Question

The appearance of colour in solid alkali metal halides is generally due to :

• A. F-centres
• B. Schottky defect
• C. Frenkel defect
• D. interstitial positions

Answer 1

The Correct Option is A

The appearance of color in solid alkali metal halides is generally due to F-centres.

Let's delve into why this is the case:

1. Explanation of F-centres: The term "F-centre" (where "F" stands for Farbe, meaning color in German) refers to an anionic vacancy in an ionic crystal that has trapped one or more electrons to maintain electrical neutrality. These trapped electrons absorb light in the visible spectrum, resulting in the appearance of color in the crystal.
2. Formation of F-centres: In alkali metal halides, when an alkali metal halide crystal is exposed to radiation or heat, the halide ions (such as Cl^- in NaCl) can be displaced, creating a vacancy. An electron can occupy this vacancy, creating an F-centre.
3. Color Appearance: The presence of these trapped electrons allows the crystal to absorb certain wavelengths of light, giving the material its characteristic color. For example, pure NaCl is colorless, but with F-centres, it may appear yellow.
4. Ruling Out Other Options:
   • Schottky defect: This involves a pair of vacancies (both anions and cations are missing), which does not contribute to color changes directly by trapping electrons.
   • Frenkel defect: Involves atoms or ions displaced to interstitial sites in the lattice, and does not create color due to lack of electron trapping.
   • Interstitial positions: Refers to extra atoms or ions occupying spaces between normal lattice positions, not typically associated with color changes.

Thus, the correct answer is F-centres, as they are specifically related to color changes in solid alkali metal halides by trapping electrons and affecting visible light absorption.