Ferroelectric crystals liquid-like behavior

E. Liquid-like Behavior in Doped Ferroelectric Crystals... 

Ferroelectric materials are a subclass of pyro- and piezoelectric materials (Fig. 1) (see Piezoelectric Polymers). They are very rarely foimd in crystalline organic or polymeric materials because ferroelectric hysteresis requires enough molecular mobility to reorient molecular dipoles in space. So semicrystalline poly(vinylidene fluoride) (PVDF) is nearly the only known compoimd (1). On the contrary, ferroelectric behavior is very often observed in chiral liquid crystalline materials, both low molar mass and poljuneric. For an overview of ferroelectric liquid crystals, see Reference 2. Tilted smectic liquid crystals that are made from chiral molecules lack the symmetry plane perpendicular to the smectic layer structure (Fig. 2). Therefore, they develop a spontaneous electric polarization, which is oriented perpendicular to the layer normal and perpendicular to the tilt direction. Because of the liquid-like structure inside the smectic layers, the direction of the tilt and thns the polar axis can be easily switched in external electric fields (see Figs. 2 and 3). 

In the early development of liquid crystals, for the most part, the study of small molecular systems dominated the field because of the close link between molecular design and commercial applications. However, it is only in the last 20 years that materials with unusual, and often hybrid structures have been investigated for their liquid-crystalline behavior. As noted, phasmidic materials, which have molecular structures that are part-disc part-rod, were found to exhibit both columnar and smectic phases. More recently, molecular systems having bent-rod-like structures have been investigated and found to exhibit a wide range of novel phases, many of which were found to be ferroelectric or antiferroelectric (without molecular chirality) due to the reduced symmetry of their mesophase structures. 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

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