In impurity defect, the higher valency cation replaces the lower valency cation. An important point to be noted is that electrical neutrality is maintained.
For e.g. In case of NaCl and SrCl2 , some of the sites of Na+ ions are occupied by Sr2+ . Two Na+ ions are replaced by Sr+2 ions. It occupies the site of one ion and the other site remains vacant. The cationic vacancies thus produced are equal in number to that of Sr2+ ions.