Schottky defect lowers the density of ionic crystals while Frenkel defect does not. Why?

Explanation

In Schottky defect, certain cations and anions are missing from lattice site resulting in decrease in mass per unit volume, i.e., density. On the other hand, in Frenkel defect, the ions do not leave the lattice but they simply change their positions from lattice points to the interstitial spaces. As a result, the density of the crystal does not change.