Question

Identify the correct statement about the crystal defects in solids:

• A. Frenkel defect is observed when the difference between sizes of cation and anion is very small
• B. Frenkel defect is not a dislocation effect
• C. Schottky defects have no effect on the physical properties of solids
• D. Trapping of electrons in lattice leads to the formation of F-centers

Hint:

Frenkel and Schottky are point defects in ionic solids; Frenkel is sometimes called a “dislocation defect” but not in the sense of a line dislocation.

- F-centers are responsible for the color in many alkali halides (e.g., NaCl turns yellow when F-centers form).

Answer 1

The Correct Option is D

Step 1: Frenkel Defect - Occurs in ionic solids with a large difference in the size of ions (e.g., small cation). - It is also called a  “dislocation defect” since the cation leaves its lattice site and occupies an interstitial site. 

Step 2: Schottky Defect - Involves equal numbers of cation and anion vacancies. - Reduces the overall density of the crystal; thus it does affect physical properties.

 Step 3: F-centers - Formed when anionic vacancies (missing negative ions) trap electrons. - These electrons absorb certain wavelengths of light, often imparting color to the crystal. Hence, among the given statements, only (4) is correct.