JEE Main 2021 — Solid State Question with Solution

To answer this question, let's analyze each statement separately.

Statement I: Frenkel defects are vacancy as well as interstitial defects.

This statement is true. A Frenkel defect, also known as a dislocation defect, occurs in a crystalline solid when an atom or ion leaves its normal site and moves to an interstitial site, creating a vacancy defect at its original position and an interstitial defect at the new position. However, it's crucial to note that the vacancy and interstitial are created by the same atom or ion moving from one site to another within the crystal. So, the Frenkel defect involves both a vacancy and an interstitial defect, but they are correlated because they involve the displacement of the same atom or ion.

Statement II: Frenkel defect leads to color in ionic solids due to presence of F-centres.

This statement is false. The color in ionic solids is usually caused by F-centers (color centers), which are defects formed when an anion vacancy is occupied by one or more electrons. These electrons can absorb visible light, giving the crystal a characteristic color. However, the Frenkel defect, involving an atom or ion moving from its lattice site to an interstitial site, does not directly lead to the creation of F-centers. Instead, the coloration associated with F-centers is most commonly linked with Schottky defects (which involve paired vacancies of cations and anions but do not include displaced atoms to interstitial sites) or directly with anion vacancies that are not necessarily related to Frenkel defects.

Conclusion: Given the explanations above, the correct answer is Option C: Statement I is true but Statement II is false.