Lattice structures are not perfect; in fact most of the time they experience defects.
In ionic crystals, the defect forms when oppositely charged ions leave their lattice sites, creating vacancies. These vacancies are formed in stoichiometry
units, to maintain an overall neutral charge in the ionic solid. The
vacancies are then free to move about as their own entities. Normally
these defects will lead to a decrease in the density of the crystal.