Color center: also

Quartz. When colorless, quart2 [14808-60-7] is also known as rock crystal. When irradiated, it becomes smoky from a color center associated with a ubiquitous Al impurity at about the 0.01% level. The name citrine [14832-92-9] is used when quart2 is colored by Fe, and irradiation of this can produce the purple-colored amethyst [14832-91-8] under certain circumstances (2). Although not signiftcandy lower priced than the natural materials, synthetic citrine and amethyst ate used in jewelry because of the abiUty to provide matched sets of stones from large, up to 7-kg, hydrothermaHy grown crystals. 

Color from Color Centers. This mechanism is best approached from band theory, although ligand field theory can also be used. Consider a vacancy, for example a missing CF ion in a KCl crystal produced by irradiation, designated an F-center. An electron can become trapped at the vacancy and this forms a trapped energy level system inside the band gap just as in Figure 18. The electron can produce color by being excited into an absorption band such as the E transition, which is 2.2 eV in KCl and leads to a violet color. In the alkaU haUdes E, = 0.257/where E is in and dis the... 

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. 

Optically active centers may also occur as a result of stractural defects. These defects are usually called colour centers, and they produce optical bands in the colorless perfect crystal. We will also discuss the main features of color centers in this chapter (Section 6.5). From the practical viewpoint, color centers are used to develop solid state lasers. Moreover, the interpretation of their optical bands is also interesting from a fundamental point of view, as these centers can be formed unintentionally during crystal growth and so may give rise to unexpected optical bands. 

Chapter 6 is devoted to discussing the main optical properties of transition metal ions (3d" outer electronic configuration), trivalent rare earth ions (4f 5s 5p outer electronic configuration), and color centers, based on the concepts introduced in Chapter 5. These are the usual centers in solid state lasers and in various phosphors. In addition, these centers are very interesting from a didactic viewpoint. We introduce the Tanabe-Sugano and Dieke diagrams and their application to the interpretation of the main spectral features of transition metal ion and trivalent rare earth ion spectra, respectively. Color centers are also introduced in this chapter, special attention being devoted to the spectra of the simplest F centers in alkali halides. 

In order to accurately determine the speed of the flash-across phenomenon, the experiment was repeated and recorded by streak camera with color film. Also thinner SPHF plates were used. In the streak camera trace, 8.5 (isec after each initial wave entered the NM, a hot spot appeared at the surface of each plate and flashed to the center of the chge each at the phenomenal speed of 35 mm/fisec. Cook (Ref 3) considers the flash-across phenomenon to be the heat pulse predicted by M.A. Cook, R. Keyes A.S. Filler (Ref 1)... 

Luminescence of Lattice Defects. Many defect centers are known in the case of the alkali-metal halides, which are derived from electrons in anion vacancies (F-centers, or color centers). Association of two or more F-centers gives new defect centers, which can each also take up an electron. These lattice defects act as luminescence centers, the emission spectra of which sometimes exhibit a large number of lines. 

However, the situation becomes already more complicated for ternary single crystals like lanthanum-aluminate (LaAlC>3, er = 23.4). The temperature dependence of the loss tangent depicted in Figure 5.3 exhibits a pronounced peak at about 70 K, which cannot be explained by phonon absorption. Typically, such peaks, which have also been observed at lower frequencies for quartz, can be explained by defect dipole relaxation. The most important relaxation processes with relevance for microwave absorption are local motion of ions on interstitial lattice positions giving rise to double well potentials with activation energies in the 50 to 100 meV range and color-center dipole relaxation with activation energies of about 5 meV. 

A color center (marked e ) in a sodium halide crystal. Note that the electronic position is an anionic site. Also, for simplicity, anions are not shown here. 

Structural colors may be caused by the diffraction or interference of light by tiny, regularly-spaced structures within a substance. Many insects and bird feathers display structural color. Structural defects in a material s crystal lattice can also affect its color. Excess or missing ions act as color centers and may affect the way the substance absorbs light. 

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