give reason schottky defect lowers the density of a solid
If in an ionic crystal of type A+B-, an equal number of cations and anions are missing from their lattice sites so that electrical neutrality as well as stoichiometry is maintained this is called a Schottky Defect.
It is a vacancy defect (due to missing ions) and also a stoichiometric defect, as the ratio of the number of cations and anions remains the same. It occurs only when there is small difference in size between cations and anions. This is produced as the result of the thermal incorporation of unoccupied lattice sites from the exterior of the crystal. The lattice undergoes thermal vibration and thermal expansion when the temperature is raised above OK. When it happens the pair of vacancies are incorporated in the crystal.
The total number of ions in a crystal with this defect is less than the theoretical value of ions, thus, the density of the solid crystal is less than normal.