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

To solve the question "Metal excess defect arises due to," we need to understand the concept of metal excess defects in ionic solids. Here's a step-by-step solution:

### Step 1: Understanding Metal Excess Defect
Metal excess defect occurs when there is an excess of metal cations in a crystal lattice. This can happen due to two primary reasons: the removal of anions or the presence of extra cations.

**Hint:** Think about what happens to the balance of cations and anions in a crystal lattice when defects occur.

### Step 2: Anion Removal
One way metal excess defects arise is through the removal of anions from their lattice positions. When anions are removed, the charge balance is disrupted, leading to an excess of cations. The vacancies left by the anions can be occupied by electrons, which are referred to as F-centers.

**Hint:** Consider how the removal of negatively charged ions affects the overall charge balance in the structure.

### Step 3: Cations in Interstitial Sites
Another way metal excess defects can occur is when smaller cations occupy interstitial sites in the crystal lattice. These interstitial cations increase the overall number of cations in the structure, contributing to the metal excess.

**Hint:** Recall the concept of interstitial sites and how smaller ions can fit into these spaces within the lattice.

### Step 4: Cationic Vacancy
Cationic vacancy refers to the absence of cations in the lattice. This would lead to a decrease in the number of metal cations, which is contrary to the definition of metal excess defects.

**Hint:** Think about how a vacancy affects the overall number of particles in the lattice.

### Conclusion
Based on the above analysis, we can conclude that metal excess defects arise due to:
1. The removal of anions (leading to the formation of F-centers).
2. The presence of extra cations in interstitial sites.

Thus, the correct answer to the question is that metal excess defect arises due to both the presence of extra cations at interstitial sites and the removal of anions.

### Final Answer
Metal excess defect arises due to:
- The presence of extra cation at interstitial sites.
- The removal of anions from their lattice positions.