Schottky defect is observed in crystals when __________.

•The Schottky defect is basically a kind of vacancy defect with the tendency to maintain electrical neutrality, therefore same no.s of cations are anions are missing from the crystal lattice.

• Other 3 options are incorrect.

• (i) some of the cations move from their lattice site to interstitial sites – this occurs in the case for Frankel defect only.

• some lattice sites are occupied by electrons- this is the case of metal excess defect.

• (iv) some impurity is present in the lattice – this is observed in crystals with impurity defects.