Which defect causes decreases in the density of crystal?

Options

A. Schottky defect
B. Frenkel defect
C. Interstitial defect
D. Metal excess defect

Answer

### Step-by-Step Solution:

1. **Understanding Crystal Defects**:
- Crystals can have various defects that affect their properties, including density. The main types of defects include Schottky defects, Frenkel defects, interstitial defects, and metal excess defects.

2. **Analyzing Each Defect**:
- **Frenkel Defect**: In this defect, a small cation leaves its original position but remains within the crystal structure. Since no particles leave the crystal, the density remains unchanged.
- **Interstitial Defect**: Here, some particles occupy interstitial sites within the crystal. This addition of particles increases the overall mass without significantly changing the volume, leading to an increase in density.
- **Metal Excess Defect**: This defect is characterized by anionic vacancies and the presence of extra cations in interstitial sites. The overall mass may not decrease, and hence, the density does not decrease.
- **Schottky Defect**: In this defect, both cations and anions leave the crystal lattice to maintain electrical neutrality. This loss of particles results in a decrease in the overall mass of the crystal, leading to a decrease in density.

3. **Conclusion**:
- Among the defects analyzed, the **Schottky defect** is the one that causes a decrease in the density of the crystal due to the loss of both cations and anions from the lattice.

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

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