Schottky defect occurs mainly in electrovalent compounds where

Options

A. Positive ions have large sizes
B. Negative ions have small sizes
C. Positive and negative ions are of similar size
D. Positive ions are big and negative ions are small

Answer

### Step-by-Step Solution:

1. **Understanding Schottky Defect**:
- A Schottky defect is a type of point defect in a crystal lattice where an equal number of cations (positive ions) and anions (negative ions) are missing from their lattice sites. This defect leads to vacancies in the crystal structure.

2. **Importance of Electrical Neutrality**:
- For a crystal to maintain electrical neutrality, the number of missing cations must equal the number of missing anions. This is crucial because if there were an unequal number of missing ions, the crystal would become charged, which is not favorable.

3. **Conditions for Schottky Defect**:
- Schottky defects typically occur in ionic compounds where the cation and anion sizes are comparable. This means that the ratio of the radius of the cation to the radius of the anion should be close to 1.

4. **Analyzing the Options**:
- **Option 1**: Positive ions have large size. (Incorrect)
- **Option 2**: Negative ions have small size. (Incorrect)
- **Option 3**: Positive and negative ions are of similar size. (Correct)
- **Option 4**: Positive ions are big and negative ions are small. (Incorrect)

5. **Conclusion**:
- The correct answer is **Option 3**, which states that positive and negative ions are of similar size. This condition allows for the formation of Schottky defects in electrovalent (ionic) compounds.