Point Defects in Ionic Crystals

Even for the most simple ionic crystals of the type A+B– like LiCl or NaCl, we can, in principle, produce arbitrary concentrations of two kinds of vacancies and two kinds of interstitials as shown on the left.

However, as we already learned in dealing with Schottky defects, global charge neutrality must be maintained. Arbitrary concentrations are thus not really allowed, we must demand that the the sum of the positively charged defects equals the sum of the negatively charged defects. In other words: we have to obey the charge conservation law.
If we also keep the number of atoms constant, we must add an A or B atom to the surface of the crystal for every pure vacancy we produce. In other (fancy) words, we have to obey the mass conservation law.

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