ABSTRACT
We are fascinated by the infinite variety of shapes of crystalline water in the form of snowflakes, we take for granted crystals of sodium chloride that we add to our food, and we decorate our bodies with highly valued crystals of carbon. Most of the modern electronic devices we use today would not be possible without crystals of silicon and germanium. Crystals are pleasing and useful to us because they are materials that have order. An ideal crystal consists of an infinite threedimensional structure that contains repeated structural units. The structural units may be single atoms, groups of atoms of single or multiple chemical elements, or many molecules. Crystals can be formed of organic as well as inorganic materials. Although crystals do have a high degree of order, they rarely have the same properties in all three dimensions. Even if they appear to have an isotropic structure, their properties can quickly be made anisotropic with the application of external electromagnetic or mechanical forces. For this reason, we must consider crystals to be anisotropic materials.
