Question

In crystals Frenkel defect is not shown by alkali metal halide but silver metal halides show. Explain why?

Answer 1

The Frenkel defect is favoured by a large difference in size between the positive and negative ions. The Metal cations are generally smaller than the anions which occupy the vacant lattice site or hole. In the case of pure alkali halides, the Frenkel defect is not common this is because the alkali metal cannot fit into the interstitial vacant positions due to their large sizes.

Whereas the silver metal halides show Frenkel defect because the cation $A{g}^{+}$ and anion $C{l}^{-}$ differ in their size to large extent. Hence, the cation $A{g}^{+}$ occupies the interstitial sites by leaving a corresponding number of lattice sites vacant.