Schottky defect is a point defect in a crystal that occurs due to the absence of equal number of cations and anions from the lattice site creating vacancies. Absence of equal number of cations and anions results in maintaining electrical neutrality and stoichiometry.
Schottky defect occurs in highly ionic compounds with high coordination number and mostly when the cation and the anion are of comparable size i.e. little difference between cation and anion size.
Examples of crystals showing Schottky defect are KCl, KBr, CsCl, etc.