F-Center

An F-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. Electrons in such a vacancy tend to 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. F-centers can be created say by passing sodium vapours over NaCl, when Cl^- ions combine with the metal ions producing non-stoichiometric defects within the lattice. The electrons released in this process diffuse to occupy the vacant places. Also ionizing radiation can produce F-centers.

References

[http://www.photonics.com/directory/dictionary/lookup.asp?url=lookup&entrynum=902&letter=c Photonics Dictionary]

W. Hayes, A.M. Stoneham "Defect and Defect Processes in Nonmetallic Solids" Wiley 1985

J. H. Schulman, W.D. Compton "Color Centers in Solids" Oxford, Pergamon 1962