Which of the following defects decrease the density ?

Options

A. Interstitial defect
B. Vacancy defect
C. Frankel defect
D. Schottky defect.

Explanation

To solve the question "Which of the following defects decrease the density?", we need to understand the types of defects in solids and their impact on density.

### Step-by-Step Solution:

1. **Identify Types of Defects**:
- There are several types of defects in solids, including vacancies, interstitials, and substitutional defects.
- **Vacancies**: These are missing atoms in the lattice structure.
- **Interstitials**: These are extra atoms that occupy spaces between the regular lattice points.
- **Substitutional Defects**: These occur when an atom in the lattice is replaced by a different type of atom.

2. **Analyze the Effect on Density**:
- **Vacancies**: The presence of vacancies decreases the number of atoms in a given volume, which leads to a decrease in density.
- **Interstitials**: The introduction of interstitial atoms increases the number of atoms in the lattice, leading to an increase in density.
- **Substitutional Defects**: The effect on density depends on the size and mass of the substituting atom compared to the original atom. If the substituting atom is smaller or lighter, it may decrease the density, while a larger or heavier atom may increase it.

3. **Conclusion**:
- Among these defects, **vacancies** are the primary type that consistently decreases the density of a solid.

### Final Answer:
The defect that decreases the density is **vacancies**.