic absence’, the orientation of this line with respect to the symmetry seen in the bright field disc allows the symmetry element responsible for it (glide plane or screw diad) to be identified. This allows 191 of the 230 space groups to be uniquely identified. The measurement of specimen thickness, extinction distance and cell parameters are also briefly discussed.