Models, Crystals, and Chemistry

Abstract

Most ceramics are crystalline. The exception is glass, which we usually discuss separately. Not only do the properties of ceramic crystals depend on how the atoms or ions are arranged, but the type and nature of defects also depend on crystal structure. You probably first encountered crystallography when discussing metals. Sixty-five (almost 90%) of the metallic elements are either cubic or hexagonal. In ceramics, many of the most important materials are neither cubic nor hexagonal, so we need to be more familiar with the rest of the subject. It is recommended that you memorize the main structures described in Chapters 6 and 7. In this chapter we provide the means to make this study more systematic. To understand why ceramics have particular structures and why certain defects form in these structures, it is really important to understand Pauling’s rules. These rules require you to visualize a tetrahedron and an octahedron and to see how they fit together. To understand properties such as piezoelectricity or the mechanisms of phase transformations, you must be able to visualize the crystal structure of the material. This is particularly important when we want to predict the properties of single crystals. We summarize the features of crystallography that we use throughout the text and give references to more specialized resources for rigorous proof of theorems and more detailed discussion. An important point to keep in mind is that the term “ceramic” generally refers to materials that have been processed in the laboratory or the factory plant but that often do exist in nature. Sometimes the natural minerals are rare such as moissanite, which is now being manufactured as a gemstone. There are far more materials and structures in nature than are used in technology. Understanding the basic principles and knowing where to learn more about minerals may help you find the next monazite or at least to know why it might be useful. A great source for further reading lies in the mineralogical literature.