Color Center

a crystal defect that absorbs light in a spectral region in which the crystal itself does not absorb light (seeSPECTROSCOPY, ).

Originally, the term “color centers” was applied only to F-centers (derived from the German word Farbenzentren, or “color centers”). F-centers were first discovered in alkali halide crystals by R. W. Pohl and co-workers in Germany in the 1930’s. According to a model that was proposed by the French scientist J. H. de Boer and that was experimentally confirmed and theoretically computed by S. I. Pekar in the USSR, an F-center is a negative-ion vacancy that has trapped an electron. The term “color centers” later came to include all point lattice defects that absorb light in a region where the crystal itself does not absorb. Such defects may constitute intrinsic centers or impurity centers. The former encompass positive-ion vacancies, negative-ion vacancies, and interstitial ions; the latter, impurity atoms and ions. Color centers are found in many inorganic crystals and in glasses. They are very common in naturally occurring minerals.

Intrinsic centers may be produced by photochemical coloration, that is, by exposing a crystal to ionizing radiation or to light corresponding to the region where the crystal itself absorbs light. Such color centers are said to be induced. In photochemical coloration, nonequilibrium charge carriers (conduction electrons and holes), which are generated upon exposure to radiation, are trapped by crystal defects and change their charge, resulting in the appearance of new bands in the absorption spectrum and a change in the color of the crystal. At least two types of color centers are usually distinguished, namely, those with trapped electrons and those with trapped holes. If the particles or photons that produce the coloration have sufficiently high energy, they may produce new defects (seeRADIATION-INDUCED DEFECTS IN CRYSTALS). The new defects also usually occur in pairs, for example, a vacancy and an interstitial ion.

Induced color centers may be destroyed by thermal bleaching —that is, by heating—or by bleaching with light—that is, by exposure to light corresponding to the spectral region in which the centers absorb light. Upon exposure to heat or light, a charge carrier—for example, an electron—is liberated from the defect that trapped it and recombines with a hole. This process may be accompanied by luminescence if the energy released is emitted as a photon. Defect pairs may also disappear upon exposure to heat; for example, an interstitial atom may occupy the corresponding vacancy. In this case, luminescence, as a rule, is not observed because all the energy released is converted into heat.

In another method of producing intrinsic centers, called additive coloration, the charge carriers that are necessary for the formation of color centers are introduced into the crystal rather than being generated in it. Hence, the process is called additive coloration, that is, coloration by the addition of something. Additive coloration is achieved by heating a crystal in a metal vapor, by introducing electrons into a heated crystal from a point cathode, or by means of electrolysis. When a crystal is heated in a metal vapor, the metal atoms diffuse into the crystal, occupy positive-ion vacancies, and form F-centers by donating electrons to negative-ion vacancies. In some cases, such as fluorite, intrinsic centers may be produced during crystallization. Color centers formed by additive coloration cannot be destroyed by heat or light alone. Thus, additively colored alkali halide crystals are bleached by heating in a halogen atmosphere, and colorless fluorite may be obtained by altering the crystallization conditions.

F-centers have been studied in the greatest detail in alkali halide crystals. However, they have also been observed in other crystals. An F-center in an alkali halide crystal gives rise to a selective bell-shaped absorption band called an F-band. The F-band usually lies in the visible region of the spectrum. For crystals with the same negative ions but different positive ions or with different negative ions but the same positive ions, the F-band is shifted to longer wavelengths as the atomic weight of the positive ions (in the former case) or of the negative ions (in the latter case) increases. For example, the F-band in NaCl has an absorption peak in the blue region (λ = 465 nm), and the color of the crystal is yellow-brown (the complementary color). The F-band in KCl has an absorption peak in the green region (λ = 563 nm), and the crystal appears violet.

Other, more complex intrinsic centers have also been observed in alkali halide crystals. Such color centers include F₂- (or M-) centers, F₃- (or R-) centers, and F₄- (or N-) centers, which are, respectively, two, three, and four bound F-centers (that is, two, three, or four negative-ion vacancies that have trapped two, three, or four electrons). F₂⁺-centers and F₃⁺-centers also exist; they are, respectively, ionized F₂-centers and ionized F₃-centers.

Color centers with trapped holes in alkali halide crystals are molecular ions of a halogen (for example, Cl) that have trapped a hole (that is, donated an electron) and that either occupy the position of two normal ions, forming a V_K-center, or occupy the position of a single normal ion. forming an H-center. The position occupied may be found in conjunction with one or two neighboring positiveion vacancies; in this case, a V_F-center or a V_t-center, respectively, is formed.

Impurity centers are foreign atoms or ions that are introduced into a crystal, glass, or some other host material. To produce impurity centers in crystals, the impurity is added to the melt or solution during crystallization or is diffused into a prepared crystal. As is the case with other point defects, impurity atoms and ions may trap electrons or holes, resulting in a change in the absorption band or the color of the crystal. In crystals and glasses that contain impurities, induced impurity centers are produced during photochemical coloration, owing to a change in the charge of the impurities. In most cases, the impurity ions that constitute induced color centers have a different valence than the ions of the host material. Thus, for example, in a Tl-doped KCl crystal, a Tl⁺ ion would be an impurity center, while Tl atoms and Tl²⁺ ions would be induced impurity centers; in a ruby (Cr-doped Al₂O₃), a Cr³⁺ ion would be an impurity center, while Cr²⁺ and Cr⁴⁺ ions would be induced impurity centers. All induced color centers may be destroyed by light or heat.

Color centers of a mixed type, namely, F_A-centers and Z-centers, have also been found in crystals that contain impurities. F_A-centers are F-centers situated adjacent to an impurity ion called an activator. Z-centers in alkali halides are F-centers associated with vacancies and divalent impurity ions, such as Ca or Sr. Complex impurity centers, which consist of two or more impurity particles of the same or different species, are also observed. For example, impurity centers in alkali halide crystals have been detected which are associated with the introduction of such ions as O^–, O₂^–, S₂^–, S₃^–, SO₂^–, PO₄^2–, and CO₃^2–. Upon exposure to light, heat, or an electric field, color centers may coagulate, forming colloid centers.

Since color centers are electron and hole trap centers, they may be luminescent centers (see). The most effective method for investigating color centers is electron paramagnetic resonance in conjunction with spectral studies; this method makes it possible to interpret the structure of color centers.

The coloring and bleaching of crystals and glasses are widely used in scientific experiments and in technology. For example, the processes are used in dosimetry, in computer technology (for information storage), and in devices that incorporate photo-chromic materials, such as glass that darkens when exposed to sunlight and clears in the dark. In archaeology and geology, studies of color centers produced by exposure to radiation from radioactive elements in the earth’s interior are used to determine the ages of clay artifacts and minerals (seeGEOCHRONOLOGY). The coloration of a number of precious and semiprecious stones—such as amethyst, citrine, diamond, and amazonite—is associated with color centers. Certain crystals and glasses that contain impurity centers are used as the active medium in lasers, including ruby lasers and Nb-doped glass lasers (seeQUANTUM ELECTRONICS and LASER).