Lattice defects solute interaction

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... 

Although Conrad et al. have frequently justified their results in terms of conventional lattice-defect theory (including the size-misfit and modulus-defect formalisms), they have gone on to consider the effects of chemical interaction between the solute and solvent atoms. In doing so, the interaction mechanism was deduced, with the aid of atomic-orbital theory, to take the form of covalent bonding between the interstitial atom and the... 

For an anisotropic defect, like crowdions or di-atomic quasi-molecules (H and Vic centres), the problem becomes much more complicated and often permits only a numerical solution (e.g., [72]). For example, an estimate of the interaction energy of a crowdion with a vacancy in Cu in the direction perpendicular to the crowdion axis is 0.1 eV at the relative distance /2ao (ao is a lattice constant) if both are in the same plane, but this energy becomes 0.02 eV only for a distance twice as large (ip = 0 in Fig. 4.8(a)). Increase of the angle

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This way of looking at defect interaction could profitably be developed further. There is a useful parallel to be drawn between solvated ions or ion-pair complexes in solution, and defects or defect complexes in their crystal lattice environment. The idea is worth using when considering the growing hints of short range order and defect association in nonstoichiometric phases. 

In addition to satisfying dangling bonds, hydrogen passivation influences many properties of silicon surfaces. For example, the hydrogen-passivated surface is generally hydrophilic and easily wetted in aqueous solutions. The interaction between hydrogen and donor or acceptor species results in a change in resistivity this effect is more pronounced in the case of p-type silicon [12]. There is also evidence that the H atoms induce surface defects into the lattice [13]. 

What is found by experiment is that, as a general rule, at substitutional concentrations close to the maximum in the conductivity isotherms, there is a minimum in the activation energy. In an early (but very comprehensive) study of ceria solid solutions with the trivalent rare earths, Faber etal. [IS] showed that the depth of the minimum, and the concentration at which it occurs, depends upon the identity of the rare earth cation (Figure 9.1). The minima have been ascribed [19] to competitive defect interactions. Initially, the effect is a weakening ofthe association energy of the dimers caused by an electrostatic interaction between the cluster and the unassociated substitutionals having an opposite effective charge in the lattice note, however, that... 

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