Lattice defect centers

The fatty acids used in these experiments were carefully purified, so that it can be asserted confidently that the protons in liquid-like motion, which contribute to the narrow line, are not mainly in impurity molecules. We have, however, suggested (, 9) that the liquid-like motion in the fatty acids centers about, and with increasing temperature grows out from, impurity centers or lattice defect centers in the crystal. An attempt has been made in the work reported here to obtain evidence that this liquid-like... 

The requirement I > 2 can be understood from the symmetry considerations. The case of no restoring force, 1=1, corresponds to a domain translation. Within our picture, this mode corresponds to the tunneling transition itself. The translation of the defects center of mass violates momentum conservation and thus must be accompanied by absorbing a phonon. Such resonant processes couple linearly to the lattice strain and contribute the most to the phonon absorption at the low temperatures, dominated by one-phonon processes. On the other hand, I = 0 corresponds to a uniform dilation of the shell. This mode is formally related to the domain growth at T>Tg and is described by the theory in Xia and Wolynes [ 1 ]. It is thus possible, in principle, to interpret our formalism as a multipole expansion of the interaction of the domain with the rest of the sample. Harmonics with I > 2 correspond to pure shape modulations of the membrane. 

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. 

It was established by steady-state luminescence spectroscopy that minerals of Mn, such as rhodonite, rhodochrosite, helvine, triplite, Mn-apatite, Mn-milarite and others, show dark red luminescence, mainly at 77 K, which is uncommon to impurity Mn ". The excitation center proved to be regular Mn ", while the emission center is Mn ", situated near some lattice defect (Gorobets et al. 1978 Gaft et al. 1981). 

Anisotropy in interface roughness and in a roughening transition. Anisotropic distribution of active centers for growth, such as lattice defects, which contribute to growth. 

If adatom-impurity atom interaction is attractive, then the impurity atom can act as a trapping center. A diffusing adatom may be trapped. In heterogeneous catalysis, the reaction rate may be changed by the trapping effect of impurities as also by lattice defects and lattice steps and so on. 

Dowden (27) considers the active centers for carbonium ion formation to be associated with surface cation vacancies. A proton, derived from water contained in the catalyst, is attracted to the anions surrounding the vacancy. A hydrocarbon molecule is assumed to be held by polarization forces above this lattice defect and the proton will be distributed between the hydrocarbon and the anions, forming a carbonium ion of a definite lifetime. 

Volkenshtein (19) and Schwab (20) suggested that the active centers of adsorption were actually lattice defects in the semiconductor. Schwab felt that the concentration of free or quasi-free electrons was an important factor in metal catalysts. 

For a given molecule on the catalyst surface, which is most often nonhomogeneous, the position of the energy levels depends on the nature of the adsorption centers ions or lattice defects of thermal origin or resulting from the previous history of the catalyst. 

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. 

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. 

The presence of lattice defects and/or intentionally placed impurities in the alkali halide crystal will cause the formation of local energy levels in the forbidden gap, called traps or activator centers. Figure 18.19 shows the energy levels of an alkali halide crystal, including the activator centers and traps. (Atomic thallium is a common activator for alkali halide crystals.)... 

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... 

Three types of solvent or solute delocalization have now been examined, as summarized in Table III for three different adsorbent types (four, if we distinguish Cig-deactivated silica from silica). The theoretical requirements on the configuration and density of adsorption sites were discussed earlier (Section II,B) for a given type of localization/delocalization to be possible. In each case the nature of adsorption sites is fairly well understood for the four adsorbents of Table III, as disucssed in Ref. / and 17 and shown in Fig. 14. Thus, in the case of alumina, surface hydroxyls do not function as adsorption sites. Although surface oxide atoms are capable of interacting with acidic adsorbate molecules (see below), in most cases the adsorbate will interact with a cationic center (either aluminum atom or lattice defect) in the next layer. As a result, we can say that in most cases adsorption sites on alumina are buried within the surface, rather than being exposed for covalent site-adsorbate interaction. These sites are also rigidly positioned within the surface. Finally, the... 

During irradiation, the steady state condition is written dn/dt) = 0, from which it results that n = At. This relation permits evaluation of the stationary concentration of the excess free carriers, provided that the value of T is known. In the semiconductors, the recombination times as a rule are small in most cases, their values are between 10 and 10 sec. However, a considerable increase of the recombination times may result from the presence of certain imperfections (impurities, lattice defects) pre-existing in the solid or generated by the radiations which act like trapping centers of the free carriers. The following calculations are therefore only valid in the absence of this trapping phenomenon. 

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