Imperfection in Solids

No real crystal is perfect -- the law of nature guarantees that every crystal carries some defects in the arrangement of its constituent particles. Far from being a flaw to be ignored, these defects have real, measurable consequences for a solid's physical and chemical properties, and even drive some of the most important technological uses of solids: doping (deliberately introducing a controlled crystal imperfection) is what increases the electrical conductivity of a semiconductor such as silicon, and the ability of a ferromagnetic material such as iron or nickel to be magnetised and demagnetised likewise depends on the presence of imperfections in its crystal structure.

Crystal defects are classified into four broad types: 1) point defects, 2) line defects, 3) interstitial defects, and 4) volume defects. This unit concentrates specifically on point defects, and more specifically still on point defects in ionic solids, which are further classified into three families: stoichiometric defects (Schottky and Frenkel defects), non-stoichiometric defects (metal excess and metal deficiency defects), and impurity defects. …