F-Center

An F-Center or Farbe (German for color) 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 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.

References

• 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. doi:10.1016/S0022-2313(97)00222-6.