Answer:
The anionic sites occupied by unpaired electrons are called F-centres. Such defects also impart colour to the crystals. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.
Metal Excess Defects
A metal excess defect is one of the defects seen in the crystal structures. The other defect is metal deficiency defect. These are the non-stoichiometric inorganic solids that contain constituent elements in non-stoichiometric ration because of the defects in their crystal structures.
- Anionic vacancies and the presence of extra cations in the interstitial sites induce metal excess defects.
- When alkali metal halides are heated in an atmosphere of vapour of the alkali metal, anion vacancies are created.
- This anion then diffuses to the crystal’s surface, where it joins newly formed metal cations.
Examples
F-centres give different colours like
- NaCl gives a yellow colour
- KCl gives a violet colour
- HCl gives pink colour
