F-center


An F-center, Farbe center or color center is a type of crystallographic defect in which an anionic vacancy in a crystal lattice is occupied by one or more unpaired electrons. Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material that is usually transparent becomes colored. This is used to identify many compounds, especially zinc oxide.

Occurrences

Color centers can occur naturally in compounds because when heated to high temperature the ions become excited and are displaced from their normal crystallographic positions, leaving behind some electrons in the vacated spaces. This effect is also exhibited by ionic compounds containing metal-excess defects.

Properties

F-centers are often paramagnetic and can be studied by electron paramagnetic resonance techniques. The greater the number of F-centers, the more intense the color of the compound. One way of producing F-centers in a crystal artificially is to heat it in an atmosphere of the metal of which it is constituted, e.g., heating NaCl in a metallic Na atmosphere.
Na0 → Na+ + e−

Na+ is incorporated into the NaCl crystal after giving up an electron.

A Cl− vacancy is generated to balance the excess Na+. The effective positive charge of the Cl− vacancy traps the electron released by the Na atom.
This trapping of the electrons by anion vacancies results in the formation of F-centers; that is, the electrons released in this process diffuse to the vacant sites where negatively charged ions normally reside. Ionizing radiation can also produce F-centers.
An H-center is in a sense the opposite to an F-center, so that when the two come into contact in a crystal they combine and cancel out both defects. This process can be photoinduced, e.g., using a laser.
The formation of F-centers is the reason that some crystals like lithium chloride, potassium chloride, and zinc oxide become pink, lilac and yellow, respectively, when heated.