Question

Explain with a diagram the formation of F-centers

Answer 1

Metal excess defect by anion vacancies (Colour or F-centres):

This type of defect imparts colour to the colourless crystal.

An F-centre in a crystal

e.g.: When NaCl crystal is heated in the atmosphere of sodium vapour, sodium atoms are deposited on the crystal surface.
Cl^– ions diffuse to the crystal surface creating vacancies at their regular sites.
These Cl^– ions combine with Na atoms on the surface to form NaCl, by releasing electron from sodium atom.
Na + Cl^– → NaCl + e^–

The electrons released diffuse into the crystal and occupy vacant sites of anions. The anion vacant sites occupied by electrons are F-centres or colour centres. NaCl shows yellow colour due to the formation of F-centre. The crystal of NaCl has excess Na. The nonstoichiometric formula of NaCl is the Na_1+xCl_1.0.