Point Defects, Charge, and Diffusion

Abstract

Point defects are particularly important in ceramics because of the role they can play in determining the properties of a material. The entire semiconductor industry is possible because of minute concentrations of point defects that are added to Si: the dopants determine if the Si is n-type, p-type, or semi-insulating: they determine the electrical properties. Solid-oxide fuel cells work because of the large concentrations of oxygen vacancies present: the vacancies provide fast ion conduction pathways. CZ is cubic because of the presence of point defects that stabilize the cubic structure.

We will address three principal questions in this chapter and leave the properties associated with point defects to later chapters where we will discuss conduction, light, and color, for example.

• What point defects are possible?

• How many point defects are present?

• How easily can they move?

To determine what defects are possible in a crystal we need to know its structure. Point defects also exist in glass, but there is the extra problem of how to define what is being “defective.” (What is the perfect structure of the glass?) To estimate the concentration of point defects, we should know (1) what types of defects are present and (2) how they form, which, in turn, determines (3) how many of each kind there will be. Answering the third question will give us many of the properties of the materials containing these defects. In some cases we want them to move quickly and in other cases we actually would rather they were immobile. To really understand this subject thoroughly you will need a good knowledge of thermodynamics and kinetics. These topics are so large that there are many texts devoted entirely to them.