A lattice defect in a crystalline solid consisting of a
vacant negative ion site and an electron bound to the site. Such defects will
absorb light and make certain normally transparent crystals appear colored.
Also called F-Center after the German word for color (Farbe).
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 electrons get excited and get
displaced from their respective positions in their crystal structure but while
doing so these electrons 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.
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