Schottky defect: equal number of cation and anion vacancies to maintain electrical neutrality. Occurs in highly ionic compounds with similar cation and anion sizes and high coordination numbers. Examples: NaCl, KCl, KBr, CsCl. Effect: density DECREASES (atoms are missing, volume stays same). Number of Schottky defects n = N exp(-$E_s$/2kT), where $E_s$ is the energy of formation. At thermal equilibrium, ~1 in 10^6 sites may be vacant in NaCl at room temperature.