Summary and Terms

Summary

Defects are imperfections in crystals that cause them to deviate from perfect crystal structures. As we illustrated in the Windscale fire example, understanding them is critical to reliably predicting the behavior of a given crystalline material.

Vacancies are points in a crystal where a host atom should be there but isn't. As we saw in our simulations, the resence of a vacancy can have significant impacts on the potential energy of the crystal overall, and can cause rearrangements (which is what caused the Windscale fire).

Substitutional impurities occur when an impurity atom sits on a lattice site. The coordination and overall effect on the crystal, like in the previously discussed case for ionic crystals, has a strong dependence on the size of the impurity atom relative to the lattice atoms.

One way to classify point defects like these is with Kroger-Vink notation, which we simplify slightly for this class. Kroger-Vink is a system that allows us to define the type of atom that comprises the point defect (including no atom, as is the case for a vacancy) and the type of site on which that atom sits. Clearly, a system like this is necessary and useful considering the different types of point defects discussed in this chapter.

Terms

Defect: Imperfections in crystal structures which have a strong influence on a material's properties. Defects are typically categorized based on their dimensionality, i.e. point, line, plane and volume. Section 6.2.1

Point Defects: crystalline imperfections that occur at lattice points in a crystal (either on lattice or interstitial sites). Section 6.2.3

Vacancy: an unfilled lattice site where a lattice atom would ordinarily be. Section 6.4.1

Substitutional Impurity: a defect where an impurity atom takes the place of one on the crystal lattice. Whether these impurities from depends heavily on the size of the impurity atom. Section 6.5.1

Interstitial Impurity: A defect where an atom occupies a position between the host atoms of the crystal. These impurities can be intrinsic, as in the case of self-interstitials, or extrinsic.

Kroger-Vink Notation: A system for classifying defects based on the site the defect is on and the identity of the impurity atom. The simplified form we'll use in this class is $x_s$ where $x$ represents the impurity atom and $s$ represents the site.