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

Options

A. cations and anions have almost equal size
B. there is a large difference in size of cations and anions
C. cations and anions have low coordination number
D. anions cannot be accommodated in voids.

Explanation

### Step-by-Step Solution:

1. **Understanding Alkali Metal Halides**:
- Alkali metal halides are compounds formed from alkali metals (Group 1 elements) and halogens (Group 17 elements). Examples include lithium fluoride (LiF), sodium chloride (NaCl), and potassium bromide (KBr).

2. **Defining Frenkel Defect**:
- A Frenkel defect occurs in a crystal lattice when an atom or ion is displaced from its normal position to an interstitial site, creating a vacancy where it was originally located. This defect typically arises when there is a significant size difference between cations and anions.

3. **Size Relationship in Alkali Metal Halides**:
- In pure alkali metal halides, the cations (alkali metal ions) and anions (halide ions) are of similar sizes. For instance, the size of lithium ion (Li⁺) is comparable to that of fluoride ion (F⁻), sodium ion (Na⁺) is similar to chloride ion (Cl⁻), and potassium ion (K⁺) is close to bromide ion (Br⁻).

4. **Reasoning for Absence of Frenkel Defect**:
- Since the cations and anions in pure alkali metal halides are of comparable sizes, there is no significant size difference that would allow a smaller ion to occupy an interstitial site. Therefore, the conditions necessary for the formation of a Frenkel defect are not met.

5. **Conclusion**:
- As a result, Frenkel defects are not found in pure alkali metal halides because the cations and anions have almost equal sizes, preventing the occurrence of this type of defect.

### Final Answer:
Frenkel defect is not found in pure alkali metal halides because cations and anions have almost equal size.

---