Point defects are imperfections in a crystal that are confined to atomic dimensions in all three directions. Depending on the chemical species involved, point defects can be considered as either intrinsic or extrinsic. Intrinsic point defects include vacancies, i.e. missing atoms, and self-interstitials, i.e. extra atoms having the same chemical species as the host crystal. Extrinsic point defects, on the other hand, are atoms having a different chemical species from the host crystal that they enter. Such point defects are often called impurity or solute atoms. Impurities usually refer to “unwanted” foreign atoms in a crystal, while solute atoms are often intentionally introduced into the crystal to alter its properties.
Point defects have a profound effect on the properties of engineering crystalline materials, either by themselves, or through their interactions with dislocations (line defects) and grain boundaries (planar defects). An example of the former situation is the semiconductor industry, whose success hinges on their ability to control the electronic properties of silicon by selective doping, through which transistors and integrated circuits can be made. An example of the latter situation is solid solution hardening, in which the elastic distortion around point defects allow them to interact with dislocations and alter the mechanical strength of the crystalline material.
In the following four chapters, we focus on the fundamental mechanics and thermodynamic principles that are needed to understand how point defects influence material properties. In Chapter 4, we study the stress and strain fields around point defects, using the elasticity theory introduced in Chapter 2. These results lead to an estimate of the energy cost of introducing point defects, as well as how point defects interact with each other and with other types of defects (e.g. dislocations) to be introduced later. In Chapter 5, the energy cost of introducing point defects is combined with the statistical thermodynamic principles of Chapter 3, to predict the concentration of point defects in a crystal at thermal equilibrium. It will be seen that, due to the entropic gain in allowing point defects, the equilibrium concentration of point defects in a crystal at finite temperature is never zero.