Lattice defects formation

Returning to the subject of lattice defect formation, we can now proceed to write a series of defect reactions for the defects which we found for our plane net ... 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

An additional problem is encountered when the isolated solid is non-stoichiometric. For example, precipitating Mn + as Mn(OH)2, followed by heating to produce the oxide, frequently produces a solid with a stoichiometry of MnO ) where x varies between 1 and 2. In this case the nonstoichiometric product results from the formation of a mixture of several oxides that differ in the oxidation state of manganese. Other nonstoichiometric compounds form as a result of lattice defects in the crystal structure. ... 

In pure and stoichiometric compounds, intrinsic defects are formed for energetic reasons. Intrinsic ionic conduction, or creation of thermal vacancies by Frenkel, ie, vacancy plus interstitial lattice defects, or by Schottky, cation and anion vacancies, mechanisms can be expressed in terms of an equilibrium constant and, therefore, as a free energy for the formation of defects, If the ion is to jump into a normally occupied lattice site, a term for... 

In addition, we have shown that further defect formation can be induced by external reacting species, and that these act to form specific types of defects, depending upon the chemical nature of the crystal lattice. 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

The promoter may slow down, or otherwise influence crystal formation and growth, or produce lattice defects. These effects may lead either to a higher activity per unit area or to a higher specific surface area. 

Alkaline earth oxides (AEO = MgO, CaO, and SrO) doped with 5 mol% Nd203 have been synthesised either by evaporation of nitrate solutions and decomposition, or by sol-gel method. The samples have been characterised by chemical analysis, specific surface area measurement, XRD, CO2-TPD, and FTIR spectroscopy. Their catalytic properties in propane oxidative dehydrogenation have been studied. According to detailed XRD analyses, solid solution formation took place, leading to structural defects which were agglomerated or dispersed, their relative amounts depending on the preparation procedure and on the alkaline-earth ion size match with Nd3+. Relationships between catalyst synthesis conditions, lattice defects, basicity of the solids and catalytic performance are discussed. 

Lattice defects and latent image formation in silver halides. Fundamental mechanisms of photographic sensitivity, p. 242. London Butterworth s Sci. Publ. 1951. 

Interstitial sites are defined as those that would usually be empty in an ideal structure. Occasionally in real structures, ions may be displaced from their lattice sites into interstitial sites Frenkel defect formation). Once this happens, the ions in interstitial sites can often hop into adjacent interstitial sites. These hops may be one stage in a long range conduction process. A schematic example is shown in Fig. 2.1(h) a small number of Na ions are displaced into the tetrahedral interstitial sites and can subsequently hop into adjacent tetrahedral sites. It should be noted, however, that while a small number of Frenkel defects may form in NaCl, conduction is primarily by means of vacancies whereas in some other structures, e.g. AgCl, Frenkel defects do predominate. 

Figure 5.7 The role of stress caused by lattice mismatch between SAM and substrate illustrated in (a) and (b) by a cross-section of a SAM (x-z plane), indicated adsorption sides (x-y plane) and the molecule-substrate interaction potential V where the solid circles indicate the energy of an adsorption site for a particular SAM molecule, (a) For rigid molecules, stress is mainly released by defect formation in SAM, which results in a layer of rather low crystallinity and small domains, (b) Molecules...

The MEG model has been extensively used to determine lattice energies and interionic equilibrium distances in ionic solids (oxides, hydroxides, and fluorides Mackrodt and Stewart, 1979 Tossell, 1981) and defect formation energies (Mack-rodt and Stewart, 1979). Table 1.21 compares the lattice energies and cell edges of various oxides obtained by MEG treatment with experimental values. 

In this case the initial act of light absorption leads to the formation of an exciton, rather than a free electron or hole. Such an exciton, as it wanders through the crystal, may meet a lattice defect and annihilate on it, the energy of the exciton being utilized to ionize the defect, i.e., to transfer an electron or hole localized on the defect to the free state (the mechanism of Lashkarev-Juze-Ryvkin, see 90, 91). If such a defect is a foreign particle chemisorbed on the crystal surface, the result will be a change in the character of the bond between this particle and the surface. Thus, the interaction of the lattice excitons with the chemisorbed particles may cause a change in the relative content of the different forms of chemisorption and... 

Make a simple estimate of the energy of defect formation in the fluorite structure (a) describe the coordination by nearest neighbours and next-nearest neighbours of an anion both for a normal lattice site and for an interstitial site at the centre of the unit cell shown in Figure 5.3(a). 

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. 

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