Properties of Antiferroelectric Crystals

Liquid crystal ferri and antiferroelectrics have many features discovered for years of comprehensive studies of corresponding crystalline substances. Thus, it would be convenient and instructive to begin with a short introduction in the structure and properties of antiferroelectric crystals. A difference between ferro-, ferri and antiferroelectrics is schematically shown in Fig. 13.15, where the three very simplified... 

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

Antiferroelectric liquid crystals were discovered in 1989.xheir antiferroelectric properties were considered as a surprise as, at that time, nobody believed that significant changes of the tilt from layer to layer were possible. In addition to the surprising antiferroelectric properties of one of the phases, several additional phases within a rather narrow temperatm-e region were found. Why such a rich variety of phases occurs within a narrow temperature region and what their structures are, has been a hot experimental and theoretical problem for a long time. The main difference between the microscopic structm-es of the phases was the period of the basic structure. Various phases will be described in more detail later but here we only mention the periods. The SmC phase is defined by the structure of a single... 

Detailed hst of the material properties for the piezoelectric crystals can be foimd in I andolt-Bomstein Tables and in other sources. Also the temperature coefficients for mai of these crystals were reported. Revised and up-dated Tables of material properties for piezoelectric rrraterials were published in recent volitmes of Landolt-Bomstein Tables. Special volumes are devoted to ferroelectric and antiferroelectric substances (Group 111, Volrrmes 3, 9,16,28 and 36). 

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... 

It was the Harvard physicist Meyer who in 1974 first recognized that the symmetry properties of a chiral tilted smectic would allow a spontaneous polarization directed perpendicular to the tilt plane [61]. In collaboration with French chemists, he synthesized and studied the first such materials [62]. These were the first polar liquid crystals recognized and as such something strikingly new. As mentioned before, substances showing a smectic C phase had been synthesized accidentally several times before by other groups, but their very special polar character had never been surmised. Meyer called these liquid crystals ferroelectric. In his review from 1977 [43] he also discussed the possible name antiferroelectric, but came to the conclusion that ferroelectric was more appropriate. 

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... 

A recent overview of the Curie principle is written in the book S.T. Lagerwall, Ferroelectric and Antiferroelectric Liquid Crystals, Wiley-VCH, Weinheim (1999). J.F. Nye, Physical Properties of Crystals, Oxford University Press, Oxford (1957). S. Garoff, R.B. Meyer, Phys. Rev. Lett., 38, 848 (1977). 

Parab SS, Malik MK, Dabrowski R, Deshmukh RR (2013) Thermodynamic and bias field characterization of quickly operating antiferroelectric liquid crystal. J Mol Liq 183 20-25 Parab SS, Malik MK, Deshmukh RR (2014a) Investigation of dielectric properties of poly(methyl methacrylate)-E7 composite films. Int J ChemTech Res 6 1836-1839 Parab SS, Malik MK, Bhatia PG, Deshmukh RR (2014b) Investigation of liquid crystal dispersion and dielectric relaxation behavior in polymer dispersed liquid crystal composite films. J Mol Liq 199 287-293... 

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. 

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... 

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... 

The chapter is organized as follows The second section discusses the prototype polar smectics the ferroelectric liquid crystals. We discuss the structure of the ferroelectric phase, the theoretical explanation for it and we introduce the flexoelectric effect in chiral polar smectics. Next we introduce a new set of chiral polar smectics, the antiferroelectric liquid crystals, and we describe the structures of different phases found in these systems. We present the discrete theoretical modelling approach, which experimentally consistently describes the phases and their properties. Then we introduce the discrete form of the flexoelectric effect in these systems and show that without flexoelectricity no interactions of longer range would be significant and therefore no structures with longer periods than two layers would be stable. We discuss also a few phenomena that are related to the complexity of the structures, such as the existence of a longitudinal, i.e. parallel to the... 

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