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.
Na0 → Na+ + e−
Na+ is incorporated at NaCl crystal.
Cl− vacancies are generated, because of the excess of Na+.
These vacancies capture available e-, neutralizing and forming F-centers; that is, the electrons released in this process diffuse to occupy the vacant places. Also, ionizing radiation can produce F-centers.
An H-center (a halogen interstitial) is in a sense the opposite, and hence a F-center and a H-center can combine and clear the lattice of a defect. This process can be photoinduced, e.g. by a laser.
- 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
- Berzina, B. (1998). "Formation of self-trapped excitons through stimulated recombination of radiation-induced primary defects in alkali halides". Journal of Luminescence. 76-77: 389. Bibcode:1998JLum...76..389B. doi:10.1016/S0022-2313(97)00222-6.
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