Schottky defects are vacancy defects shown by ionic solids in which the anion and cation are nearly of the same size. In ionic solids, vacancy at a certain site is accompanied by another vacancy at a nearby anion site to maintain electrical neutrality. Such paired vacancy called Schottky defect.
Schottky defects, preserve the electrical neutrality of the crystal. The presence of Schottky defects in a crystal lowers its density appreciably.
In sodium chloride, there are approximately 10 .10. 10. 10 .10 .10 Schottky pairs per cm cu At room temperature. There are about 10 rest to 22 ions in one cm cube of sodium chloride. Therefore, in sodium chloride at room temperature, there is one Schottky defect per 10 rest to 16 ions.
Frenkel defects are shown by ionic solids. In this defect, the smaller ion usually cation leaves its lattice site and enters an interstitial site. Frenkel defect is a hybrid defects consisting of schottky defect and interstitial defect. Such hybrid defect , (combination of schottky and interstitial ) is called as Frenkel defect. A Frenkel defect which is also called as dislocation defect.
- Frenkel defects do not alter or affect the density of the crystalline solid.
- Frenkel defect is shown by the ionic substances in which the ionic sizes differ appreciably.
- In pure alkali halides, Frankel defects are not observed, because th ions are bigger than the void size.
- Frankel defects are found in silver halides due to the small size of Ag+, and in zinc sulphide which is due to small size of Zn2+.
- Frankel defect arises in case of ionic solids with relatively smaller cationic size which can fit into interstitial space and difference in ionic radii is large.This defect is also called as dislocation defect.