Which of the following is a non-stoichiometric defect ?

Options

A. Frenkel defect
B. Schottky defect
C. metal deficiency defect
D. interstitial defect

Answer

To determine which of the given options is a non-stoichiometric defect, we will analyze each option based on the definitions of stoichiometric and non-stoichiometric defects in solid-state chemistry.

### Step-by-Step Solution:

1. **Understand the Types of Defects**:
- **Stoichiometric Defects**: These defects do not change the overall stoichiometry of the compound. The ratio of the different types of ions remains constant.
- **Non-Stoichiometric Defects**: These defects alter the stoichiometry of the compound, meaning there is a change in the ratio of the ions present.

2. **Analyze Each Option**:
- **Option 1: Frankel Defect**: This is a type of point defect where an ion (usually a cation) is displaced from its lattice site to an interstitial site. This defect maintains the overall stoichiometry, so it is a stoichiometric defect.
- **Option 2: Schottky Defect**: This is another point defect where an equal number of cations and anions are missing from the lattice. This defect also maintains the stoichiometry of the compound, making it a stoichiometric defect.
- **Option 3: Interstitial Defect**: This defect occurs when extra ions occupy interstitial sites in the lattice. Like the previous defects, it does not change the overall stoichiometry, thus it is also a stoichiometric defect.
- **Option 4: Metal Deficiency Defect**: This defect occurs when there are fewer metal ions than what is required by the stoichiometric formula of the compound. This leads to a change in the stoichiometry, making it a non-stoichiometric defect.

3. **Conclusion**: Based on the analysis, the correct answer is **Option 4: Metal Deficiency Defect**, as it is the only non-stoichiometric defect among the options provided.

### Final Answer:
**Metal Deficiency Defect** is a non-stoichiometric defect.