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An F-center or color 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 defect 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. The greater the number of F centers, the more intense the color of the compound. F centers are a type of color center.
This is used to identify many compounds, especially zinc oxide (yellow).
The discolorations were later named F centers, as in Farbe, the German word for color. These defects were extensively studied by Robert Wichard Pohl and his institute at the University of Göttingen since 1920. (1992). 019505329X, Oxford University Press. 019505329X One of his assistants, concluded in 1933 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 by Pohl in 1937 but still was considered tentative. It was formalized theoretically by Nevill Mott and Ronald Wilfred Gurney in 1940. It took until 1957 to prove find conclusive experimental evidence using electron spin resonance.
Often F centers are paramagnetic and can be studied by electron paramagnetic resonance techniques. The F centers most commonly studied are those that occur in alkali metal halides. Alkali metal halides are normally 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 studied. 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. 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.
Though F centers have been observed in other materials, they are generally not the cause for coloration in those materials. There are few examples of naturally occurring F centers causing colorations. 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. (2025). 9780470077948, John Wiley & Sons Inc. ISBN 9780470077948
Another example of an F center found in nature is a relatively long-lived F center found in sapphire through luminescence, which had a duration of about 36 ms in one study.
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. Similar to the FA is the FB center, which consists of an F center with two neighbouring positive ions replaced by a positive ion of a different kind. The FB centers are also divided into two groups, FB(I) and FB(II), with similar behaviour to the FA(I) and FA(II) centers. Due to the statistical nature of the distribution of impurity ions, FB centers are much more rare than FA centers.
An F2 center can also be ionised, and form an F2+ center. When this type is found next to a cation impurity, this is an (F2+)A center.
There are three mechanisms of energy absorption by radiation:
a) Exciton formation. This amounts to an excitation of a valence electron in a halide ion. The energy gained (typically 7 or 8 eV) will partly be lost again through the emission of a luminescent photon. The rest of the energy is available for displacing ions. This energy radiates through the lattice as heat. However, it turns out that his energy is too low to move ions and therefore not capable of generating F centers.
b) Single ionization. This corresponds to separating an electron from a halide ion; the energy required is about 2 eV more than exciton formation. One can imagine that the halide ion which lost an electron, is not properly bound on its lattice site any more. It is possible that it will move through the lattice. The created vacancy can now trap the electron, creating the F center. If the halide ion recaptures the electron first, it can release more thermal energy than by exciton formation (2 eV more) and it could cause other ions to move also. c) Multiple ionization. This process requires the most energy. A photon interacts with a halide ion, ionizing it twice, leaving it positively charged. The ion remaining is very unstable and will quickly move to another position, leaving a vacancy which can trap an electron to become an F center. To free two electrons, about 18 eV is required (in the case of KCl or NaCl). Research suggests about one double ionization occurs in ten single ionizations. However, the created positive halide ion will easily and quickly adopt an electron; making it unable to create the F center.
The most likely mechanism of F center creation is not yet determined. Both are possible and likely, but which once occurs the most is unknown.
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.
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.
In oxides it is possible to additively color a crystal with a different metal than the cation. The resulting absorption spectra are substantially the same as if the component metal was used.
Only certain F centers are suitable for application in color center lasers, known as laser-active F centers. Simple F centers are not laser-active, but more complex F centers have been shown to form stable color center lasers. These are namely FA(II), FB(II), F2+ and (F2+)A centers. Other even more complex F centers are potentially laser-active, but they do not play a significant role in color center lasers physics. Examples of a material with FA centers used in color center lasers are crystals of potassium chloride (KCl) or rubidium chloride (RbCl) doped with lithium chloride (LiCl), containing FLi-centers. These crystals have been found to be good materials for color center lasers with emission lines of wavelengths between 2.45 and 3.45 μm.
F centers usually have an absorption band in visible range, and the emission is to longer wavelengths. The differences in peaks is often larger than a factor of 2 and the resulting emissions are near infrared. However at lower temperatures the shift becomes smaller, though there are some crystals that are able to emit visible light. One such example is powdered MgO, treated with additive coloring, emitting violet-blue light by absorbing violet light in vacuum. (1972). 9781468429725, Plenum Press. ISBN 9781468429725
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