Schottky defects generate an equal number of cation and anion vacancies while doping produces only cation vacancies and not anion vacancies. Why?
Schottky defect also exists in pair to maintain electroneutrality. Therefore, it generates equal number of cation and anion vacancies. In case of doping of NaCl by CdCl2, CI– ions enter Cl– ions site in NaCl while Cd2+ displace Na+. In doping so, one positive charge becomes extra. Therefore one Na+ is displaced so as to maintain electro-neutrality. That is doing only cation vacancies exist.