Frenkel defect is not found in halides of alkali metals.why?

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

In Frankel defect there is a large difference between the size of cation and anion so that if a ion is missing from correct lattice site it occupies neighbouring interstitial site but in case of alkyl halide it can`t fit into the interstitial site due to their large sizes so alkyl halide do not show Frankel defect