Schottky defect is defined as which of the following?

• A. A cation and anion vacancy pair
• B. A cation interstitial pair
• C. A dislocation
• D. A substitutional impurity

Answer: (A)

In ionic crystals, defects can remove ions from their lattice sites while keeping the overall charge balance intact. A Schottky defect is the creation of a matched pair of vacancies: one missing cation site and one missing anion site. This preserves both the stoichiometry and electrical neutrality of the crystal, which is why such vacancies appear together in compounds like NaCl. It’s different from a Frenkel defect, where a cation moves into an interstitial site leaving behind a vacancy (a vacancy plus an interstitial of the same ion). It’s also not a dislocation (a line defect) or a substitutional impurity (a foreign atom replacing a host atom). Thus, a cation and anion vacancy pair best defines a Schottky defect.