Polarity in crystals

The main source of spontaneous polarization in crystals is the relative freedom of cations that fit loosely into the crystal s octahedral cavities. The number of degrees of freedom of the octahedrons affects the spontaneous polarization value and hence influences the crystal s ferroelectric properties. Abrahams and Keve [389] classified ferroelectric materials into three structural categories according to their atomic displacement mechanisms onedimensional, two-dimensional and three-dimensional. 

Orientational polarization is not a resonant process since the molecular dipoles have inertia. The response of the orientational polarization to a charge of the electric field is, therefore, always retarded. This process is called dielectric relaxation. The characteristic time constant of such a relaxation process—this is the time for reaching new equilibrium after changing the excitation—is called relaxation time (r). It is strongly temperature dependent, since it is closely related to the viscosity of the material. At room temperature, the relaxation times of the orientational polarization in crystals are of 10 -10 s. In amorphous solids and polymers, however, they can reach a few seconds or even hours, days, and years, depending on the temperature. 

Second-harmonic generation for nonlinear optics, ferroelectricity, and piezoelectricity are all properties that are dependent on the pre.sence, magnitude, and orientation of bulk polarity in crystals and films. Therefore, the issue of how to design a polar solid from basic principles remains a challenge that has immense potential relevance to materials science. Obviously, a polar solid is guaranteed if a pure enantiomer is used as a component of a compound. However, the presence of polarity does not in any way imply that optimal packing will occur and, further-... 

Pauling rationalization of chemical bonding in solids Perovskite type polarization in crystals... 

Tagantsev AK (1991) Polarization in crystals and its response to thermal and elastic perturbations. Phase Trans 35 119-203... 

In crystallizing fatty acids, solvent polarity does not influence crystal form as much as temperature and concentration (9). Infrared (9,10) and wide-line nmr spectra (11) as well as x-ray methods (12,13) can be used to detect the various crystalline forms. 

Fig. 14. A sample of a lamellar liquid crystal between crosses polarized in an optical microscope gives a pattern of "oily streaks" and Maltese crosses (a) while the Hquid crystal consisting of an array of cylinders shows the characteristic sectional pattern (b).

Polarization effects are another feature of Raman spectroscopy that improves the assignment of bands and enables the determination of molecular orientation. Analysis of the polarized and non-polarized bands of isotropic phases enables determination of the symmetry of the respective vibrations. For aligned molecules in crystals or at surfaces it is possible to measure the dependence of up to six independent Raman spectra on the polarization and direction of propagation of incident and scattered light relative to the molecular or crystal axes. 

In general, ILs behave as moderately polar organic solvents with respect to organic solutes. Unlike the organic solvents to which they are commonly compared, however, they are poorly solvating and are rarely found as solvates in crystal structures. 

In crystals that belong to the three-dimensional category, reorientation of the polarization occurs due to displacements that appear to be relatively equal in all three directions. Such displacements are observed in the case of island-type crystals. 

Since then, the vibrational spectrum of Ss has been the subject of several studies (Raman [79, 95-100], infrared [101, 102]). However, because of the large number of vibrations in the crystal it is obvious that a full assignment would only be successful if an oriented single-crystal is studied at different polarizations in order to deconvolute the crystal components with respect to their symmetry. Polarized Raman spectra of samples at about 300 K have been reported by Ozin [103] and by Arthur and Mackenzie [104]. In Figs. 2 and 3 examples of polarized Raman and FTIR spectra of a-Ss at room temperature are shown. If the sample is exposed to low temperatures the band-widths can enormously be reduced (from several wavenumbers down to less than 0.1-1 cm ) permitting further improvements in the assignment. 

Fig. 2 Raman spectra of a single-crystal of orthorhombic Sg at three different polarizations in which the off-diagonal elements of the Raman scattering tensor are non-zero big, b2g, b g), after [105]. However, Raman intensities of other polarizations like flg components ( 54 cm ) penetrate in the spectra due to optical anisotropy in the crystal...

Of the five bending vibrations of the Sg molecule three are Raman active (V2, Vg, Vii) and two are IR active (V4, Vg). Most of the Raman active modes in the crystal could clearly be resolved in spectra at low temperatures and by polarization measurements. For example. Fig. 6 shows the Raman active factor group components of the Vg mode obtained at three different polarizations. In Fig. 7 an analogous IR spectrum is presented. 

The samples of l,6-T2-DBpD and l,6-T2-2,3,7,8-Cl4-DBpD are useful in metabolism and mode of action studies. For example, when incubated with rabbit liver microsomes, l,6-T.>-DBpD is extensively metabolized to polar product(s) but only when these preparations are fortified with reduced nicotinamide-adenine dinucleotide phosphate. Under the same conditions l,6-T2-2,3,7,8-Cl4-DBpD is completely resistant to metabolic attack. In some types of studies, a higher specific activity possibly is desirable i.e., >1 Ci/mmole), and this can be achieved, with the methodology already developed, by using larger amounts of tritium gas or working on a larger synthetic scale so that it is not necessary to add unlabeled materials to assist in crystallization steps where a certain minimum amount of compound is necessary. 

Having investigated the electrochemical behavior of ZnSe, and in view of the well-known blue luminescence of the compound, the previous authors extended their work to study electroluminescence from I-doped n-ZnSe crystals under anodic polarization in aqueous media containing metal ions such as Cu(II) and Sn(II) [123]. 

Investigation of the differences in crystal packing between (431) and (426) from comparison of their respective X-ray structures, revealed that (431) was more tightly packed than (442), reflected in their respective melting points of 235 and 170 °C. It was postulated that the absence of in vivo activity for (431) may be explained by the resultant reduction in water solubility and dissolution rate compared with (426). The comparatively high calculated polar surface area of (431) (122.5A ) compared with (426) (89.3 A ) was also proposed as a factor influencing the marked difference in bioavailability between the two related compounds. Compound (426) (SLV-319) is currently being developed with Bristol-Myers Squibb for the potential treatment of obesity and other metabolic disorders. Phase I trials for obesity were started in April 2004. Earlier Phase I clinical trials for the treatment of schizophrenia and psychosis, which commenced in April 2002, appear to have been abandoned. 

Papoular, R.J. and Gillon, B. (1990) Maximum entropy reconstruction of spin density maps in crystals from polarized neutron diffraction data, Europhys. Lett., 13(5), 429 134. 

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