Frenkel defects are not found in alkali metal halides. Explain.

Frenkel defect arises when an ion is missing from its lattice site and occupies an interstitial position. The crystal as a whole remains electrically neutral because the number of anions and cations remain same. Since cations are usually smaller than anions, they occupy interstitial sites. This type of defect occurs in the compounds which have low co-ordination number and cations and anions of different sizes. e.g., ZnS, AgCl and AgI etc.

Frenkel defect are not found in pure alkali metal halides because due to larger size of the cations of the alkali metals, they cannot get into the interstitial sites (gaps or spaces in the structure).