Why is Frenkel defect not found in pure alkali metal halides ?

Explanation

To understand why Frenkel defects are not found in pure alkali metal halides, we need to break down the concept of Frenkel defects and the characteristics of alkali metal halides.

### Step-by-Step Solution:

1. **Definition of Frenkel Defect**:
- A Frenkel defect occurs when a smaller ion (typically a cation) is displaced from its lattice position to an interstitial site, creating a vacancy at the original lattice point. This defect is characterized by the presence of both a vacancy and an interstitial ion.

2. **Size of Ions**:
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