Selected Area Diffraction Pattern or SADP are widely used for the determination of crystal structures, orientations, examine crystal defects or material textures. Majorly, two kind of diffraction pattern can be observed. A regular pattern of bright spots (spot pattern) can be seen in the diffractogram if it is obtained from one or a few single crystals (Figure on the left). On the other hand, when numerous differently orientated crystallites are covered by the aperture area, their diffraction patterns superimpose to form an image of concentric rings which are often referred to as ring pattern (Figure on the right). As this spots or the rings represents the two-dimensional projection of reciprocal crystal lattice, the distance between the centre point and the spots can be utilized to measure the interplanar distance and the orientation relationship between the corresponding planes.


Figure 1. Different types of SADP (a) Spot pattern (b) Ring pattern.

As diffraction reflects the crystal symmetry, any defects present in the crystal or sample such as dislocations, twins or stacking fault can affect the shape or size of the diffraction patterns in the form artefacts. As for example, if stacking fault (which is basically a missing stacking sequence of atoms in lattice) is present in the sample, the diffraction points from the area may come as “streaks” because of very small thickness of the sample (typically few nanometres). Whereas in case of twin also extra reflections can be observed. In case of the SADO from the twin planes, an orientation relationship can be observed. A typical bright field TEM micrograph of stacking fault and twins and corresponding SADP patterns are represented below.


Figure 2. SADP pattern from stacking faults in Hexagonal NbTiCrZrB2 sample.


Figure 3. SADP pattern from twins in FCC-CoCrFeMnNiSi0.02 sample.