Polar crystals, surface effects

Water molecules, by virtue of their great polarity and their very small, compact shape, can very effectively surround the individual ions as they freed from the crystal surface. 

Some of the difficulties encountered in establishing the effect of solvent on crystal growth may be circumvented by focusing on polar crystals. This is because the difference in the rates of growth of opposite faces (hid) and (hkl) along a polar direction must arise primarily from differences in their solvent-surface interactions. Thus, one generally does not have to be concerned with faces other than the hemihedral ones in question. We illustrate below an approach to understanding solvent-surface interactions in the polar crystals of resorcinol (102). 

Surface Stabilized Ferroelectric Liquid Crystals (SSFLC)116 Here all three vectors of spontaneous polarization (Fs) are initially aligned by surface effects in thin cells (ca 2 pm). The switchability is due to 180° rotation of the Fs vectors on a cone. 

If a piezoelectric plate (Fig. 6.1), polarized in the direction indicated by P, carries electrodes over its two flat faces, then a compressive stress causes a transient current to flow in the external circuit a tensile stress produces current in the opposite sense (Fig. 6.1(a)). Conversely, the application of an electric field produces strain in the crystal, say a negative strain reversal of the field causes a positive strain (Fig. 6.1(b)). The changes in polarization which accompany the direct piezoelectric effect manifest themselves in the appearance of charges on the crystal surface (see Eq. (2.71)) and, in the case of a closed circuit, in a current. 

In the case of ionic crystals, the presence of different ionic species and the possibility of polarization effects inherently complicate the estimation of surface distortion. Previous considerations of the depth of penetration of surface effects in these materials (7, 10) have used oversimplified interaction potential functions or have imposed rather severe constraints on the form of relaxation permitted. A fairly detailed treatment of the distortion in the outermost layer of a free 100 ... 

Conversion of electromagnetic wave (EW) polarization provides an efficient and powerful method for diagnostics of media a nd s tructures with reduced symmetry (e.g. anysotropic crystals, media with natural and artificial gyrotropy, periodic structures, solid-state surfaces and thin films). On the other hand, such media and structures can be used as polarization converters. The conversion of the polarization in surface layers and thin films is usually small [1,2] and achromatic because in this case the region of interaction of the EW with the polarization active medium is small and the interaction itself is non-resonant. However, the effect may increase substantially (resonantly) and the polarization converted radiation becomes colored when the external EW excites eigen-oscillations on optically active surface or in an optically active film. For example, under the non-uniform cyclotron resonance excitation in two-dimensional (2D) electron system, high conversion efficiency can be reached [3]. 

In ionic crystals, reconstruction effects can also be involved in the stabilization of polar surfaces (Tasker s type 3). For instance, the (100) surface of the fluorite-type crystal of Li20 becomes stable if half of the Li atoms are moved from the bottom face of the slab to the top face above the oxygen atoms to produce a zero-dipole structure (Figure 39). In fact, this kind of surface has been observed experimentally. ... 

These effects have been used by investigators to reduce the background radiation levels in x-ray spectroscopy. For Bremsstrahlung radiation, the maximum polarization vector is parallel to the electron path in the x-ray tube. In many conventional-wavelength spectrometer designs, the specimen and crystal surfaces will become parallel when 26 equals the takeoff angle 2 ( 45°). In addition, the x-ray tube axis and the rotational axes of the crystal and detector are parallel. This so-called parallel optics cannot take advantage of the polarization of the white spectrum. If, however, the surface plane of the crystal and specimen remain perpendicular... 

As shown in Chapter 2, to optimize the contrast in the IR spectrum of an ultrathin film, it is necessary in many cases to use nonnormal angles of incidence and p-polarized radiation, which creates specific difficulties in the interpretation of the spectrum. The problems stem from the appearance of additional bands in the spectra of samples that are small relative to the wavelength these bands are due to the surface charges resulting from the polarization of the samples. The dependence of the transverse vibrational frequency of a polar crystal on the crystal size, called the size effect, was discovered by Frohlich [2]. A convincing explanation of this effect in the IR spectra of thin films was presented in 1963 by Berreman [3] while studying the transmission of 325-348-nm LiF layers. Consequently, this size effect in the IR spectra of ultrathin films became known as the Berreman effect by Harbecke et al. [4]. 

As mentioned earlier, the helical structure of the smectic C phase should be untwisted by an electric or magnetic field, or suppressed by a surface effect, to observe ferroelectric properties of the phase. In the first publication on ferroelectric liquid crystals [5] an approach to nontwisted ferroelectric LC materials was suggested. By mixing two individual ferroelectric liquid crystals having opposite signs of P but different absolute values, one can compensate the helical twisting without zeroing the polarization. That has been done for low-molar-mass liquid crystals... 

In 1949 Waser [23a] tried to establish the absolute configuration of (D) - tartaric acid by correlating the relative rates of growth of the hemihedral (hkl) and (hkl) faces with the ease of attachment of the free molecule at either face in terms of intermolecular distances between the crystal and the molecule to be attached. In fact, as Turner and Lonsdale pointed out [23b], given an asymmetric molecule X-A in a polar crystal, as in Scheme 4, there is no a priori reason why, on the basis of intermolecular distances only, the attachment of an X to an A face should take place more readily than that of an A to an X face . The observed differences in development of hemihedral faces may be explained primarily in terms of surface-solvent interactions or polarizability effects. 

The second turn of the discussion around the nature of a SHG in nematic liquid crystals arised when the SHG was observed in oriented layers of 4-methoxybenzylidene-4 -butylaniline (MBBA)/ The phenomenon has been explained by the lack of the symmetry center in the nematic phase. The zero-field SHG in MBBA was also investigated but the nature of the effect was connected with the flexoelectric polarization of surface layers. Such a polarization has to remove the inversion center in surface liquid crystalline layers and to allow the SHG to be detectable. Another explanation of the zero-field SHG in terms of the electric quadrupolar interaction was suggested in. ... 

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