Complex Crystal and Glass Structures

Abstract

This chapter is separated from the previous one just to make it less overwhelming. We have demonstrated the principles in Chapter 5 and considered some of the simpler ceramic structures in Chapter 6. Now we are considering structures that have more than two chemically different atoms in the unit cell (like YBa₂Cu₃O₇), although some will still only have two components. We will include materials (like SiO₂) in which covalent bonds are particularly important and encounter materials involving secondary bonds such as van der Waals interactions (especially in the clay minerals).

It is a little difficult to learn these structures by heart but some, like cristobalite and perovskite, you should know. For others, you may survive by just knowing the basic ideas involved. This emphasizes the reason for this chapter (and Chapter 6)—if you understand the building blocks, you can better appreciate the properties of more complex structures that are composed of combinations of such building blocks. The logic behind the order in which these are discussed is first cubic, then the silicates (starting with silica), then the complicated ones, and finally some new materials that challenge our perception of what a ceramic is.

Glass has often been treated separately from ceramics, but today few programs in materials science have the time for a specialized course on glass. We include a discussion of glass structures in this chapter since they link so closely with the crystal structure of crystalline silicates and the general concept of coordination polyhedra. We will discuss the properties of glass later. Remember that the structure of glass is not random; it just lacks long-range order. We have point defects and other defects in glass just as we do in crystals; the challenge is to define the nondefective structure to which we can relate them. What makes a point defect in glass a defect and not just part of the glass?

A common mantra throughout this chapter is that diagrams are essential. A difficulty is that you generally need more than one diagram (view) to appreciate a three-dimensional (3D) structure. Computer programs can make the 3D aspects much more apparent.

In this and the previous chapter, the xyz-axes in the schematics of cubic crystal structures lie along the cube edges; the length of the cube edge is the lattice parameter.