Schottky defect in a crystal is observed when

Options

A. Density of the crystal is increased
B. Unequal number of cations and anions are missing from the lattice
C. An ion leaves its normal site and occupies an interstitial site
D. Equal number of cations and anions are missing from the lattice

Answer

**Step-by-Step Solution:**

1. **Understanding Schottky Defect**: A Schottky defect in a crystal occurs when there are vacancies created by the removal of equal numbers of cations and anions from the crystal lattice. This leads to a decrease in the density of the crystal.

2. **Analyzing the Options**:
- **Option 1**: "Density of the crystal is increased."
- This is incorrect because the removal of ions (cations and anions) leads to a decrease in density.

- **Option 2**: "Unequal number of cations and anions are missing from the lattice."
- This is also incorrect. In Schottky defects, the number of cations and anions removed is equal.

- **Option 3**: "An ion leaves its normal site and occupies an interstitial site."
- This describes a Frenkel defect, not a Schottky defect. Therefore, this option is incorrect.

- **Option 4**: "Equal number of cations and anions are missing from the lattice."
- This is the correct description of a Schottky defect.

3. **Conclusion**: Based on the analysis of the options, the correct answer is that a Schottky defect is observed when equal numbers of cations and anions are missing from the lattice.

**Final Answer**: The correct option is **D**: Equal number of cations and anions are missing from the lattice.

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