If cation loses its original lattices site and occupies an interstitial  void, this defect results. It creates a vacancy defect at its original lattice site and an interstitial defect at its new location.

It is found in crystals:

(i) with low co.no.

(ii) Where there is large difference in the size of cation and anions e.g. ZnS, AgCl, AgBr, AgI.

Due to this defect, the density doesn’t change. Due to the presence of holes, crystals conduct electricity stability decreases. Dielectric constant increases $\because$ similar charges come closer.