Explain f-centre

A compound may have excess metal ion. When negative ion is absent from its lattice side  and compound have excess metal ion and leaving a hole which occupied by electron to maintain electrical neutrality , the ionic sites are called F center .Colour in these crystals are impart by unpaired electrons.
These types of defects are found in crystals which have electric neutrality and Schoottky defects .The hole occupied by electron is called F–center. This F center is responsible for most of the interstitial properties of compound  .
For example We observe this effect when Crystals of NaCl  heated in an atmosphere of sodium vapour.

An F-Center or Farbe (German for colour) center is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more electrons, depending on the charge of the missing ion in the crystal or mainly due to the reason that when a compound is heated to a high temperature (mainly metallic oxides) the ions get excited and get displaced from their respective positions in their crystal structure but while doing so these ions leave behind some electrons in the vacated spaces. This leads to the phenomenon of colouring of compounds. This is used to identify many compounds, especially zinc oxide (yellow). Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material that is usually transparent becomes colored. Thus the origin of the name, F-center, which originates from the German Farbzentrum. The translation of this term also provides the synonym color center, which can also refer to such defects. F-centers are often paramagnetic and can then be studied by electron paramagnetic resonance techniques. The greater the number of F-centers, the more intense is the color of the compound. A way of producing F centers is to heat a crystal in the presence of an atmosphere of the metal that constitutes the material, e.g.: NaCl heated in a metallic Na atmosphere.