F-орталықтары: кристалдық ақаулар және түс пайда болуы

Type of crystallographic defect

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. 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).

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 non colloidal 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. It took until 1957 to prove that this was true using electron spin resonance.

F 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. 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. 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.

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. 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.

Sometimes the F center might acquire an additional electron, making the F center negatively charged, such that it is called an F− center. Similarly, when the F center misses an electron, when it is ionised, 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. 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.

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. 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. They tend to protrude from the surface compared to regular lattice points as well. With F centers being less bound than electrons at regular lattice sites, they work as a catalyst for adsorption. However this means that these defects quickly deteriorate in open air by absorbing oxygen, but are reversible by removing the oxygen from the environment. The ESR spectrum of Fs center is temperature dependent in the hyperfine structure in oxides. This must arise from an increasing overlap of the unpaired electron wave function at the Nucleus of the positive ion. Fs center can be changed or destroyed by heating. The defects in alkali halide crystals are destroyed at low temperatures. crystals start to slowly discolour at 200 K. For oxides temperatures to destroy these defects is substantially higher, 570 K for CaO. In oxides it is possible to create complex Fs centers by annealing.