Introduction and Outline

Energy comes in many forms, and in this chapter we'll explore the effects thermal energy has on point defects in crystals, specifically vacancies. We first described vacancies in Chapter 6 as empty spaces where atoms would normally be in a perfect crystal. We should expect, because a missing atom is a departure from a perfect crystal, that the presence of a vacancy has some effect on the overall energy of said crystal. In this chapter, we'll explore the distribution of kinetic energy within a crystal and its dependence on temperature. We'll also take a close look at vacancies - why they exist and the models we use to describe how many are present in a crystal.

• Section 8.3 - Exploration: Atomic Terraces and Temperature Independence Here we'll just observe an interesting phenomenon that will motivate and anchor the following discussions.
• Section 8.6 - Vacancy Concentration - A Thermodynamic Argument using language that should be familiar if you have worked with spontaneity in chemical reactions. Here, we look at enthalpic (bond breaking) and entropic (configurational) contributions to Gibbs free energy to make an argument about why vacancies exist, and how we might expect their populations to change with temperature.
• Section 8.6 - Vacancy Concentration - Arrhenius Behavior Vacancy concentration can be mathematically modeled with a general Arrhenius behavior, which allows us to predict how many vacancies we expect in a crystal, depending on activation energies and temperature.