Any deviation from the perfectly ordered arrangement constitutes a defect or imperfection. The compounds in which the number of irregularity are present in the arrangement of atoms or ions are called atomic imperfections or point defects. These may be stochiometric or non - stoichlometric. Stoichiometric defects are further classified into Schottky defects in which number of positive and negative ions are missing in equal ratio and Frenkel defects in which an ion leaves its correct lattice sites and occupies on interstitial site. Schottky defect is more common in ionic compounds with high co-ordination number and where the size of positive and negative ions are almost equal, while in Frenkel defects are common in ionic compounds, which have low co-ordination number and in which there is large difference in size between positive and negative ions.
Which of the following defect, if present, lowers the density of the crystal ?

Options

A. Frenkel
B. Schottky
C. Edge dislocation
D. Constitution of F - centres

Answer

To solve the question regarding which defect lowers the density of the crystal, we will analyze the two types of stoichiometric defects mentioned: Schottky defects and Frenkel defects.

### Step-by-Step Solution:

1. **Understanding Point Defects**:
- Point defects are irregularities in the arrangement of atoms or ions in a crystal lattice. They can be classified into two categories: stoichiometric and non-stoichiometric defects.

2. **Stoichiometric Defects**:
- These defects maintain the overall stoichiometry of the compound. They include Schottky and Frenkel defects.

3. **Schottky Defect**:
- In a Schottky defect, equal numbers of cations and anions are missing from the lattice. This means that there are vacancies created in the crystal structure.
- Since ions are missing, the total mass of the crystal decreases, leading to a reduction in density.

4. **Frenkel Defect**:
- In a Frenkel defect, an ion (either cation or anion) leaves its original lattice position and occupies an interstitial site.
- Although there is a rearrangement of ions, the total number of ions in the crystal remains the same, and thus the density does not change.

5. **Conclusion**:
- Among the defects discussed, the Schottky defect is the only one that lowers the density of the crystal due to the absence of both cations and anions.

### Final Answer:
The defect that lowers the density of the crystal is the **Schottky defect**.

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