Question

Impurity defect is a type of point defect. It can occur __________

• A. in one way, as substitution impurity defect
• B. in one way, as interstitial impurity defect
• C. in two ways, as substitution impurity defect or as interstitial impurity defect
• D. in two ways, as vacant impurity defect or as interstitial impurity defect

Answer 1

The correct option is C in two ways, as substitution impurity defect or as interstitial impurity defect

Defects in ionic crystals, can be introduced by adding impurities.​ Similar sized cation substitute​ the existing cation of ionic crystal.​

For example ; When molten NaCl is crystallised having a small amount of $SrC{l}_2.$​ Some $N{a}^{+}$ ions’ locations are occupied by $S{r}^{2+}$ ions​
Each $S{r}^{2+}$ replaces two $N{a}^{+}$ sites ​by occupying a site of one $N{a}^{+}$ ​and other site remaining vacant.​
Number of cationic vacancies generated​ is equal to number of $S{r}^{2+}$ ​in the crystal​
Other example:​ solid solution of ​$CdC{l}_2$ and AgCl. ​

Interstitial Impurity Defect: When some small foreign atoms (like B, C, N, H) are trapped in interstitial voids of the lattice​ without any chemical reaction. Formula remains the same. in this case.
Hence option (c) is correct.