Question

Given below are two statements :
Statement I : Frenkel defects are vacancy as well as interstitial defects.
Statement II : Frenkel defect leads to colour in ionic solids due to presence of F-centres.
Choose the most appropriate answer for the statements from the options given below :

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Remember that Frenkel defects are "dislocation" defects where stoichiometry is maintained and density remains constant, whereas color due to F-centres is a feature of metal excess defects.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The Frenkel defect is a type of point defect in crystalline solids where an atom or ion (typically a smaller cation) leaves its original lattice site, creating a vacancy, and moves to an interstitial position.
Step 3: Detailed Explanation:
1. Analysis of Statement I: Since the Frenkel defect involves the migration of an ion from its lattice site to an interstitial site, it simultaneously creates a vacancy at the original site and an interstitial defect at the new site. Thus, it is considered both a vacancy and an interstitial defect. Statement I is true.
2. Analysis of Statement II: Colour in ionic solids is often caused by F-centres (Farbe centres), which are anionic vacancies occupied by one or more unpaired electrons. These are typical of non-stoichiometric metal excess defects or Schottky defects in some cases, but not Frenkel defects. Frenkel defects do not change the density or create such electron-trapped anionic vacancies. Statement II is false.
Step 4: Final Answer:
Statement I is true but Statement II is false.