Which of the following defect causes decrease in density of the crystal?

Options

A. Frankel
B. Schottky
C. Interstitial
D. F centre,

Answer

To determine which defect causes a decrease in the density of a crystal, we need to analyze the different types of point defects in crystals. The two main defects we will consider are the Frenkel defect and the Schottky defect.

### Step-by-Step Solution:

1. **Understanding Point Defects**:
- Point defects are imperfections in the crystal lattice that occur at a single point. The two common types are Frenkel defects and Schottky defects.

2. **Frenkel Defect**:
- In a Frenkel defect, an ion leaves its normal lattice position and occupies an interstitial site (a space between the regular lattice points).
- This means that while one ion is missing from its original position, it is not lost from the crystal; it is simply relocated.
- Therefore, the overall number of ions in the crystal remains the same, and the density does not change.

3. **Schottky Defect**:
- In a Schottky defect, an equal number of cations and anions leave their lattice sites and create vacancies in the crystal.
- This results in a reduction in the total number of ions present in the crystal lattice because these ions are not just relocated but are completely removed from the lattice.
- As a result, the density of the crystal decreases due to the presence of these vacancies.

4. **Conclusion**:
- Based on the analysis, the Schottky defect is the defect that causes a decrease in the density of the crystal.

### Final Answer:
The defect that causes a decrease in the density of the crystal is the **Schottky defect**.

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