About Frenkel defect
Frenkel defect:
This defect was discovered by russian scientist Frenkel in 1926 in ionic crystals. This defect arises when certain ion found missing from its normal sites and occupy positions elsewhere in the interstitial sites in the crystal lattice. In this case, the crystal remain electrically neutral as the number of cation and anions remain same. This defect is shown below where a cation A+ is found missing from its normal site and present in one of the interstitial sites in the crystal lattice.