Impurity Defects

If molten NaCl containing a little amount of SrCl₂ is crystallized, some of the sites of Na+ ions are occupied by Sr²⁺ (Fig.).

Each Sr²⁺ replaces two Na⁺ 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.

Another similar example is the solid solution of CdCl₂ and AgCl.