Introduction

In this chapter, we will take a closer look at the geometry of electron diffraction patterns. First we will discuss spot patterns and how to index them. Superpositions of spot patterns and double diffraction are explained in detail, as well as Moiré patterns and the corresponding diffraction effects. After a brief discussion of ring patterns we introduce linear features, such as streaks, Kikuchi lines, and HOLZ lines. Convergent beam electron diffraction and symmetry determination (both point group and space group) form the topic of Section 9.5. Diffraction effects in modulated structures are introduced; displacive and compositional modulations as well as interface modulations are analyzed, and the difference between commensurate and incommensurate structures is highlighted. Various types of diffuse intensity distributions are introduced next, and we conclude this chapter with a discussion of the shape function of polyhedral particles.

Spot patterns

Indexing of simple spot patterns

Indexing a spot pattern is perhaps one of the most important tasks of an electron microscopist and it is hence useful to outline a possible procedure. The procedure below can be used if the crystal structure of the material is known a priori. Needless to say, it is important to obtain diffraction patterns in the correct experimental conditions (eucentric height, focused SAD aperture and image, focused spots), so that the calibrated value for the camera constant can be used.