Compounds antiferroelectric

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). 

In this section, we will present the crystal structures of chiral mesogenic compounds exhibiting ferroelectric liquid crystalline phases which are listed in Table 18 [152-166]. Moreover, four compounds of the list show antiferroelectric properties and two compounds form only orthogonal smectic phases. The general chemical structures of the investigated chiral compounds are shown in Fig. 27. 

Zareba et al. [165] described the crystal structure of the chiral 4-(l-methyl-heptyloxycarbonyl)-phenyl 4-heptyloxytolane-4 -carboxylate (C7-tolane) which shows monotropic antiferroelectric and ferroelectric phases. The single-crystal X-ray analysis of this compound shows that the crystal has a smectic-like layer structure composed of largely bent molecules where the chain of the chiral group is almost perpendicular (86°) to the core moiety. Within the layers, the molecules are tilted. The central tolane group of the molecule is roughly planar. 

Table 18) must show a bent structure. This may be one reason for the interlocking and the occurrence of the 21-axis and the antiferroelectricity. But bent structures are possible as well as by gauche conformations like in the described solid state of compound 4-[(S)-2-methylheptyloxy]phenyl 4 -oct-ylbiphenyl-4-carboxylate [153]. 

This situation changed dramatically in 1996 with the discovery of strong electro-optic (EO) activity in smectics composed of bent-core, bowshaped, or banana-shaped achiral molecules.4 Since then, the banana-phases exhibited by such compounds have been shown to possess a rich supermolecular stereochemistry, with examples of both macroscopic racemates and conglomerates represented. Indeed, the chiral banana phases formed from achiral or racemic compounds represent the first known bulk fluid conglomerates, identified 150 years after the discovery of their organic crystalline counterparts by Pasteur. A brief introduction to LCs as supermolecular self-assemblies, and in particular SmC ferroelectric and SmCA antiferroelectric LCs, followed by a snapshot of the rapidly evolving banana-phase stereochemistry story, is presented here. 

The method has been applied to various hydrogen-bonded ferro- and antifer-roelectrics (see Chap 2 in this volume). The first results have, however, been obtained for the antiferroelectric compound H2C4O4 (squaric acid) and its deuterated analogue D2C4O4. 

Squaric acid (H2SQ) has been chosen as a first test compound because it has a very simple molecular structure. Planar sheets of the squarate (C4O4) groups are linked to each other in a two-dimensional network through O - H...0 bonds (Fig. 1) with weak van der Waals forces [52,53]. The protons perform an order/disorder motion above the antiferroelectric phase transi-... 

On the other hand, the proton potential of the 5-bromo compound is exactly symmetrical with reference to the reaction coordinate of the tautomerization. Consequently, the proton transfer can proceed through the tunnelling mechanism. This is the reason why the paraelectric behaviour is maintained even at 4 K. The suppression of the antiferroelectric phase transition may be derived from a quantum tunnelling effect. Such paraelectric behaviour can be regarded as quantum paraelectricity , which is a notion to designate the phenomenon that (anti)ferroelectric phase transitions are suppressed even at cryogenic temperatures due to some quantum-mechanical stabilization, proton tunnelling in this case. 

In their analysis of ferroelectric compounds using QSD, Villars etal studied 175 ternary ferroelectric and antiferroelectric oxides. These compounds were divided into three sets F with Tc > 500 K with 50 representatives, F2 with 22 compounds with 300 K < Tc < 500 K, and F3 with 103 representatives containing pseudotemaries and quaternary oxides with F > 500 K. The calculation of AX and AR for... 

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 NMR spectrum of KOH shows a typical quadrupolar powder pattern which changes with temperature as the compound is cooled through the temperature at which an antiferroelectric structural transition occurs (Figure 8.23A). Simulation of these... 

In the compounds that are close to PbZr03 in composition, room-temperature structures are orthorhombic (Fig. 12), but the octahedral cations do not uniformly displace parallel to [110]. Rather, for very Zr" cation that displaces parallel to [110], a neighboring Zr" shifts parallel to [TTO]. The net polarization therefore is zero, and the material is classified as antiferroelectric. Antiferroelectric materials also exhibit higher-than-average dielectric constants, but they are not so extreme as those observed in ferroelectric compounds. 

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. 

Nematic materials are only one member of a large family of a variety of structurally different compounds forming liquid crystalline mesophases. Although only nematics have yet found really widespread use, mostly for display applications, some structurally highly diverse smectic phases also have unique electrooptical characteristics, for example ferroelectricity or antiferroelectricity, which can be modulated by selective fluorination [5, 51]. For 20 years intensive effort has been devoted to making practical use of these phenomena. 

Sodium and potassium formate, together with copper formate (anhydrous and as the tetrahydrate) have been used as model compounds for surface formates [56] and 7.4.3. Copper formate tetrahydrate undergoes an antiferroelectric transition at 235.5 K. The INS spectra clearly show that above this temperature the water molecules are disordered and below it they are ordered [57]. 

Initially the octyl to dodecyl compounds were prepared and these were found to exhibit relatively normal behavior, i.e. smectic A phases were found for the lower homologues with smectic and ferroelectric smectic C phases occurring for the higher members. However, when the tetradecyl homologue was examined in the polarizing transmitted light microscope, an iridescent helical mesophase was observed which upon cooling underwent a further phase transition to a ferroelectric smectic phase. In addition, this compound was also found to exhibit antiferroelectric and ferrielectric phases. 

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