F-center

thumb|220px|F-center in an NaCl crystal

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 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).

History

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 Wilhelm 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. These defects were extensively studied by Robert Wichard Pohl and his institute at the University of Göttingen since 1920. One of his assistants, concluded in 1933 that these F centers are atomic crystal defects. 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 in the blue part of the visible spectrum, giving a sodium chloride crystal with sufficient F center defects a yellow tinge. In other alkali chlorides the wavelength 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 color is caused by electron F centers. It is thought that this F center is formed due to nearby uranium deposits in the rock: the radiation from radioactive decay is energetic enough 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.

Types

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.

thumb|Simple F center. Positive ions are shown as + and negative halide ions as -. The electron e is in the anion vacancy.

Single vacancy F center

Sometimes the F center might acquire an additional electron, making the F center negatively charged, such that it is called an F− center. Similarly, an F center missing an electron is 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 neighboring 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 neighboring positive ions replaced by a positive ion of a different kind. FB centers are also divided into two groups, FB(I) and FB(II), with similar behavior 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.

thumb|Configuration of F2 center. The electrons are in diagonally neighboring lattice sites.

Complex F center

Combinations of neighboring F centers due to neighboring anion vacancies will be called, for two and three neighbors respectively, F2 and F3 centers. Larger aggregates of F centers is certainly possible, but the details of its behavior are yet unknown.

An F2 center can also be ionized, 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 more weakly 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 wavefunction at the Nucleus of the positive ion.

Fs centers can be changed or destroyed by heating. The defects in alkali halide crystals are destroyed at low temperatures. Crystals start to slowly discolor at 200 K. Higher temperatures are required to destroy Fs centers in oxides (570 K for CaO). In oxides it is possible to create complex Fs centers by annealing.

Fabrication

Irradiation

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.

There are three mechanisms of energy absorption by radiation:

This is done by heating the crystal to a high temperature in the vapour of the corresponding metal. The temperature is bounded by its melting point and the temperature at which colloids form, e.g. for KCl between ~400 and 768°C. Metal atoms are captured on the surface of the crystal, where they are ionized, and the valence electron is shunted to the crystal lattice. Since this process happens at high temperatures, the mobility of ions is also high. A negative ion will move towards the newly formed ion. This leaves behind an anionic vacancy which can trap the electron to form an F center. Afterwards the crystal is quenched to prevent the F centers moving through the crystal to form colloids.

An example of this process is 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.

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.