ABSTRACT

In this chapter, the authors show how electron diffraction can help us to determine the crystal system of a material, its lattice type, its point group and its space group. With the large tilt angles that are available in modern TEMs it is possible to examine large volumes of reciprocal space. It is, in principle, straightforward matter to decide whether or not the crystal is cubic, hexagonal or has lower symmetry from observation of the geometries of a series of spot patterns tilted through large angles. It is vitally important for the applications described below that the zone axis of interest is exactly parallel to the electron beam. It is usually most convenient do the preliminary sample tilting with a large spot size so that there is plenty of illumination on screen and Kikuchi lines can be more easily seen. The final stage of centring is difficult to accomplish with the goniometer alone, especially if crystal of interest is small.