Lattice color centers

A second kind of electronic defect involves the electron. Let us suppose that the second plane of the cubic lattice has a vacancy instead of a substitutional impurity of differing valency. This makes it possible for the lattice to capture and localize an extraneous electron at the vacancy site. This is shown in the following diagram. The captured electron then endows the solid structure with special optical properties since it ean absorb photon energy. The strueture thus becomes optically active. That is, it absorbs light within a well-defined band and is called a "color-center" since it imparts a specific color to the crystal. 

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. 

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. 

Figure 6.12 The structures of some typical color centers in alkali halide crystals (such as NaCl). The defects are represented on a plane of the alkali halide crystal. The circles represent the lattice ions and a is the anion-cation distance.

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. 

With the introduction of the lattice structure and electroneutrality condition, one has to define two elementary SE units which do not refer to chemical species. These elementary units are l) the empty lattice site (vacancy) and 2) the elementary electrical charge. Both are definite (statistical) entities of their own in the lattice reference system and have to be taken into account in constructing the partition function of the crystal. Structure elements do not exist outside the crystal and thus do not have real chemical potentials. For example, vacancies do not possess a vapor pressure. Nevertheless, vacancies and other SE s of a crystal can, in principle, be seen , for example, as color centers through spectroscopic observations or otherwise. The electrical charges can be detected by electrical conductivity. 

COLOR CENTERS. Certain crystals, such as the alkali halides, can be colored by the introduction of excess alkali metal into the lattice, or by irradiation with x-rays, energetic electrons, etc. Thus sodium chloride acquires a yellow color and potassium chloride a blue-violet color. The absorption spectra of such crystals have definite absorption bands throughout the ultraviolet, visible and near-infrared regions. The term color center is applied to special electronic configurations in the solid. The simplest and best understood of these color centers is the F center. Color centers are basically lattice defects that absorb light. 

Electron donors and acceptors for reversible redox systems must invariably exhibit at least two stable oxidation states, or the net result will be an irreversible chemical reaction. The donor or acceptor components of the redox system need not be confined to independent atoms, ions, or molecules but could even be imperfections in crystal lattices capable of functioning as electron traps. The well-known color centers in alkali halides are just such acceptor systems. 

Color Centers. Lattice defects in alkali halide crystals provide ideal trapping sites for electrons which in turn cause marked color changes in the system. Symons and Doyle (112) have reviewed the research on color centers in alkali halide crystals to about 1960. In... 

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. 

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. 

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. 

The formation of color centers in the alkali halides, especially silver, has been studied extensively and in great detail, in an attempt to understand the photographic process. At least half a dozen color centers have been identified in these materials, of which the most widely studied is probably the F center, defined as an electron trapped at an anion vacancy. The name comes from the German word for color Farbe. In the case of KBr, the F center (Fig. 16.7) is believed to be an electron trapped at a bromine vacancy. The F center can be modeled by assuming the electron is trapped in a box of side d, which scales with the lattice parameter of the alkali halide. The F center transition is believed to be between the ground and first excited state of this particle in a box. This model, while crude, qualitatively explains the data for some of the alkali halide F center spectra. 

Equation (53) describes Debye relaxation. Magnesium and calcium-doped lithium fluorides have a characteristic Debye relaxation diagram from vhich the dopant concentration and the relaxation time can be deduced. Many others crystals containing mobile lattice defects have similar Debye s relaxation processes. Major understanding of the structure of color centers results from dielectric relaxation spectra. Nuclear magnetic resonance, optical and Raman spectroscopy can be used efficiently in conjunction vith dielectric spectroscopy. 

Centers due to impurities. Another type of color centers are impurity ions, the bands of which are caused by electronic transitions to neighboring ions of the host lattice (U-centers) or of electron defects trapped by impurities (p2-cen-ters). In the case of high impurity concentrations colloidal segregations may form. 

Band structure details of insulators can be determined from their UV/VIS spectra. Defects in the crystal produce electronic levels within the gap between the conduction and the valence bands. Spectroscopic measurements at low temperature allow the investigation of the phonon structure of a crystal. Absorptions due to lattice or point defects can be used to describe the optical and electronic properties of the insulator. For example, Cr in AI2O3 crystals leads to an intense color change of the crystal. Many so-caUed color centers are based on lattice defects caused by intercalation of atoms in the crystal lattice. 

Many defects are charged, they diffuse through the lattice, if the temperature is high enough, and they can associate with each other to form defect clusters or color centers. Such associates strongly affect the properties of the solid and materials engineers control the defect concentrations in solids to obtain the wanted properties. The following are examples of reactions between defects. 

Color centers are simple point defects in crystal lattices, consisting of one or more electrons trapped at an ionic... 

While color centers exist in many different crystal lattices, most research to date has been done on point defects in alkali halide crystals. This review will concentrate on the alkali halide centers because they form the basis of practically all the useful color center lasers, and they are well understood. A representative sample of color centers in... 

The F-center is the most fundamental color center defect in the alkali halide lattice. Although it is not laser-active, the optical properties of the F-center are important in understanding the laser physics of other color center lasers. The fundamental absorption band of the F-center, called the F band, corresponds to a transition fi om the Is-like ground state to the 2p-like first excited state of the square-well potential. The F-band transition is very strong, and dominates the optical spectrum of the alkali-halide crystal. In fact, the term F-center comes fi om the German word Farbe, meaning color, and refers to the strong color imparted to the otherwise transparent alkali-halide crystals. 

The F J centers can be associated with certain defects in the crystal lattice to form more stable color centers with output characteristics similar to the F J center. To date, four types of stabilized centers have been reported the (Fj)a, (FJ), (F+ p+. q2- centers. The (FJ)A center is an Fj... 

A stable laser-based (FJ)a center has been demonstrated in several lattices, and currently is the only color center laser able to tune beyond 3.9 /xm. (FJ)a center lasers combine the best characteristics of the Fa and Fj laser (1) they are operationally stable with no fading (2) they can be stored at room temperature because they are addi-tively colored (see section III) and (3) they are reasonably powerful. 

The transition involved in the Tl°(l) laser is not related to other color center transitions discussed so far. This laser transition occurs between the perturbed Pi/2 and P3/2 states of the free Tl atom. This transition is normally parity forbidden, but the strong odd symmetry perturbation caused by the positive ion vacancy mixes the P states with higher lying states and allows for a modest electric dipole to appear between these states. The absorption band caused by this transition in KCl is centered around 1.06 /xm, and the emission band is centered at 1.5 /u,m. The Stokes shift comes about through lattice relaxation... 

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