Question

Which of the following defects are shown by KCl crystal?
(A) Schottky defect
(B) Frenkel defect
(C) Metal excess defect
(D) Metal deficiency defect
Choose the correct answer from the options given below:

• (A) and (B) only
• (A) and (C) only
• (B) and (C) only
• (B) and (D) only

Hint:

KCl exhibits Schottky defects and to a lesser extent, Frenkel defects, where equal numbers of cations and anions are displaced or vacated from their lattice positions. Metal excess and metal deficiency defects are not commonly observed in KCl.

Answer 1

The Correct Option is A

In KCl (potassium chloride) crystal, ionic defects can occur due to the misplacement or absence of ions in the crystal structure. Let's analyze each option: 
- Schottky defect (A): The Schottky defect is a vacancy defect that occurs when equal numbers of cations and anions are missing from the lattice. This type of defect reduces the density of the crystal, and is commonly found in KCl. 
Therefore, KCl shows a Schottky defect.
 - Frenkel defect (B): The Frenkel defect occurs when an ion (usually a cation) is displaced from its normal position and occupies an interstitial site. This defect is also possible in KCl, but it is more commonly seen in AgCl and ZnS. KCl does exhibit Frenkel defects to some extent, though they are less common compared to Schottky defects. 
- Metal excess defect (C): In a metal excess defect, extra metal cations are added to the crystal and occupy interstitial positions, usually to balance the charge. KCl does not exhibit this defect under normal conditions. 
- Metal deficiency defect (D): In a metal deficiency defect, there is a lack of metal cations in the crystal. KCl does not typically show metal deficiency defects under normal conditions. 
Thus, the correct answer is Option (1), as KCl can exhibit both Schottky defects and Frenkel defects. 
Reactions for Defects: - Schottky defect: The Schottky defect occurs due to the removal of equal numbers of cations and anions from the lattice: \[ \text{K}^+ \text{(lattice site)} + \text{Cl}^- \text{(lattice site)} \rightarrow \text{Vacancy of K}^+ + \text{Vacancy of Cl}^- \] - Frenkel defect: The Frenkel defect occurs when a cation is displaced to an interstitial site: \[ \text{K}^+ \text{(lattice site)} \rightarrow \text{K}^+ \text{(interstitial site)} \]