F - centres in an ionic crystals are

Options

A. Lattice sites containing electrons
B. Interstitial sites containing electrons
C. Lattice sites that are vacant
D. Interstitial sites containing cations

Answer

### Step-by-Step Solution:

1. **Definition of F-Center**:
- An F-Center, or Frenkel center, is a type of defect in ionic crystals where an anionic vacancy (missing anion) is occupied by an electron. This creates a neutral defect that contributes to the color and electrical properties of the crystal.

2. **Formation of F-Centers**:
- F-Centers are typically formed when an ionic crystal is heated in the presence of a vapor of the cation (e.g., sodium vapor in the case of NaCl). The heat provides energy that allows some of the anions (e.g., Cl-) to leave their lattice positions, creating vacancies.

3. **Example with Sodium Chloride (NaCl)**:
- In a NaCl crystal, Na+ and Cl- ions are arranged alternately. When sodium vapor is introduced and the crystal is heated, sodium atoms deposit on the surface. Some Cl- ions can diffuse away from their lattice sites, creating vacancies.

4. **Electron Trapping**:
- When a Cl- ion leaves its position, it creates an anionic vacancy. The sodium atoms that are deposited can ionize to form Na+ and free electrons. These free electrons can then occupy the anionic vacancies, forming F-Centers.

5. **Resulting Structure**:
- The resulting structure has an anionic vacancy that is now occupied by an electron, which stabilizes the vacancy and affects the properties of the crystal, such as its color.

### Final Answer:
F-Centers in ionic crystals are anionic vacancies that are occupied by electrons. They are formed when an ionic crystal is heated in the presence of a vapor of the cation, leading to the diffusion of anions and trapping of electrons in the resulting vacancies.

---