Two or more cations of lower valency are replaced by a cation of higher valency to maintain electrical neutrality. Hence some cation vacancies are created. For example : In an ionic solid ‘NaCl’, if impurity of Sr2+ is added (as SrCl2) then two Na+ ions left thus lattice sites. To maintain electrical neutrality one lattice site is occupied by Sr2+ ion while other lattice site will remain vacant.
