Ferroelectric antiferroelectrics

The compositional perovskite series that has served as the basis for much of this research is the so-called PBZT system. The quadrilateral that joins the endmembers PbZr03-BaZr03-BaTi03-PbTi03 exhibits virtually complete solid solution (Fig. 12). Nevertheless, this system displays a number of morphotropic phase transitions that involve transformations of several kinds ferroelastic, ferroelectric, antiferroelectric, and relaxor. Understanding the nature of these isothermal transitions requires a review of the thermal distortions that occur as these perovskites are cooled. 

As noted earlier, the incorporation of chiral groups in the liquid crystal moieties can have the effect of inducing non-linear properties, which include thermochromism, ferroelectricity, antiferroelectricity, electrostriction, and flexoelectricity. In a now classical study, Hult [82] demonstrated that it was possible for supermolecular material 34 to exhibit two-state ferroelectric switching. The remarkable material he investigated, shown in Fig. 30, was found to exhibit two hitherto unclassified mesophases between the smectic... 

Fig. 13.25 Geometry for discussion of the electric field-induced ferroelectric-antiferroelectric transition. Antiferroelectric structure (a) in the zero field and ferroelectric structure at the field exceeding the F-AF transition threshold (b)...

Table A..5-1 The 72 families of ferroelectric materials. The number assigned to each family corresponds to the number used in LB III/36. The numbers in parentheses (A sub>. f+a ) after the family name serve the purpose of conveying some information about the size and importance of the family. The numbers indicate the following A sub the number of pure substances (ferroelectric, antiferroelectric, and related substances) which are treated as members of this family in LB III/36 A f+A the number of ferroelectric and antiferroelectric substances which are treated as members of this family in LB III/36 n, the number of representative substances from this family whose properties are surveyed in Sect. 4.5.4. For some of these families, additional remarks are needed for instance, because the perovskite-type oxide family has many members and consists of several subfamilies because the liquid crystal and polymer families have very specific properties compared with crystalline ferroelectrics and because the traditional names of some families are apt to lead to misconceptions about their members. Such families are marked by letters a-m following the parentheses, and remarks on these families are given under the corresponding letter in the text in Sect. 4.5.3.1...

Questions still exist the SmX phase exhibiting V-shaped switching is a new phase or there are some conventional phases such as ferroelectric, antiferroelectric, and ferrielectric phases. One of the most important experiments remaining is the quantitative measurement of the tilting correlation between adjacent layers. This is a future problem. 

These data together with the refractive indices provide one of the most effective structural characterizations of a solid. Optical methods are particularly sensitive in detecting pseudosymmetric forms of a higher symmetry phase and, therefore, for detecting phase changes with temperature in ferroelectric, antiferroelectric, and transparent ferrimagnetic crystals. 

Ionic crystals are sometimes ferroelectric, antiferroelectric, or paraelectric. Ferroelectric crystals belong to one of the ten polar classes (1, 2, m, mm2, 3, 3m, 4, 4mm, 6, 6mm) and are therefore pyroelectric and piezoelectric. The spontaneous polarization of a ferro-... 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

Landolt-Bitmstein, Ferroelectric and Antiferroelectric Substances, Vol. 9, New Senes Group III, Springer-Vedag, Berlin, Germany, 1977. 

U.N. Venevtzev, E.D. Politova, S.A. Ivanov, Ferroelectrics and antiferroelectrics of barium titanate family, Khimiya, Moscow, 1985 (in Russian). 

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. 

Fig. 17a-c. Sketches of the molecular arrangements for the smectic structure with alternating layer-to-layer tilt a conventional and chevron smectic C layering in low molecular mass mesogens b ferroelectric hilayer chevron structures for achiral side-chain polymers c antiferroelectric hilayer chevron structures for achiral side-chain polymers. Arrows indicate the macroscopic polarization in the direction of the molecular tilt... 

Dalai NS, Gunaydin-Sen O, Bussmann-Holder A (2007) Experimental Evidence for the Coexistence of Order/Disorder and Displacive Behavior of Hydrogen-Bonded Ferroelectrics and Antiferroelectrics. 124 23-50 Dalai NS, see Bussmann-Holder A (2007) 124 1-21 Daul CA, see Atanasov M (2003) 106 97-125... 

In the general case of arbitrary two-dimensional Bravais lattices (not rectangular and rhombic), the ground state, depending on the lattice parameters (x0 and y0 in Fig. 2.13), is characterized by ferroelectric (0.25 < x0 <0.5) or stratified bisublattice antiferroelectric ordering (0 < x0 < 0.25). 

Fig. 2.13. Two-dimensional Bravais lattice with the basis vectors a)s a2, and the reciprocal lattice vectors bi, b2. The solid and dashed arrows at angles A and 0A give the ferroelectric (k = 0) and antiferroelectric (k = bi/2) configurations of dipoles in the ground state.

In lead zirconate, PbZr03, the larger lead ions are displaced alternately from the cube comer sites to produce an antiferroelectric. This can readily be converted to a ferroelectric by the substitution of Ti4+ ions for some of the Zr4+ ions, the maximum value of permittivity occurring at about the 50 50 mixture of PbZrC>3 and PbTiC>3. The resulting PZT ceramics are used in a number of capacitance and electro-optic applications. The major problem in the preparation of these solid solutions is the volatility of PbO. This is overcome by... 

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. 

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. 

Along with the prediction and discovery of a macroscopic dipole in the SmC phase and the invention of ferroelectric liquid crystals in the SSFLC system, the discovery of antiferroelectric liquid crystals stands as a key milestone in chiral smectic LC science. Antiferroelectric switching (see below) was first reported for unichiral 4-[(l-methylheptyloxy)carbonyl]phenyl-4/-octyloxy-4-biphenyl carboxylate [MHPOBC, (3)],16 with structure and phase sequence... 

Figure 8.12 Longitudinal sheets with antiparallel polar symmetry are illustrated for achiral SmCA and SmC phases. Since it is not possible to switch to ferroelectric state in such system upon application of electric field, these structure should not be considered antiferroelectric.

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