Molecular point defects

FIRST-PRINCIPLES STUDY OF MOLECULAR POINT DEFECTS IN ICE IH INTERSTITIAL VS. VACANCY... 

In general, however, identification of the crystal cell is only part of the problem of characterizing the structure of crystalline polymers. Crystals are never perfect and the units cells do not infinitely duplicate through space even when they are grown very carefully from dilute solution using low molecular mass materials. As with the organic crystals considered in Chapter 3, a variety of defects can be observed and are associated with chain ends, kinks in the chain and jogs (defects where the chains do not lie exactly parallel). The presence of molecular (point) defects in polymer crystals is indicated by an expansion of the unit cell as has been shown by comparison of branched and linear chain polyethylene. The c parameter remains constant, but the a and b directions are expanded for the branched polymer crystals. Both methyl and... 

The presence of molecular (point) defects in polymer crystals may be indicated by an expansion of the unit cell. The unit cell parameters of branched polyethylene have been extensively studied and compared with those of linear polyethylene (Fig. 7.9). The c parameter remains constant and the branched polymer crystals are expanded in the a (mostly) and b directions. Both methyl and ethyl branches cause... 

Electrical Properties. Generally, deposited thin films have an electrical resistivity that is higher than that of the bulk material. This is often the result of the lower density and high surface-to-volume ratio in the film. In semiconductor films, the electron mobiHty and lifetime can be affected by the point defect concentration, which also affects electromigration. These effects are eliminated by depositing the film at low rates, high temperatures, and under very controUed conditions, such as are found in molecular beam epitaxy and vapor-phase epitaxy. 

Structure Crystal structures, Point defects, Dislocations Crystal structures, Defect reactions, The glassy state Configuration, Conformation, Molecular Weight Matrices, Reinforce- ments Biochemistry, Tissue stracture... 

Let us re-examine the notion of a point defect in this context. If a molecular subgroup of a molecule is imperfect, this damaged molecule constitutes a point defect in the crystal, although the defect has no immediate influence on the molecule s translational mobility. Point defects that induce (translational) motion are vacancies or interstitials. We can infer from the form of the Lenard-Jones potential that vacan-... 

In the case of semiconductors doped with f elements a different kind of an energy transfer process can be observed, namely from extended band states or excitonic states to the highly localized f-element states. Such a process is different from the cases discussed in the preceding sections, where the energy transfer from point defects (or at the most molecular states) was considered. The interest in semiconductors doped with f elements is obvious, because of their potential to combine sharp f-element luminescence with the possibility of simple electrical excitation via the semiconductor host. However, a quenching of the luminescence with... 

While the effective g value is expressed in terms of three principal values directed along three axes or directions in a single crystal, only the principal values of g can be extracted from the powder spectrum rather than the principal directions of the tensor with respect to the molecular axes. (Therefore it is more correct to label the observed g values as gi, g2, g3 rather than g gyy, in a powder sample.) In the simplest case, an isotropic g tensor can be observed, such that all three principal axes of the paramagnetic center are identical (x = y = z and therefore gi= gi = g-i). In this case, only a single EPR line would be observed (in the absence of any hyperfine interaction). With the exception of certain point defects in oxides and the presence of signals from conduction electrons, such high symmetry cases are rarely encountered in studies of oxides and surfaces. 

The 3-/im infrared absorption band in wet synthetic quartz is due to high-pressure clusters of molecular water rather than point defects (Aines and Rossman 1984 Gerretsen et al. 1989). 

Methods that probe local structure and defects, such as STM and AFM, show that nearly full eoverage on the molecular scale is easy to achieve [36, 39, 40]. Defects for long-chain alkanethiol monolayers are restricted primarily to point defects such as missing single adsorbate molecules, grain boundaries between domains and step edges usually related to discontinuities in the substrate [39, 41]. 

As we have seen, the expressions for the rate constant obtained for different models describing the lattice vibrations of a solid are considerably different. At the same time in a real situation the reaction rate is affected by different vibration types. In low-temperature solid-state chemical reactions one of the reactants, as a rule, differs significantly from the molecules of the medium in mass and in the value of interaction with the medium. Consequently, an active particle involved in reaction behaves as a point defect (in terms of its effect on the spectrum and vibration dynamics of a crystal lattice). Such a situation occurs, for instance, in irradiated molecular crystals where radicals (defects) are formed due to irradiation. Since a defect is one of the reactants and thus lattice regularity breakdown is within the reaction zone, the defect of a solid should be accounted for even in cases where the total number of radiation (or other) defects is small. 

In atomic or molecular sohds, common types of point defects are the absence of an atom or molecule from its expected position at a regular lattice site (a vacancy), or the presence of an atom or molecule in a position which is not on the regular lattice (an interstitial). In ionic solids, these point defects occur in two main combinations. These are Schottky defects, in which there are equal numbers of cation and anion vacancies within the crystal, and Frenkel defects, in which there are cation vacancies associated with an equal number of "missing" cations located at non-lattice, interstitial positions. Both are illustrated in Figure 1.1. Point defects are also found in association with altervalent impurities, dislocations, etc., and combinations of vacancies with electrons or positive holes give rise to various types of colour centres (see below). 

Vibrational frequency shifts due to the presence of the three point defects. Horizontal axis describes the energy hv of each vibrational mode in the defect-free ciystal Vertical axis describes the energy shift in the presence of the defect. Panel a) Vacancy, b) Tu interstitial, c) Be interstitial. Symbols Vj, Vr, vj and vj- vs denote, respectively, groups of molecular translational, molecular librational, intramolecular bending and intramolecular stretching modes, c.f Ref. 1. 

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