An F-center or Farbe center (from the original German Farbzentrum, where Farbe means color and zentrum means 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 in a crystal lattice tend to absorb light in the visible spectrum such that a material that is usually transparent becomes colored. F-centers are a type of color center.
This is used to identify many compounds, especially zinc oxide (yellow).
Color centers can occur naturally in compounds (particularly metallic oxides) 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.
The color centers most commonly studied are those that occur in alkali metal halides. Alkali metal halides are transparent, they do not show absorption from the far ultraviolet into the far infrared. Thus any changes in optical absorption can easily be detected and studies. The absorption band of F-centers in sodium chloride is located around blue light, giving a sodium chloride crystal with sufficient F-center defects a yellow tinge. In other alkali chlorides the location of the F-center absorption band ranges from violet to yellow light.
F-centers have been observed in other materials, though they are generally not the cause for coloration in those materials. One such example is a relatively long-lived F-center found in sapphire through luminescence, it had a duration of about 36 ms.
Before the discovery of point defects it was already known that some crystals can be discolored using various methods. In 1830 T.J. Pearsall discovered that fluorspar could be discolored using violet light. Thirty years later similar results were achieved by melting crystals together with a specific metal. In 1921 W. Röntgen extensively measured rock salts. One set of these tests measured a photoelectric conductivity 40,000 times larger, after the salt was radiated with x-rays. A similar result to x-rays was accomplished by coloring the crystals with metal vapors. The photoelectric effect mainly happened around specific wavelengths, which was later found to be noncolloidal in nature. The discolorations were later named F-centers, as in Farbe the German word for color. In 1933 Mollwo concluded that these F-centers are atomic crystal defects. Around this time people started to assert these defects were unpaired electrons. The vacancy model was first described in 1937 but still was considered tentative. However it took until 1957 to prove that this was true using electron spin resonance.
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 sodium 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 (i.e., anions) normally reside. Ionizing radiation can also produce F-centers.
An H-center (a halogen interstitial) 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.
There are different types of electron centers, depending on the material and radiation energy. An F center is usually a position in a lattice where an anion, a negatively charged ion, is replaced by an electron.
Sometimes the F center might acquire an addition electron, making the F center negatively charged, such that it is called an F- center. Similarly, when the F center misses an electron, it will be an F+ center. It is also possible to have a -2e charged anion, needing 2 electrons to form an F center. Adding or taking away an electron will make it an F- or F+ center respectively according to the convention.
Another type of a single vacancy F center is the FA center which consists of an F center with one neighbouring positive ion replaced by a positive ion of a different kind. These FA centers are divided into two groups, FA(I) and FA(II) depending on the type of replacement ion. FA(I) centers have similar properties as regular F centers, whereas FA(II) centers cause two potential wells to form in the excited state due to the repositioning of a halide ion.
Combinations of neighbouring F centers due to neighbouring anion vacancies will be called, for two and three neighbours respectively, F2 and F3 centers. Larger aggregates of F centers is certainly possible, but the details of its behaviour are yet unknown.
F-centers can appear anywhere in the crystal but as they are often formed by radiating a crystal many of the defects form near the surface. As Fs-centers are often created with radiation they tend to form near the surface. Electrons bound in Fs have smaller transition energies compared to bulk F-centers. They tend to protrude from the surface compared to regular lattice points as well.
There are few examples of naturally occurring F-centers. One possible candidate is the mineral Blue John. This is a form of fluorite, CaF2. Although it has not been confirmed, it is believed that the colour is caused by electron F centers. It is thought that this F center is formed due to nearby uranium deposits in the rock; the radioactive decay radiation caused the energy necessary to form the F center. The first F-centers created were in alkali halide crystals. These halides were exposed to high-energy radiation, such as X-rays, gamma radiation or a tesla coil.
An. F- center can be created by irradiating a material with F centers. Some electrons in F centers will become ionized and move through the material until they will be absorbed by a F center, becoming an F- center with two electrons. The formation of an F2 center is very similar. An F center is ionized and becomes a vacancy; the electron moves through the material to bind to another F center, which becomes an F- center. The electron vacancy moves through the material and ends up next to the F- center, which gives its electron back to the vacancy, forming two neighbouring F centers, i.e. an F2 center.