Point Defects
Index
Point defects are atomic scale, 'zerodimension' defects in a crystal structure. They include missing atoms, "vacancies," impurity atoms on lattice sites, "substitutional impurities," impurity atoms in non-lattice locations, "interstitial impurities," and interstitial atoms from the host lattice, "self-interstitials." 

In ionic solids it is necessary to conserve overall charge neutrality and point defects must be created in neutral groups. Schottky defects vacancies on both the negative and positive ion lattices. In an alkali halide, NaCl, one Na+ and one Cl- vacancy must be created. The sodium vacancy has an efective negative charge as the ion was positive, and the chlorine vacancy has an effective positive charge as the ion was negative. If an impurity atom such as Ca++ is substituted for a Na+ ion in the lattice, this extra positive charge must be compensated by creating a sodium vacancy. Frenkel defects are vacancy-interstitial pairs. In AgCl silver interstitials and silver vacancies form Frenkel pairs and the group is charge neutral, the vacancy being negative and the interstitial positive.

Point defects are favored thermodynamically as they increase the configurational entropy of the structure. If the energy of vacancy formation is Q eV/atom, the equilibrium concentration of vacancies at a temperature T is given by: XV = exp(-Q/kT), where k is Boltzmann's constant. Similar expressions hold for the other point defects.