Question

what is f - centre ?​

Answer 1

An F-center, Farbe center or color center is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more unpaired electrons. Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material that is usually transparent becomes colored. 

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Answer 2

f centre us defined as the one constituent particle is present at the centre of each face