Antiferroelectric material

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). 

A less well-documented effect is that of the phase-transition temperature of certain crystals which are very sensitive to deuteration. Some crystals of ferroelectric and antiferroelectric materials, and in particular dihydrogen phosphates and hydrogen selenites, which are extensively hydrogen bonded, display this effect (Blinc and Zeks, 1974). For some crystals, such as caesium... 

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. 

Ayyub, R, Chattopadhyay, S., Pinto, R., Multani, M. Ferroelectric behavior in thin films of antiferroelectric materials. Phys. Rev. B 57(10), R5559-R5562 (1998)... 

In the field-off state the macroscopic polarization of the antiferroelectric phase is zero. With increasing field, the induced polarization, at first, increases linearly with field and then, at a certain threshold, the antiferroelectric (AF) structure with alternating molecular tilt transforms in the ferroelectric one (F) with a uniform tilt, see Fig. 13.24a. Correspondingly, the macroscopic polarization jumps from a low value to the level of the local polarization Po [34]. The process is quite similar to that observed in crystalline antiferroelectrics. With a certain precaution we can speak about a field-induced AF-F non-equilibrium phase transition . The magnitude of the switched polarization in some antiferroelectric materials can be quite... 

The antiferroelectric phase, discovered in 1989, is currently the snbject of mnch research because of the great potential in display devices. The application of an electric field to an antiferroelectric material indnces a ferroelectric ordering which can be switched in the usnal way by a reversed pnlse. Removal of the field will regenerate the antiferroelectric phase. Such a system has the advantage over the normal ferroelectric system of well-defined electric field thresholds and accordingly shonld be mnch easier to multiplex. 

Antiferroelectric materials have an exchange interaction between unpaired electrons that results in antiparallel alignment of electron spins. The magnetic moments of ions in adjacent planes are equal in magnitude, aligned, but in opposite polarity therefore, the result is no net magnetization. A few materials such as FeO, MnO, NiO, and CoO exhibit this behavior. 

The applications of liquid crystals have unquestionably added incentive to the quest for new liquid crystal materials with superior properties such as viscosity, elastic constants, transition temperatures, and stability. In recent years this has catalyzed work on chiral materials as dopants for ferroelectric displays and for antiferroelectric materials with structures avoiding the number of potentially labile ester groups that were present in the original materials in which... 

Actually, the first solid antiferroelectric material had been discovered at Tokyo Institute of Technology [40]. The discovery of the first antiferroelectric liquid crystal at the very same university was quite unintentional, but can be considered quite fortunate. In this chapter, I will introduce some details of what led to the discovery of antiferroelectric liquid crystals from the study of ferroelectric liquid crystals and also discuss the history of the discovery of the ferroelectric phase. 

Materials which do not display spontaneous polarization are called paraelectric (antiferroelectric materials, which will be dealt with in section 11.2.2.4 should... 

Since P must remain normal to z and n, the polarization vector forms a helix, where P is everywhere normal to the helix axis. While locally a macroscopic dipole is present, globally this polarization averages to zero due to the presence of the SmC helix. Such a structure is sometimes termed a helical antiferroelectric. But, even with a helix of infinite pitch (i.e., no helix), which can happen in the SmC phase, bulk samples of SmC material still are not ferroelectric. A ferroelectric material must possess at least two degenerate states, or orientations of the polarization, which exist in distinct free-energy wells, and which can be interconverted by application of an electric field. In the case of a bulk SmC material with infinite pitch, all orientations of the director on the tilt cone are degenerate. In this case the polarization would simply line up parallel to an applied field oriented along any axis in the smectic layer plane, with no wells or barriers (and no hysteresis) associated with the reorientation of the polarization. While interesting, such behavior is not that of a true ferroelectric. 

Figure 8.8 Structure and phase sequence of (R)-MHPOBC is shown. One of most famous smectic LCs, antiferroelectric switching in SSFLC cells was first discovered with this material.

Figure 8.13 Hypothetical smectic mesogen with hinge in center of core is illustrated. Such material could in principal switch to ferroelectric state, which we term the SmAPp, upon application of electric field in plane of layers. If this state exists in well on configurational hypersurface, then ground-state structure is antiferroelectric, denoted SmAPA.

In detailed studies of this mixture, strong evidence was obtained suggesting that the new achiral smectic phase is antiferroelectric. This consisted mainly of the observation of a double hysteresis loop in the polarization vs. applied electric field curve for the material.30 In addition, it was shown that the mixture is a... 

Figure 8.17 Structure and phase sequence of first banana-phase mesogen, reported by Vorlander in 1929, is given. Liquid crystal phase exhibited by this material (actually Vorlander s original sample) was shown by Pelzl et al.36a to have B6 stmeture, illustrated on right, in 2001. Achiral B6 phase does not switch in response to applied fields in way that can be said to be either ferroelectric or antiferroelectric.

Figure 8.18 Smectic dimer of Watanabe, possessing an odd number of methylene units in linking group. This material self-assembles into intercalated smectic structure very similar to B6 banana phase. As for B6 phase, this achiral phase is also neither ferroelectric nor antiferroelectric.

The highly complex and unusual textures observed for B7 materials is complemented by unusual X-ray diffraction behavior. While the beautiful mystery of the B7 texture is not understood in detail, MHOBOW shows EO behavior, which allows some definitive statements regarding its nature. Thus, while some B7 materials are reported to be EO-inactive (no EO switching),54 and some are reported to exhibit antiferroelectric EO behavior,59 MHOBOW exhibits a unique texture change upon application of electric fields. 

Antiparallel dipole ordering to produce an antiferroelectric crystal is also commonly encountered. Other ways of ordering electric dipoles are not so well characterized, but parallels with the situation in magnetic materials occur. 

In PbZrOs, which also has a perovskite structure, the offset atoms are arranged alternately in opposite directions. This produces an antiferroelectric state. PbZrOs with some zirconium replaced by titanium gives the widely used ferroelectric materials, PZT (PbZr, Ji/93). 

Pure KTaOs has a perovskite structure but is not ferroelectric or antiferroelectric. Replacing some K ions by Li does, however, produce a ferroelectric material. Explain why the substitution of Li might have this effect. 

Extensive research has already been carried out on incorporation of fluorine into molecules which can lead to profound and unexpected results on biological activities and/or physical properties [ 1 - 5]. In particular, optically active fluorine-containing molecules have been recognized as a relatively important class of materials because of their interesting characteristics and potential applicability to optical devices such as ferroelectric or antiferroelectric liquid crystals [6-11]. Recent investigations in this field have opened up the possibility for the... 

Fig. 5 Induction of the blue phase by doping a N material with (a) a rod-shaped molecule MHPOBC and (b) a bent-shaped molecule P8-PIMB. In both cases, the blue phase is induced above the N phase. The bent-shape of the antiferroelectric molecule is responsible for the blue phase induction in (a), since the doping of a real rod-shaped molecule (TBBA) does not induce the blue phase [26]...

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