Electronic induced distortions

The remaining sections of this chapter examine particular classes of cations that show electronically induced distortions. Section 8.2 explores the distortions caused by lone pairs and Section 8.3 explores the distortions found in transition metals. 

One of the best known examples of electronically induced distortion is the steric effect of the non-bonding valence-shell electrons found in main group cations in low oxidation states, cations such as S" " ", and Sn, in which one or more pairs of valence electrons are not involved in chemical bonding. Such nonbonding electrons are popularly known as lone pairs because they occur as localized spin-paired electrons. 

Fig. 8.9(b), ZnV20g is able to crystallize with the brannerite structure whose theoretical bond valences, calculated from the network equations ((3.3) and (3.4)) and shown in Fig. 8.9(b), already predict an out-of-centre distortion for the ion. ZnV20g thus adopts a bond graph that supports the electronically induced distortion. In this case the adoption of a lower symmetry bond graph is favoured because it is able to reduce the bond strain. 

Lattice-induced strains are characterized by large values of the GII because the environments around some atoms are stretched and around other atoms are compressed but, since the valence is still distributed as uniformly as possible among the bonds, the BSI remains small. This contrast with the electronically driven distortions discussed in Chapter 8 where the GII is small (the valence sum rule is obeyed) but the BSI is necessarily large. 

The moleeules or atoms comprising the dielectric exhibit a dipole movement distanee. An example of this is the stereochemistry of covalent bonds in a water molecule, giving the water molecule a dipole movement. Water is the typical case of non-symmetric molecule. Dipoles may be a natural feature of the dielectric or they may be induced. Distortion of the electron cloud aroimd non-polar molecules or atoms through the presence of an external electric field can induce a temporary dipole movement. This movement generates friction inside the dielectric and the energy is dissipated subsequently as heat[l]. 

Consequently, development with a nonpolar solvent selectively dissolves the unexposed, nonpolar polymer, providing high-resolution negative patterns with no evidence of swelling-induced distortion. The PSTTF system is reported to show good sensitivity to Al Ka X-ray (50 mj/cm ) and electron beams (10 xC/cm ) (101). The DUV lithographic performance of PSTTF has not yet been published. 

In this expression, Hg represents a purely electronic interaction between an undistorted system and the surface, and are electronic operators determining the interaction between a vibration (irrep F, component y) and the surface. Uiese latter operators must therefore have transformation properties dictated by the symmetry of both the adsorbed molecule and the surface. The form of (2) is suggestive of the standard method by which JT theory is developed, and this may be a desirable approach for future work. As a first approximation, however, we ignore the additional complication of surface-induced distortion and concentrate on the zeroth order term . 

Transmission electron microscopy is valuable because it allows imaging of even the smallest supported clusters of a heavy metal such as platinum. Dark field microscopy has been used to advantage for platinum clusters consisting of < 20 atoms each in zeolite KLTL [30]. Even single platinum atoms can be detected on zeolite supports thinner than about 20 nm, but the precision with which clusters can be pinpointed in the structure is limited by beam damage-induced distortion of the zeolite framework [30]. 

Less data are available on decay after-effects from "Te. A matrix of PbTe irradiated with neutrons was found to have a chemical isomer shift of over +0-1 mm s relative to a PbTe absorber [56]. Furthermore this shift difference decreased exponentially with a time constant of about ten days, the implication being that the difference is due to radiation damage causing a defect structure which anneals at room temperature. It was proposed that a radiation-induced distortion of the band structure in the PbTe semiconductor alters the s-electron density at the nucleus. 

Although LIGA technology is able to make microstmctures with very high aspect ratio and very vertical sidewalls, there are some factors which have limited further improvements in pattern accuracy, minimum feature size, and sidewall verticality in LIGA. These factors include X-ray diffiraction and photoelectron scattering, penumbra effect, non-vertical sidewall in absorber patterns, mask distortion, and secondary electrons induced by X-ray in substrate material [6]. 

The loss or acquisition of an electron generates a radical cation or a radical anion, respectively. They are called positive and negative polarons, respectively polaronic states arise within the band-gap. Considering, for the sake of brevity, the case of p-doping, the arising of a polaron requires oxidation and induces distortion in a low number of structural units over which the polaron is delocalized. Further electron extraction requires that different polarons are confined in the same polymer chains, inducing a more marked chain distortion. As a result of distortion and... 

Figure 8.4. Illustration of attractive effective interaction between two electrons mediated by phonons. Left the distortion that the first electron induces to the lattice of ions. Right the second electron with opposite momentum at the same position but at a later time. The lattice distortion favors energetically the presence of the second electron in that position.

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