Question

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

• A. f-centres
• B. Frenkel defect
• C. Schottky defect
• D. interstitial position

Answer 1

The correct option is A f-centres

Alkali halides like $NaCl$ and $KCl$ show metal excess defect. When crystals of $NaCl$ are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The $C{l}^{-}$ ions diffuse to the surface of the crystal and combine with $Na$ atoms to give $NaCl$. This happens by the loss of an electron by sodium atoms to form $N{a}^{+}$ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result, the crystal now has an excess of sodium.

The anionic sites occupied by unpaired electrons are called F-centres. They impart a yellow colour to the crystals of $NaCl$. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.

Similarly, excess of lithium makes $LiCl$ crystals pink and excess of potassium makes $KCl$ crystals violet (or lilac).