Question

On heating lithium chloride crystal, pink colour appears due to:

• A. Frenkel defect
• B. Metal excess defect
• C. Impurity defect
• D. Metal deficiency defect

Answer 1

The correct option is B Metal excess defect

When a $LiCl$ crystal is heated in an atmosphere of lithium vapour, $C{l}^{-}$ ions diffuse to the surface of the crystal and interact with the lithium atom to form $L{i}^{+}\text{and}C{l}^{-}$ ions. Electrons released by the reacted lithium atoms occupies the vacant sites of chloride ions that had moves to the surface to react with $Li$.
The vacant site of crystal lattice, which are occupied by electrons are called F-centres.
These free electrons gets excited to higher energy levels, give rise to absorption spectra. So, the compounds are often coloured. In $KCl$, F-centres imparts violet colour radiation. This is called metal excess defect.