Q.Schottky defect in a crystal is observed when

Both are stoichiometric point defects (the crystal's overall formula is unchanged), but they differ in what happens to the missing ion and in their effect on density:

• Schottky defect: EQUAL numbers of cations and anions go missing from the lattice altogether. Seen when the cation and anion are of similar size (e.g. NaCl). Because mass is genuinely removed from the cell while its volume stays the same, Schottky defects lower the crystal's density -- e.g. VO's theoretical density (6.5 g/cm3, from unit-cell dimensions) vs its actual density (5.6 g/cm3) indicates ~14% Schottky defect. The resulting vacancies also give ions/atoms an easy pathway to migrate through the lattice. …