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 o f 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.