Which of the following defects, if present, lowers the density of the crystal ?

Options

A. Frenkel defect
B. Schottky defect
C. F-centres
D. Interstitial defect

Explanation

To determine which defect lowers the density of a crystal, we need to analyze the types of defects present in crystalline solids. The two main types of defects we will consider are Frenkel defect and Schottky defect.

### Step-by-Step Solution:

1. **Understanding Frenkel Defect**:
- In a Frenkel defect, a cation is displaced from its lattice site and occupies an interstitial site within the crystal.
- This means that while one cation is missing from its original position, it is still present in the crystal structure, just in a different location (interstitial site).
- Since the total number of ions remains the same, the overall mass of the crystal does not change.
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