When a crystal having rock salt type geometry is heated in the presence of it's metal vapour then defect in it will be

Options

A. Stoichiometric defect
B. Metal excess defect
C. Anion excess defect
D. Frenkel defect

Answer

To solve the question about the type of defect that occurs when a crystal with rock salt type geometry is heated in the presence of its metal vapor, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Crystal Structure**:
- Rock salt (NaCl) has a face-centered cubic (FCC) structure where sodium ions (Na⁺) and chloride ions (Cl⁻) alternate in a 1:1 ratio.

2. **Heating in Metal Vapor**:
- When the rock salt crystal is heated in the presence of its metal vapor (for example, sodium vapor), additional metal atoms (Na) are introduced into the crystal lattice.

3. **Identify the Type of Defect**:
- The introduction of extra metal atoms leads to a situation where there are more cations (Na⁺) than anions (Cl⁻) in the crystal structure. This creates an imbalance in the stoichiometry of the crystal.

4. **Determine the Defect Type**:
- The defect created by the addition of extra cations (metal ions) is known as a **metal excess defect**. This is specifically referred to as a **metallic cation defect** or **metal axis defect**.

5. **Conclusion**:
- Therefore, the correct answer to the question is that the defect formed is a **metal axis defect**.

### Final Answer:
- The defect in the crystal when heated in the presence of its metal vapor is a **metal axis defect**.

---