The appearance of the diffraction pattern can reflect the nature of the crystalline phases in the specimen. For example, if the material is microcrystalline or amorphous the diffraction pattern consists of a series of concentric rings rather than spots/discs.
When the electron beam interacts with the sample when the sample is oriented with a zone axis pattern parallel to the electron beam, then the diffraction pattern form in the back focal plane of the objective lens is a regular array of reflections. This is seen projected onto the viewing screen as an array of reflections organized in a predictable manner based on the crystal structure of the sample.
The image shows a classic spot diffraction pattern obtained from Nd13CaO7.
Click here to find out how to set up the TEM lenses in order to obtain such an image.
Convergent Beam Electron Diffraction (CBED)
When the electron beam is converged on the sample to a point (method = convergent beam), instead of using a parallel stream of electrons through the sample, the diffraction pattern forms discs instead of spots in the back focal plane of the objective lens.
These discs can contain detail that provides information about the crystal structure of the specimen. In these images we see almost perfect mirror symmetry in the patterns within the discs. These symmetries can be used to determine the point and space group of the crystal.
This image shows Kikuchi Patterns with a convergent beam diffraction pattern. The star-like peaks show mirror symmetry in a regular crystal matrix that is well aligned along the zone axis.
Sometimes, instead of intensity spots, the electron diffraction pattern is composed of concentric rings.
Materials that contain no long-range order in the atomic lattice produce diffuse ring diffraction patterns with no discrete reflections and one or possibly two diffuse rings of maximum intensity. Amorphous samples, e.g. polymers and metallic glasses, produce this kind of pattern.
Some thin films used as substrates to hold samples are made of materials such as amorphous SiN. This is the ring pattern from such a film.
The bar across the pattern is the “beam stop” used to cover the bright central beam spot so that the more diffuse rings can be captured as a digital image.
If the material is a collection of a large number of crystals, with different orientations, then individual reflections are seen within the rings.
This image is a selected area diffraction (SAD) pattern from a Zr/Ni/Cu-based alloy and zirconia