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Explain stoichiometric defects shown by ionic solids. |
Answer» Solution : (i) Schottky DEFECT : Schottky defects is basically a vacancy defect in ionic solids in which the electrical neutrality is maintained by absence of equal number of cations and anions from lattice sites.  Like simple vacancy defect, Schottky defect also decreases the density of the substance. The number of such defects are quite significant in ionic solids. For Ex. : In NaCl at room temperature, there are `10^6` Schottky pairs PER cm and there are about `10^22` ions per `cm^3`, thus, there is one Schottky defect per `10^16` ions. Ionic compounds with high co-ordination number and having cation and anion of COMPARABLE size shows Schottky defect. Ex. : NaCl, KCl, CsCl and AgBr shows Schottky defects. (ii) Frenkel Defect: Ionic compounds with low co ordination number and having large difference in size of cation and anion shows Frenkel defects. In Frenkel defect, the smaller ion generally the cation is dislocated from its normal site to an interstitial site.  Frenkel defects Frenkel defect is also called dislocation defect and because of dislocation of cation, a vacancy defect is created at its original site and interstitial defect is created at new location. Frenkel defect doesnot alter density of crystal. Ex. : Zns, AgCl, AgBr and Agl shows Frenkel defects due to SMALL size of `Zn^(2+)` and `Ag^(+)` ions. It is to be NOTED that AgBr shows both Frenkel and Schottky defects. |
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