Question

(i) What type of non-stoichiometric point defect is responsible for the pink colour of LiCI ?

(ii) What type of stoichiometric defect is shown by NaCI ?

Answer 1

(i) Metal excess defect caused due to anion vacancies is responsible for the pink colour of LiCl. When LiCl is heated in an atmosphere of Li vapour, the excess of Li atoms deposit on the surface of LiCl crystal $C{l}^{-}$ ions diffuse to surface to combine with Li atoms which ionize by losing electrons. The electrons diffuse back into the crystal and occupy the vacant sites created by $C{l}^{-}$ ions. The sites containing electrons trapped in anion vacancies are called F-centres.

(ii) Schottky defect is shown by NaCl crystal because in this compound the coordination number is high and there is small difference in the size of cations and anions.