A schottky defect is a type of point defect in a crystal lattice. In non – ionic crystals it means a lattice vacancy defect. In ionic crystals, the defect forms when oppositely charged ions leave their sites, creating vacancies. There vacancies are formed in stoichiometric units, to maintains an overall neutral change in the ionic solid.
The vacancies are then free to move about their own entities. Normally these defects will lead to a decrease in the density of the crystal.