To explain this, let us take an example of NaCl doped with SrCl, impurity when SrCl₂ is added to NaCl solid as an impurity, two Na⁺ ions will then be replaced and one of their site will be occupied by Sr^2- while the other will remain vacant. Thus, we can say that when a cation of higher valance is added as an impurity to an ionic soil, two or more cations of lower valency are replaced by a cation of higher valence to maintain electrical neutrality. Hence, some cationic vacancies are created.