Schottky Defects This is where a pair of ions are removed from their sites in a lattice. They are removed as a pair so as to maintain the charge balance in the lattice, for example is the NaCl rock salt structure both a Na+ and a Cl- are removed- if only one was removed then the the charge would not balance. The pair of ions do not have to be adjacent, they can be from completely different regions of the crystal. Halides of the alkali metals commonly have schottky defects at low concentration. If the temperature of crystal is increased the number of defects is found to increase according to an Arrenius equation.
Frenkel defects This is where a displaced ion or molecule in the lattice moves to a interstitial site and leaves its previous site empty.what is an interstitial site? it is simply the space or cavity made when a group of atom or ions pack together.Octahedral site are ones where the site is surrounded by 8 ions-imagine a Co(Cl-)6 complex without the metal ion. A tetrahedral one has four ions or atoms surrounding the site. These defects can only occur when the ion is small enough to fit the gap and also easily polarised-as adjacent ions are of the same charge. this sort of defect is common in the silver halides- where the Ag+ ion moves to an interstitial site.
