Hole color centers

Color Centers. Characteristics of a color center (1,3,7) include production by irradiation and destmction by heating. Exposure to light or even merely time in the dark may be sufficient to destroy these centers. Color arises from light absorption either from an electron missing from a normally occupied position, ie, a hole color center, or from an extra electron, ie, an electron color center. If the electron is a valence electron of a transition element, the term color center is not usually used. 

The electron can be trapped, for example by an interstitial which is converted to an H atom. The AlO is the hole color center which absorbs light and gives the color to smoky quart2. Bleaching is the result of thermal energy releasing the trapped electron, which then produces the reverse of reaction 1. The amethyst color center in quart2 is exactly like the smoky, except that Fe " replaces. ... 

The Fe(III)0 4 hole color center gives the purple color. On being heated, the trapped electron is released and the reverse of reaction 2 occurs producing Fe(III)0 4, which provides the pale yellow color of citrine. 

An example of a hole color center is smoky quart2 [14808-60-7]. Here itradiation (either produced by nature or in the laboratory) of Si02 containing a trace of A1 ejects an electron from an oxygen adjacent to the A1 or, in customary nomenclature [AlO ] — [AlO ]" + e the ejected electrons are... 

Defects produced by X-ray or neutron bombardment. Irradiation of colorless quartz crystals with X-rays will result in the formation of a dark brownish gray to black color of smoky quartz. Coloration is due to the formation of a color center, a hole color center this time. If the smoky quartz is heated to several hundred °C it will become clear again. 

Any material which can form a color center contains two types of precursors as shown in Figure 2a. The hole center precursor is an atom, ion, molecule, impurity, or other defect which contains two paired electrons, one of which can be ejected by irradiation, leaving behind a hole center (Fig. 2b). The electron center precursor is an atom, ion, etc, which can produce an electron center by trapping the electron ejected from the hole center precursor. A hole and an electron center are thus formed simultaneously. Either or both can be the color center. Almost all materials have hole center precursors. If there is no electron center precursor, however, the displaced electron returns to its original place and the material remains unchanged. 

Fig. 2. (a) Irradiation of a material containing A, a hole precursor having an electron pair, and B, an electron precursor, to form (b) a color center having... 

In a more general application, thermoluminescence is used to study mechanisms of defect annealing in crystals. Electron holes and traps, crystal defects, and color-centers are generated in crystals by isotope or X-ray irradiation at low temperatures. Thermoluminescent emission during the warmup can be interpreted in terms of the microenvironments around the various radiation induced defects and the dynamics of the annealing process (117-118). ... 

Vo) in the crystal. (Vo) can catch electrons to form F and centers. (Pb) is also able to attract electrons while (Vb)" can trap holes to give rise to color centers. They vdll make a contribution to the X-ray irradiation-induced absorption. Of course, the charge balance of the crystal is kept by charge compensation among these defects. Regretfully, the detailed characterization of these defects is too difficult to cover here and further experiments need to be performed. 

The alkali halides cire noted for their propensity to form color-centers. It has been found that the peak of the band changes as the size of the cation in the alkali halides increases. There appears to be an inverse relation between the size of the cation (actually, the polarizability of the cation) and the peak energy of the absorption band. These are the two types of electronic defects that are found in ciystcds, namely positive "holes" and negative "electrons", and their presence in the structure is related to the fact that the lattice tends to become charge-compensated, depending upon the type of defect present. 

Since the original studies of F centers many other color centers have been characterized that may be associated with either trapped electrons or trapped holes. These are called electron excess centers when electrons are trapped and hole excess centers when holes are trapped. 

The color center is the [A104]4 group, which can be thought of as [A104]5 together with a trapped hole. The color arises when the trapped hole absorbs radiation. 

So far, we have dealt with optically active centers based on dopant ions, which are generally introduced during crystal growth. Other typical optically active centers are associated with inhinsic lattice defects. These defects may be electrons or holes associated with vacancies or interstitials in ionic crystals, such as the alkali halide matrices. These centers are nsually called color centers, as they prodnce coloration in the perfect colorless crystals. 

Salts of the bases MOH are crystalline, ionic solids, colorless except where the anion is colored. For the alkali metal ions the energies required to excite electrons to the lowest available empty orbitals could be supplied only by quanta far out in the vacuum ultraviolet (the transition 5p6 —5p56s in Cs+ occurs at 1000 A). However, colored crystals of compounds such as NaCl are sometimes encountered. Color arises from the presence in the lattice of holes and free electrons, called color centers, and such chromophoric disturbances can be produced by irradiation of the crystals with X rays and nuclear radiation. The color results from transitions of the electrons between energy levels in the holes in which they are trapped. These electrons behave in principle similarly to those in solvent cages in the liquid ammonia solutions, but the energy levels are differently spaced and consequently the colors are different and variable. Small excesses of metal atoms produce similar effects, since these atoms form M+ ions and electrons that occupy holes where anions would be in a perfect crystal. 

When electrons or holes are trapped at defects in a solid, both the electronic and the optical properties are modified. Such composite defects are called color centers. Large numbers of these defects have been characterized, and the term includes defects that have absorption maxima outside the visible spectrum. The expression color center is thus used... 

One of the commonest forms of hole-excess center imparts color to the minerals smoky quartz and amethyst. These minerals are forms of silica, containing aluminium as an impurity. The AP+ substitutes for 81 +, and to preserve charge neutrality equal amounts of H+ are incorporated into the crystal. The smoky purple color arises in the electron deficient [A104] group. It is formed when an electron is liberated from a neutral [A104] group by ionising radiation is trapped on one of the H+ ions present. Other hole centers have been described in a variety of crystals. 

Radiation sources including X-rays, y-rays, and ultraviolet produce preferential light absorption at color centers in glass by formation of free electrons and holes. These are trapped at a defect such a vacancy, an interstitial atom, a multivalent impurity, or a nonbridging oxygen T The types of color center induced in siliea, soda-lime-silica, borate, and phosphate glasses are elucidated by optical and electron spin resonance studies of irradiated samples. Table 1 summarizes composition variables and reaction types that induce damage. 

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