Which of the following is/are correct ?

Options

A. Schottky defect lowers the density
B. Frenkel defect increases the dielectric constant of the crystals
C. Stoichiometric defects make the crystals electrical conductors
D. In the Schoottky defect, equal number of extra cations and electrons are present in the interstitials sites

Answer

To determine which statements regarding defects in solids are correct, we will analyze each statement one by one based on our understanding of solid-state chemistry.

### Step-by-Step Solution:

1. **Statement 1: Schottky defect lowers the density.**
- **Explanation:** A Schottky defect occurs when an equal number of cations and anions are missing from their lattice sites in a crystal. This results in vacancies, which effectively reduces the mass of the unit cell without changing its volume significantly. Therefore, the density of the crystal decreases.
- **Conclusion:** This statement is **correct**.

2. **Statement 2: Frenkel defect increases the dielectric constant of the crystal.**
- **Explanation:** A Frenkel defect occurs when an ion (either cation or anion) is displaced from its lattice site to an interstitial site. This creates a situation where ions are closer together, which can enhance the electrical permittivity of the material. An increase in electrical permittivity leads to an increase in the dielectric constant of the crystal.
- **Conclusion:** This statement is **correct**.

3. **Statement 3: Stoichiometric defects make the crystal electrical conductors.**
- **Explanation:** Stoichiometric defects, which include both Schottky and Frenkel defects, introduce vacancies and interstitials that allow for ionic movement. This ionic mobility can facilitate the conduction of electricity in the crystal, making it conductive.
- **Conclusion:** This statement is **correct**.

4. **Statement 4: In the Schottky defect, equal numbers of extra cations and electrons are present in the interstitial sites.**
- **Explanation:** This statement is misleading. In a Schottky defect, there are vacancies created for cations and anions, but it does not imply that there are extra cations or electrons in interstitial sites. Instead, the defect involves missing ions rather than the presence of extra ions or electrons.
- **Conclusion:** This statement is **incorrect**.

### Final Conclusion:
The correct statements are:
- Statement 1: Correct
- Statement 2: Correct
- Statement 3: Correct
- Statement 4: Incorrect

Thus, the correct options are 1, 2, and 3.