Write a short note on impurity defects.

(N/A) - If molten $NaCl$ containing a small amount of $SrCl_2$ is crystallized,some of the sites of $Na^+$ ions are occupied by $Sr^{2+}$ ions.
- Each $Sr^{2+}$ ion replaces two $Na^+$ ions. It occupies one site,and the other site remains vacant.
- The number of cationic vacancies produced is equal to the number of $Sr^{2+}$ ions added.
- Example: Solid solution of $CdCl_2$ and $AgCl$.