What type of defect causes F-centers in a crystal?

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ESE Electrical 2017 Official Paper

Answer 1

A point in a crystalline compound at which a negative ion missing from the crystal lattice has been replaced by an electron is called F-centres defect.
Metal excess defect due to anionic vacancies causes F-centres in a crystal.
This type of defect is observed in those crystals which are likely to form schottky defects.
Alkali metal halides like NaCl and KCl show this type of defect.
F-centre has been identified by electron spin resonance as an electron bound at a negative iron vacancy.