Metal excess defect arises due to

Options

A. Anionic vacancies
B. The presence of extra cations at interstitial sites
C. Cationic vacancies
D. Both (1) & (2)

Answer

**Step-by-Step Solution:**

1. **Understanding Metal Excess Defect**:
- A metal excess defect occurs when there is an excess of metal cations in a crystal lattice compared to the number of anions. This leads to a situation where the positive charge from the metal cations exceeds the negative charge from the anions.

2. **Formation of Anionic Vacancy**:
- An anionic vacancy is created when an anion (negatively charged ion) is removed from the lattice. For example, if we have a lattice with 4 metal cations (M⁺) and 4 anions (A⁻), removing one anion results in 4 M⁺ and only 3 A⁻. This creates an imbalance where there are 4 positive charges and only 3 negative charges.

3. **Resulting Charge Imbalance**:
- With 4 positive charges and 3 negative charges, the overall charge is now positive (4 - 3 = +1). This indicates that there is an excess of positive charge, which corresponds to a metal excess defect.

4. **Adding Extra Cation**:
- To further illustrate, if we add an extra metal cation (M⁺) to the lattice, we now have 5 M⁺ and still only 4 A⁻. This further increases the excess of metal cations, confirming the presence of a metal excess defect.

5. **Cationic Vacancy**:
- If instead of creating an anionic vacancy, a cationic vacancy (missing metal cation) is formed, the situation would be different. In this case, the number of positive charges would decrease, potentially leading to a metal deficiency defect rather than an excess.

6. **Conclusion**:
- Therefore, a metal excess defect arises due to the formation of an anionic vacancy or the addition of extra cations, resulting in more positive charges than negative charges in the crystal lattice.