Electron Diffraction
The TEM is capable of detecting electrons that are diffracted after passing through the sample. Images of the resultant diffraction patterns are very useful for determining whether or not a specimen is crystalline or amorphous. The diffraction pattern can provide further crystallographic information, such as the presence and identity of different phases and crystal types, crystalline symmetry, space groups, and the orientation of crystals and phases.
There are two main techniques used to collect diffraction patterns: selected area diffraction (SAD) and convergent beam electron diffraction (CBED). CBED is advantageous because this method is capable of providing three-dimensional crystallographic information.
The primary limitations of electron diffraction are specimen size and stability, as well as the methods used to prepare the specimen. Another complicating factor of electron diffraction involves the nature of electrons themselves. Because electrons carry a negative charge, they interact with the negative electron clouds and positive nuclei of atoms. Electrons are also affected by magnetic fields because they represent a moving charge. Overall, electron diffraction is suitable for determining crystal symmetry but is not accurate enough to determine lattice parameters or atomic positions unless the experimental conditions are optimal.
X-ray diffraction is another commonly used crystallographic technique, but because electrons have a shorter wavelength than X-rays, electron diffraction patterns are capable of providing more information about the distribution of lattice points. In terms of user interaction, the level of user knowledge of the TEM required for a successful electron diffraction experiment is much greater than that necessary to carry out X-ray diffraction experiments, which are highly automated.
