Which one of the following cannot be called as a 'non-stoichiometric defect'?

Options

A. Metal excess defect due to anion vacancies.
B. Metal excess defect due to presence of extra cations.
C. Metal deficiency due to absence of cations
D. Combination of vacancy and interstitial defects

Answer

To determine which of the given options cannot be classified as a 'non-stoichiometric defect', we first need to understand the definitions and types of defects in solids, particularly in ionic compounds.

### Step-by-step Solution:

1. **Understanding Defects in Solids**:
Defects in solids can be classified into two main categories: stoichiometric defects and non-stoichiometric defects.

2. **Stoichiometric Defects**:
- **Metal Excess Defect**: This occurs when there are extra metal cations in the crystal lattice. This can happen due to:
- **Anion Vacancies**: Where anions are missing from their lattice positions, leading to an excess of metal cations.
- **Interstitial Cations**: Where extra metal cations occupy interstitial sites in the lattice.
- **Metal Deficiency Defect**: This occurs when there are fewer metal cations than required by the stoichiometric ratio, often due to the absence of cations.

3. **Non-Stoichiometric Defects**:
These defects do not maintain the stoichiometric ratio of the compound. They can arise from:
- An imbalance in the number of cations and anions.
- Presence of vacancies or interstitials that disrupt the stoichiometry.

4. **Analyzing the Options**:
We need to analyze the options provided to identify which one does not fall under non-stoichiometric defects:
- **Option 1**: Metal excess defect due to anion vacancies (This is a known stoichiometric defect).
- **Option 2**: Metal excess defect due to presence of extra cations (This is also a known stoichiometric defect).
- **Option 3**: Metal deficiency due to absence of cations (This is a stoichiometric defect).
- **Option 4**: Combination of vacancy and interstitial defect (This is typically a non-stoichiometric defect).

5. **Conclusion**:
Based on the analysis, the option that cannot be called a 'non-stoichiometric defect' is **Option 4** (Combination of vacancy and interstitial defect), as it is a combination that maintains stoichiometry.

### Final Answer:
The option that cannot be called a 'non-stoichiometric defect' is **Option 4**.