Question

Which is not a stoichiometric defect?

Hint:

Stoichiometric defects keep the chemical composition unchanged, whereas non-stoichiometric defects alter the cation-anion ratio.

Answer 1

Step 1: Understanding stoichiometric defects.
Stoichiometric defects are those which do not disturb the stoichiometry of the compound. The ratio of cations to anions remains the same. Common examples are Schottky defect, Frenkel defect, and interstitial defect.

Step 2: Identifying non-stoichiometric defect.
Non-stoichiometric defects occur when the ratio of cations to anions changes. Metal excess defect is a classic example of this, where extra metal ions create an imbalance, changing the stoichiometry.

Step 3: Analyzing the options.
- (A) Interstitial defect: A stoichiometric defect caused by small ions occupying interstitial sites.
- (B) Frenkel defect: A stoichiometric defect caused by displacement of an ion to an interstitial site.
- (C) Metal excess defect: Not a stoichiometric defect. It changes the cation-anion ratio.
- (D) Schottky defect: A stoichiometric defect due to equal number of missing cations and anions.

Step 4: Conclusion.
Thus, the defect which is not stoichiometric is the
metal excess defect.

Final Answer: The correct answer is (C) Metal excess defect.