The appearance of colour in solid alkali metal halides is generally due to:
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 $$Cl^-$$ 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 $$Na^+$$ 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).