Question

What type of non-stoichiometric point defect is responsible for the pink colour of LiCl ?

Answer 1

If as a result of the imperfections in the crystal, the ratio of the cations to the anions becomes different from that indicated by the ideal chemical formula, the defects are called non-stoichiometric defects

LiCl exhibit non-stoichiometric defect due to metal excess by anionic vacancies. A negative ion may be missing from its lattice site, leaving a hole which is occupied by an electron, thereby maintaining the electrical balance. The sites containing the electrons thus trapped in the anion vacancies are called F-centres because they are responsible for imparting colour to the crystals.

The Li atoms get deposited on the surface of LiCl crystal. Cl- ions then diffuse to the surface where they combine with the Li atoms which become ionize by losing electrons. These electrons diffuse back into the crystal and occupy the vacant sites created by the Cl- ions. These electrons absorb some energy of the white light, giving pink colour to LiCl