Q.What are f-centers?

F-centres are electron-occupied anion vacancies that make an ionic crystal coloured.

Explanation

When an ionic crystal such as NaCl is heated in an atmosphere of the corresponding metal vapour (sodium vapour, in this case), the metal atoms deposit on the surface and diffuse into the crystal. Na atoms lose electrons to become Na+ ions, which occupy normal cation sites, and the released electrons diffuse into the crystal and occupy anion vacancies. These anion vacancies occupied by unpaired electrons are called F-centres (from German 'Farbenzentre', meaning colour centres).

Effect …