Which of the following is true regarding metal excess defect?

Both Options 1 and 2 are correct

The correct answer is option 4. Both Options 1 and 2 are correct.

Metal excess defect is a type of non-stoichiometric defect in crystalline solids, and it occurs when there is an excess of metal ions compared to the stoichiometric ratio.

Understanding Metal Excess Defect:
Metal excess defect can arise in two main ways:

A. Anionic Vacancies: This occurs when an anion is missing from its lattice site, creating a vacancy. To maintain electrical neutrality, an electron occupies this vacancy, leading to an overall excess of metal ions. This type of defect is often seen in alkali halides like sodium chloride when they are heated in the presence of sodium vapor. The missing anions can be replaced by electrons, forming F-centers (color centers) that absorb light and impart color to the crystals.

B. Interstitial Cations: This occurs when extra metal cations occupy interstitial sites in the crystal lattice. The additional cations increase the number of metal ions relative to anions. This type of defect can be observed in compounds such as zinc oxide (ZnO) when heated, where some Zn atoms occupy interstitial sites and electrons are also present to maintain electrical neutrality.

Evaluating the Statements:
1. It is caused due to anionic vacancies: This is correct. Anionic vacancies lead to metal excess defects by leaving behind electrons that maintain charge neutrality.

2. It is caused due to the presence of extra cations at interstitial sites: This is also correct. Extra cations at interstitial sites contribute to metal excess defects by increasing the number of metal ions.

3. It is caused due to cationic vacancies: This is incorrect. Cationic vacancies would lead to metal deficiency, not metal excess.

Given the above explanations, the correct statement regarding metal excess defect is 4. Both Options 1 and 2 are correct.