Discuss in details the following type of crystalsa) Schottky defectb) Frenkel Defectc) Metal excess defect

a) Schottky Defect: This type of defect is usually observed in ionic crystals. In a Schottky defect, equal numbers of cations and anions are missing from the crystal lattice, thereby maintaining the electrical neutrality. This defect decreases the density of the substance. It is more common in highly coordinated crystals such as NaCl and KCl.

b) Frenkel Defect: This defect is also observed in ionic crystals, particularly in those where the difference in size between the cation and anion is large. In a Frenkel defect, a cation leaves its normal site in the crystal lattice and moves to an interstitial site. This creates a vacancy defect at the original site and an interstitial defect at the new site. This defect does not change the density of the substance. It is more common in crystals such as AgCl, ZnS, and AgBr.

c) Metal Excess Defect: This type of defect occurs in crystals when there is a surplus of metal atoms. This can happen due to two reasons: either some of the lattice sites are occupied by extra metal atoms, or some of the sites are occupied by electrons (also known as F-centers). These defects usually impart color to the crystals. For example, alkali halides like NaCl and KCl show this type of defect. When heated in the presence of sodium vapor, the sodium atoms get diffused into the crystal and occupy the vacant sites, resulting in a yellow color.