COLOUR CENTRES / F-CENTRES :
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 forcolor
(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 colorcenter, 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.
