(i) Schottky defect: It arises due to the missing of equal number of cations and anions from their normal positions leaving behind a pair of holes. This defect is usually observed in ionic compounds having high coordination number and ions of almost similar size. The density of the crystal is lowered due to the presence of vacancies in the crystal lattice. Schottky defect increases slightly the electrical conductance of the crystal. E.g. NaCl, KCl, CsCl. 

(ii) Frenkel defect: It arises w’hen an ion, usually cation, leaves its normal site and occupies an interstitial site. This defect is usually observed in ionic ‘ compounds having low co-ordination number , and crystals with anions are much larger in size than the cations. Since no ions are missing from the crystals as whole, it doesn’t affect the density of the crystal. E.g.: AgCl, ZnS, AgBr 

(iii) Interstitials: Atoms or ions which occupy the normally vacant interstitial positions in a crystal are called interstitials. 

(iv) F-centres: The electron occupying holes, created by missing of anions from the lattice sites are called F-centres. These F-centres are responsible for colour of compound.