Ionic solids, which have anionic vacancies due to metal excess defect, developed colour. Explain with the help of a suitable example.

Explanation

Let us take and example of NaCl crystal is heatd in presence of Na vapour, some `Cl^(-)` ions leave their lattice sites to combine with Na to form NaCl. The `e^(-1)` lost by Na to form `Na^(+)(Na^(-)+Cl^(-)-gtNaCl)` then diffuse into the crystal to occupy the anion vacancies. these sites are called F-centres. these `e^(-)` s absorb energy from visible light, get excited to higher energy level and when they fall back to ground state, they impart yellow colour to NaCl crystal.