Frenkel defect is not found in the halides of alkali metals because alkali metals have
1. high electropositivity
2. high ionic radii
3. high reactivity
4. ability to occupy interstitial sites
Frenkel defect is not found in pure alkali metal halides. This is because of the larger size of the cations of the alkali metals, due to the large size the cations cannot fit into the interstitial sites.