Question

The pink colour of lithium chloride crystal is due to:

• A. frenkel defect
• B. metal excess defect
• C. metal deficiency defect
• D. impurity defect

Answer 1

The correct option is B metal excess defect

LiCl has non-stochiometric metal excess defect due to anion vacancies. The negative ions $C{l}^{-}$ are missing from their lattices sites leaving the holes in which electrons are entrapped so that electrical neutrally is maintained.
When $LiCl$ is heated, Li atoms gets deposited on the surface of the crystal. The $C{l}^{-}$ ions diffuse into the surface and combine with Li atoms to give $LiCl.$ This is so because of loss of electrons by Li atoms to form $L{i}^{+}$. The released electons diffuse excess into crystal and occupy anionic sites. As a result, there is an excess of Li. The anionic sites occupied by unpaired electrons are F-centres which imparts a pink color to $LiCl$ crystals. The color is observed as a result of excitation of these electrons when they absorb energy from visible light falling on crystals.