Which defect is not found in pure alkali metal halides ?

Options

A. Frenkel defect
B. Schottky defect
C. Both (a) and (b)
D. Point defect

Answer

To determine which defect is not found in pure alkali metal halides, we need to analyze the types of defects that can occur in ionic crystals, particularly focusing on alkali metal halides.

### Step-by-Step Solution:

1. **Understanding Ionic Defects**: Ionic solids can exhibit various types of defects, including Schottky defects, Frenkel defects, and interstitial defects. Each of these defects involves the arrangement and positioning of cations and anions in the crystal lattice.

2. **Identifying the Defects**:
- **Schottky Defect**: This defect occurs when an equal number of cations and anions are missing from their lattice sites, creating vacancies. This defect is common in ionic solids.
- **Frenkel Defect**: This defect occurs when a cation or anion is displaced from its lattice position to an interstitial site, creating a vacancy and an interstitial ion.

3. **Analyzing Alkali Metal Halides**: Alkali metal halides (like NaCl, KBr) consist of larger cations (like Na⁺, K⁺) and anions (like Cl⁻, Br⁻). The size of these ions plays a crucial role in the types of defects that can occur.

4. **Frenkel Defect in Alkali Metal Halides**: In alkali metal halides, the larger size of the cations makes it difficult for them to fit into interstitial sites. Therefore, Frenkel defects, which require the cation to move to an interstitial site, are not typically found in these compounds.

5. **Conclusion**: Since alkali metal halides do not exhibit Frenkel defects due to the size of the cations, we conclude that this is the defect that is not found in pure alkali metal halides.

### Final Answer:
The defect that is not found in pure alkali metal halides is the **Frenkel defect**.