Ionic displacement polarization

Polarization processes are extremely important in HR lluicls. Generally, there are four kinds of polarizations in a non-aqueous system containing no electrolytes or ions. They are electronic, atomic, Debye and the interfacial polarizations (the Wagner-Maxwell polarization). If the particulate material is an ionic solid, ionic displacement polarization should also be considered. The Debye and the intcrfacial polarizations arc rather slow processes as compared with electronic and the atomic polarizations. Usually, the former two polarizations arc called the slow polarizations, appearing at low frequency fields, whereas the last two are termed fast polarizations, appearing at high frequencies. 

Eq. (41) is a general expression for a solid material of a very low conductivity and marked ionic displacement polarization. Differentiating Eq. (41) with respect to o, one would find the tgS has a maximum value. 

We now wish to discuss displacements by nucleophilic reagents (Y ) on alkyl derivatives (RX). These are ionic or polar reactions involving attack by a nucleophile at carbon. A typical example is the reaction of hydroxide ion with bromomethane to displace bromide ion ... 

Values of s and s" are calculated using equations in which the contributions of dipolar effects, ionic displacements, and electrode polarization effects are additive. If conduction effects predominate, i.e., when neither interfacial effects nor dipolar effects are significant, the loss factor is given by (Kranbuehl et al., 1986) ... 

Among the 32 classes of single-crystal materials, 11 possess a centre of symmetry and are non-polar. For these an applied stress results in symmetrical ionic displacements so that there is no net change in dipole moment. The other 21... 

Figure 3.88. Schematic representation of the polarization of a particle with a Stem layer only. The distribution of the cation concentration and the potential in the Stem layer are sketched. Arrows Indicate ionic displacements upon polarization.

Another recent addition to the fluorous biphase toolbox is the discovery of fluorous phase transfer catalysts for halide substitution reactions in aqueous-fluorous systems.This class of reactions is academically intriguing, as an ionic displacement reaction has taken place in one of the least polar solvents known. They make use of fluorous phosphonium salts under biphasic conditions but can also make use of non-fluorous phosphonium salts in a triphasic system. Further information and reactions using such systems will no doubt be reported in the next few years. 

Temperature will influence only the polarization mechanisms that depend on long-range ionic displacement such as dipolar polarization. Ionic polarization is not strongly affected by temperature since long-range mobility of the ions is not required for it to be operative." ... 

When a metal body is exposed to an electric field, free electrons are displaced by electric forces until the field in the body vanishes. In an ideal dielectric (dc conductivity is zero), there exists only bound charges (electrons, ions) that can be displaced from their equilibrium positions until the field force and the oppositely acting elastic force are equal. This phenomenon is called displacement polarization (electronic or ionic polarization). A dipole moment is induced in every atom or between ion pairs. The molecular dipoles can only be rotated by an electric field. Usually, their dipole moments are randomly oriented. In an external held, however, an orientation parallel to the field direction is preferred so that a dipole moment is induced. This process is called orientational polarization. 

In an alternating electric field, the displacement polarization leads to electric oscillations. This is a resonant process with resonant frequencies of lO -lO " Hz for the electronic and of 10 -10 Hz for the ionic polarization. 

FIGURE 3.57 Schematic illustrations of polarization mechanisms, (a) Electronic displacement, (b) Ionic displacement, (c) Dipole orientation, (d) Space charge. 

The ion displacement polarization usually takes place in an ER suspension having ionic solid particulate. If the conductivity of the solid material is not loo high, then the maximunn of the dielectric loss tangent, due to the ion displacement polarization, can be expressed [28,43]... 

Depending on the investigated material and the frequency of the applied electric field, determined polarization can be electronic and atomic (very small translational displacement of the electronic cloud in THz frequency range), orientational or dipolar (rotational moment experienced by permanently polar molecules in kHz-MHz frequency range), and ionic (displacement of ions with respect to each other in Hz-kHz frequency region). 

On the basis of an examination of precise structural and ferroelectric data for a variety of ferroelectric crystals, Abrahams et derived the following empirical equation relating Curie temperature 7 and relative ionic displacement Az of the homopolar metal ion along the polar direction (e.g., Ti in BaTiOj, Sb in SbSI, and Nb in LiNbOj) at T and atmospheric pressure ... 

Piezoelectric response is related to ionic displacement dielectric response. In a heteropolar (partially ionic) material that lacks a center of inversion symmetry, displacement of atoms of one polarity with respect to atoms of another polarity results in a change in shape of the material. A relationship between shape and applied electric field is termed a piezoelectric response. When the unit cell of the lattice includes inversion symmetry such a displacement moves charge but does not change the shape. Consequently, such materials are not piezoelectric. An example of how a material can lack an inversion center is found in all zincblende-structure materials. In these materials, a cation and anion lie at opposite ends of each bond and the structure is not symmetric around this bond. Furthermore, all bond pairs are... 

The simplest explanation for the hydrogen bond is based upon the polar nature of F—H, O—H, and N—H bonds. In a molecule such as H20, the electron pair in the O—H bond is displaced toward the oxygen nucleus and away from the hydrogen nucleus. This partial ionic character of the O—H bond lends to the hydrogen atom some positive character, permitting electrons from another atom to approach closely to the proton even though the proton is already bonded. A second, weaker link is formed. 

Figure 2.8 Shell model of ionic polarizability (a) unpolarized ion (no displacement of shell) (b) polarized (displaced shell) (c) interactions 1, core-core 2, shell-shell 3, core-shell.

Ionic polarization occurs in ionic materials because an applied field acts to displace cations in the direction of the applied field while displacing anions in a direction opposite to the applied field. This gives rise to a net dipole moment per formula unit. For an ionic solid, the atomic polarization is given by... 

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