why are frenkel defects not found in pure alkali halides?
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 case of pure alkali halides, the frenkel defect is not common. because the the alkali metal cannot fit into the interstitial vacant positions due to their large sizes.