what is F centre?
An F-Center or Farbe 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..
e.g.:NaCl heated in a metallic Na atmosphere.
Na 0 → Na + + e −
Na + is incorporated at NaCl crystal.
Cl − vacancies are generated, because of the excess of Na + ...
When Potassium chloride is heated in sodium vapour, we see that is acquires a yellow colour. The heating in excess sodium creates new lattice sites for sodium. To maintain the structure, an equal number of chloride sites are also created, but the latter are vacant. The electron of the potassium atom that is ionized in the KCl matrix is trapped by the chloride ion vacancy. This trapped electron can be freed into the crystal by absorbing visible light and hence the colour. A trapped electron at an anion vacancy is an electronic defect and it is known as F centre (from German Farbe meaning colour).
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anionic sites occupied by unpaired electrons in a crystal with metal excessb defect is called fcentres
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.
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