Mixtures antiferroelectrics

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. 

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... 

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... 

In some crystals the location of dipole moments can even be more complicated. For example, in Fig. 13.15c, one layer with the dipoles looking down alternates with two layers where the dipoles are looking up. Therefore we have three-layer periodicity 3/ with two antiparallel layers and one extra polar layer. Such a structure may be considered as a mixture of the ferroelectric and antiferroelectric structures and is called ferrielectric. In case (c), the ferroelectric fraction is one part per period, qp = 1/3 and the spontaneous polarization is finite, Pg = (l/3)Po. For pure antiferroelectric phase qp = 0/2 and for pure ferroelectric one qp = 1/1 = 1. More generally, for different ferrielectric structures qp = nim, where m is the number of layers in the unit cell (period) and m is the ferroelectric layer fracture per unit cell, both being integers. Then, for both n and m oc, nim 1, the difference between n and m become smaller and smaller and the so-called Devil s staircase forms. 

Achiral smectic materials with anticUnic molecular packing are very rare [40] and their antiferroelectric properties have unequivocally been demonstrated only in 1996 [41]. The antiferroelectilc properties have been observed in mixtures of two achiral components, although no one of the two manifested this behaviour. In different mixtures of a rod like mesogenic compound (monomer) with the polymer comprised by chemically same rod-like mesogenic molecules a characteristic antiferroelectric hysteresis of the pyroelectric coefficient proportional to the spontaneous polarization value has been observed for an example see Fig. 13.27a. Upon application of a low voltage the response is linear, at a higher field a field-induced AF-F transition occurs. 

Fig. 13.27 Achiral antiferroelectric. Voltage dependence of pyroelectric coefficient describing the double hysteresis loop (a) and dependence of the field-induced polarization on the content of a monomer in the polymer-monomer mixtures (b)...

As shown by the X-ray diffraction, polymer-monomer mixture consists of SmC bilayers. A bilayer is the principal unit cell having either non-polar C2h or polar C2v (b) S5mimetry. The former is incompatible with both ferroelectricity or antiferroelectricity, because such a structure has an inversion centre. On the contrary, in sketch (b) each bilayer is polar with Pq vector located in the tilt plane along the y-axis. In a stack of such layers the direction of Pq alternates and the stmcture (b) is antiferroelectiic in its ground state. Only strong electric field Ey causes the transition to the ferroelectric structure shown in sketch (c) as observed in experiment. Note that both the Pq and P = X Pq vectors are always lying in the tilt plane. 

Isozaki, T., Fujikawa, T., Takezoe, H., Fukuda, A., Hagiwara, T., Suzuki, Y., Kawamura, I. Competition between ferroelectric and antiferroelectric interactions stabilizing varieties of phases in binary mixtures of smectic liquid crystals. Jpn. J. Appl. Phys. 31, L1435-L1438 (1992)... 

Link, D.R., Clark, N.A., Ostrovskii, B.I., Soto Bustamante, E.A. Bilayer-by-bilayer antiferroelectric ordering in freely suspended films of an achiral polymer-monomer liquid crystal mixture. Phys. Rev. E 61, R37-R39 (2000)... 

Soto Bustamante et al. [103,104] have reported antiferroelectricity in the smectic A phase of polymer-monomer mixtures. Mixtures of the achiral polyacrylate... 

In fact, two new representatives of polar achiral systems have been discovered quite recently antiferroelectric polymer-monomer mixtures [286] and ferroelectric biaxial smectic A phases composed of ba-nana-like molecules [287]. 

The synthesis of nonchiral smectic liquid crystals is a broad topic for discussion, however, it can be divided into subsections in two different ways. For example, smectic systems can be split into metallomesogens and nonmetallomesogens, alternatively, they can be divided into materials for (1) meso-phase structure elucidation and classification [ 1 ], (2) property-structure correlations [2] and (3) host systems for ferroelectric and antiferroelectric mixtures. In the following sections template structures used for the synthesis of smectic materials will be described, followed by discussions of the syntheses of materials that have extensive histories in the elucidation of smectic phase structures, and finally of the syntheses of smectogens that are useful in applications. 

The synthesis of the first achiral host material that exhibits a smectic Cau phase [36] (achiral version of the antiferroelectric phase) is shown in Fig. 42. It can be seen that this material has three aromatic rings and two ester groups, but in addition it has a swallow-tail unit. This unit (for no apparent reason at the moment) stabilizes the formation of smectic Cai, phases. The material like a ferroelectric host can be doped with a chiral dopant to give an antiferroelectric mixture. 

Note added in proof Quite recently, Itoh et al. reported that the phase in these pyrimidine host mixtures is not antiferroelectric but ferroelectric and that the typical tristable switching results from an ultra-short pitch of about 50 nm. (K. Itoh, Y. Takanishi, J. Yokoyama, K. Ishikawa, H. Takezoe and A. Fu-kuda, Jpn. J. Appl. Phys. 1997, 36, L784). 

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