Neighboring defects

Shluger, A. and Stefanovich, E., Models of the self-trapped exciton and nearest-neighbor defect pair in SiOj, Phys. Rev. B 42,9664 (1990). 

For a defect to be considered as isolated, sufficiently large supercells must be adopted to avoid spurious interactions among neighboring defects because of both relaxation/reconstruction and long-range electrostatic effects. 

The very simple and somewhat ad hoc form of Glarum s assumptions coupled with the one dimensional diffusion model used have led to several extensions of the original treatment these include relaxation by next nearest and other neighboring defects again by diffusion or random walk models and to the three dimensioanl case by such models. These go well beyond the motivation of the original idea to see whether a simple cooperative mechanism could account for marked short time deviations from simple models of rotational relaxation of a single dipole in a mean field approximation. For a review of much of this see B ttcher-Bordevijk (48). Particularly in the three dimensional case however there is an increasing question as to how far one can or should invoke isotropic diffusion processes to relax the component of a molecular electric moment parallel to that at an earlier time. 

The protection current produced by the usual full-wave rectifier has a 100-Hz alternating component of 48%. There are receivers with selective transmission filters for 100 Hz, which corresponds to the first harmonic of the cathodic protection currents [45]. With such a low-frequency test current, an inductive coupling with neighboring pipelines and cables is avoided, which leads to more exact defect location. 

The same theory, i.e. Eqs. (86) and (87), allows us to understand why CO and similar molecules adsorb so much more strongly on under-coordinated sites, such as steps and defects on surfaces. Since the surface atoms on these sites are missing neighbors they have less overlap and their d band wUl be narrower. Consequently, the d band shifts upwards, leading to a stronger bonding. 

In the heterogeneous solid, a different mechanism concerning charge dominates. If there are associated vacancies, a different type of electronic defect, called the "M-center", prevails. In this case, a mechanism similcu to that already given for F-centers operates, except that two (2) electrons occupy neighboring sites in the crystal. The defect equation for formation of the M-center is ... 

The last defect is one involving two nearest neighbor cation lattice-sites The following Table presents the defect reactions governing this case. 

Point defects in solids make it possible for ions to move through the structure. Ionic conductivity represents ion transport under the influence of an external electric field. The movement of ions through a lattice can be explained by two possible mechanisms. Figure 25.3 shows their schematic representation. The first, called the vacancy mechanism, represents an ion that hops or jumps from its normal position on the lattice to a neighboring equivalent but vacant site or the movement of a vacancy in the opposite direction. The second one is an interstitial mechanism where an interstitial ion jumps or hops to an adjacent equivalent site. These simple pictures of movement in an ionic lattice, known as the hopping model, ignore more complicated cooperative motions. 

Defects in ferrimagnetic structures often lead to noncollinear (canted) spin structures. For example, a diamagnetic substitution or a cation vacancy can result in magnetic frustration which leads to spin-canting such that a spin may form an angle 6c with the collinear spins in the sample [80, 81]. Similarly, the reduced number of neighbor ions at the surface can also lead to spin-canting [80-83]. 

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