Ferro-and antiferroelectricity

Thiophenes of type 31 (X-Y = CH) were generated via Lawesson s reagent-mediated cyclization of 1,4-dicarbonyl compounds 30 under microwave irradiation in the absence of solvent [37]. The reaction was carried by mixing the two solid reagents in a glass tube inserted inside a household microwave apparatus and irradiating until the evolution of H2S ceased. An interesting application of this method is the preparation of liquid crystals and other ferro- and antiferroelectric material such as compound 33 (Scheme 10). 

As is obvious from the table, Tc is almost doubled upon deuteration. These isotope effects are one of the largest observed in any solid state system. The question arises about isotope effects in non-hydrogen-bonded ferro- and antiferroelectrics. As already mentioned in the Introduction, within a mean-field scheme and in a purely ionic model it was predicted that these systems should not exhibit any isotope effect in the classical limit, which has been verified experimentally. Correspondingly, there was not much effort to look for these effects here. However, using a nonlinear shell-model representation it was predicted that in the quantum limit an isotope effect should... 

In the following sections, we shah demonstrate that the observed behavior of electro-optic activity with chromophore number density can be quantitatively explained in terms of intermolecular electrostatic interactions treated within a self-consistent framework. We shall consider such interactions at various levels to provide detailed insight into the role of both electronic and nuclear (molecular shape) interactions. Treatments at several levels of mathematical sophistication will be discussed and both analytical and numerical results will be presented. The theoretical approaches presented here also provide a bridge to the fast-developing area of ferro- and antiferroelectric liquid crystals [219-222]. Let us start with the simplest description of our system possible, namely, that of the Ising model [223,224]. This model is a simple two-state representation of the to-... 

Experimentally ferro- and antiferroelectricity of this type was discussed for rare earth compounds in [20-22]. Unoki and Sakudo [20] were the first who found the antiferroelectric anomaly in the DyV04 crystal that is simultaneously ferroeleastic and antiferroelectric. 

The pseudospin methodology is widely used not only for the description of hydrogen containing ferro- and antiferroelectrics, but also for the study of many other systems with hydrogen bonds. In particular, the pseudospin methodology was applied by Silbey and Trommsdorff [99] for examining the influence of two-phonon process on the rate constant of molecular compounds. In the next sections we will also employ the pseudospin formalism for the investigation of some problems where protons are exemplified by the cooperative behavior. 

Ferro- and Antiferroelectricity Volume Editors Dalai, N. S., Bussmaim-Holder, A. 

Venevtsev, Yu.N., Politova, E.D., and Ivanov, S.A. (1985) Ferro- and Antiferroelectrics of Barium Titanate Family, Chemistry, Moscow (in Russian). 

With increasing temperature the order of dipoles in each sublattice decreases and, at a certain temperature, a phase transition into the paraelectric phase occurs. It may be either second or first order transition. In the paraelectric phase local polarization Pq vanishes. The nature of the spontaneous polarization is similar in solid ferro- and antiferroelectrics. In both cases, the dipole-dipole interactions are dominant. For example, if dipoles are situated in the points of the body-centred cubic lattice, they preferably orient parallel to each other and such a structure is ferroelectric. However, the same dipoles placed into the points of a simple cubic... 

A difference between ferro- and antiferroelectrics may also be discussed in terms of the soft elastic mode [3], In the infinite ferroelectric crystal, there is no spatial modulation of the spontaneous polarization (only dipole density is periodic). Therefore, at the transition from a paraelecttic to the ferroelectric phase, both the wavevector q for osciUatimis of imis responsible for polarization and the correspondent oscUlatimi frequency co = Kef tend to zero. We may say that the soft elastic mode in ferroelecttics condenses at q 0. In antiferroelectrics, the sign of the local polarization Pq alternates in space with wavevector qo = 2nl2l = n/l and the corresponding imi oscillation frequency is finite, m = Kqf = Kn ll. It means that in antiferroelectrics the soft mode condenses at a finite wavevector n/l and rather high frequency. As a result, in the temperature dependence of the dielectric permittivity at low frequencies, the Curie law at the phase transitimi between a paraelecttic and antiferroelectric phases is not well pronounced. 

Ferro- and Antiferroelectric Compounds Based on the Bent-Shape Molecules... 

In conclusion of this chapter it should be stated that bistable and tristable switching of ferro- and antiferroelectric liquid crystals is very fast and provides long memory states. The latter allows one to design displays without semiconductor... 

Part k covering functional materials is organized in a two-step approach. The first step corresponds to searching for the substance of interest, that is, the relevant group of substances. The second step corresponds to the physical property of interest. Materials covered are semiconductors, superconductors, magnetic materials, dielectrics and electrooptics, and ferro- and antiferroelectrics. 

Landolt-Bomsteirr Tables, Neue Serie Vol. in/3 (Editors K.H.Hellwege, A.M. Hellwege), Springer Verlag. Mitsui T, Abe R, Fumhata Y, Gesi K, Ikeda T, Kawabe K, Makita Y, Mamtake M, Nakamura E, Nomura S, Sawaguchi E, Shiozaki Y, Tatsuzaki L Toyoda K (1969) Ferro- and Antiferroelectric Substances. 

As we have seen, most liquid crystals have too high a symmetry to be macroscop-ically polar if they obey the n - -n invariance (which all civilized liquid crystals do, that is, all liquid crystal phases that are currently studied and well understood). The highest symmetry allowed is C2 (monoclinic), which may be achieved in materials which are liquid-like at most in two dimensions. Even then external surfaces are required. Generally speaking, a polar liquid crystal tends to use its liquid translational degrees of freedom so as to macroscopical-ly cancel its external field, i.e., achieve some kind of antiferroelectric order. For more liquid-like liquids, piezo-, pyro-, ferro-, and antiferroelectricity are a fortiori ruled out as bulk properties. These phenomena... 

Surowiak, Z., On the technology of deposition of polycrystalline thin films of ferro- and antiferroelectrics on metallic substrates. Acta Phys. Polon. M3 543-50 (1973). 

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