Question

The pink colour of lithium chloride crystal is due to:

• A. Impurity defect
• B. Metal excess defect
• C. Frenkel 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 $L{i}^{+}$ ions. Electrons released by the reacted lithium atoms occupy the vacant sites of chloride ions that had moved to the surface to react with $Li$.
The vacant sites of crystal lattice, which are occupied by electrons are called F-centres.
These free electrons get excited to higher energy levels and give rise to absorption spectra. So, the compounds are often coloured. In $LiCl$, F-centres imparts pink colour to it.
This is called metal excess defect.
Solids containing F-centres are paramagnetic, as the electrons occupying the vacant sites are unpaired.