Question

Which of the following compounds is likely to show both Frenkel and Schottky defects in its crystalline form?

• A. $\mathrm{ZnS}$
• B. $\mathrm{CsCl}$
• C. $\mathrm{KBr}$
• D. $\mathrm{AgBr}$

Answer 1

The correct option is D

$\mathrm{AgBr}$

• The explanation for the correct option:

D) $\mathrm{AgBr}$

• Frenkel defect is a point defect that arises when the smaller ion leaves its position and enters an interstitial site forming a vacancy.
• For example - $\mathrm{AgBr},\mathrm{ZnS},$etc.,
• In this case, the cation which is ${\mathrm{Ag}}^{+}$ is smaller in size as compared to the anion${\mathrm{Br}}^{-}.$
• There is a large difference between the size of the cation and the anion.
• Therefore, the cation leaves its position and enters an interstitial site showing a Frenkel defect.
• A Schottky defect is a point defect in which an equal number of cations and anions are missing.
• This defect happens when there is a small difference in the size of cations and anions.
• An equal number of ${\mathrm{Ag}}^{+}$ions and ${\mathrm{Br}}^{-}$ ions leave their normal lattice sites.
• So, $\mathrm{AgBr}$ show both Frenkel and Schottky defects.

The explanation for the incorrect options:
A) $\mathrm{ZnS}$

• In this case, the cation which is${\mathrm{Zn}}^{2+}$ is smaller in size as compared to the anion ${\mathrm{Br}}^{-}.$
• Therefore, the cation leaves its position and enters an interstitial site showing a Frenkel defect.
• It does not show Schottky defect as there is a huge difference in the size of the cation and anion.

B)$\mathrm{CsCl}$

• Here, the size of the cation and anion i.e., ${\mathrm{Cs}}^{+}\mathrm{and}{\mathrm{Cl}}^{-}$ are nearly similar.
• Therefore, it only shows Schottky's defect and not Frenkel's defect.

C) $\mathrm{KBr}$

• Here, the size of the cation and anion i.e., ${\mathrm{K}}^{+}\mathrm{and}{\mathrm{Br}}^{-}$ are nearly similar.
• Therefore, it only shows Schottky's defect and not Frenkel's defect.

Therefore, D) $\mathrm{AgBr}$is correct. Only $\mathrm{AgBr}$shows both Frenkel and Schottky defects.