Response antiferroelectric

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.

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]...

The dielectric response of PBSQ 2H2O derived from tautomerization is observed under atmospheric pressure and at ambient temperature. Furthermore, the dielectric constant turns out to be almost temperature-independent in the temperature range 4-300 K. When PBSQ was deuterated, the dielectric constant obeyed the Curie law, and an antiferroelectric phase transition was observed at 30 K. This result is strong supporting evidence for a significant contribution from the tunnelling mechanism to the dielectric response of the hydrogenous sample. 

The presence of clusters in BC nematics is now well established from various measmements. Recent studies " have in fact indicated a ferroelectric or an antiferroelectric response to an applied electric field, and an unusual low-frequency (presumably collective) mode has been detected in the dielectric spectra of bent-core nematics, which might also be related to clusters. In spite of the intense studies, however, the exact structure and the physical properties of the clusters are still unknown. Therefore, not surprisingly, a precise physical model for the role of polar clusters in the flexoelectric response of BC nematics and a quantitative estimation of the resulting increment of the flexocoefiicients has not yet been worked out. 

Kumar et al. proposed that a chain formed by intercalated bent-core molecules with alternating dipoles may have a considerable quadrupolar moment and they assumed that this idea is transferable to antiferroelectric layers. They estimated an increment in the quadrupole moment proportional to the number of molecules (m) in such clusters. As the typical cluster size indicated by X-ray measurements is about 4-5 layers of 20-30 nm, m may be of the order of 1,000. Therefore, on the one hand, the presence of clusters with an antiferroelectric SmCP structure may cause a huge increase in the quadrupolar contribution to the flexoelectric response compared to that of individual molecules. 

A difference between ferro- and antiferroelectrics may also be discussed in terms of the soft elastic mode [3], In the infinite ferroelectric crystal, there is no spatial modulation of the spontaneous polarization (only dipole density is periodic). Therefore, at the transition from a paraelecttic to the ferroelectric phase, both the wavevector q for osciUatimis of imis responsible for polarization and the correspondent oscUlatimi frequency co = Kef tend to zero. We may say that the soft elastic mode in ferroelecttics condenses at q 0. In antiferroelectrics, the sign of the local polarization Pq alternates in space with wavevector qo = 2nl2l = n/l and the corresponding imi oscillation frequency is finite, m = Kqf = Kn ll. It means that in antiferroelectrics the soft mode condenses at a finite wavevector n/l and rather high frequency. As a result, in the temperature dependence of the dielectric permittivity at low frequencies, the Curie law at the phase transitimi between a paraelecttic and antiferroelectric phases is not well pronounced. 

By a proper treatment of the electrodes, one can obtain a texture with a uniform orientation of the smectic normal in one direction within the cell plane. Between the crossed polarizers such a cell will be black if a polarizer is installed parallel to the smectic normal. Upon application of the electric field, the antiferroelectric structure becomes distorted. At low voltages of any polarity, the electrooptic response is proportional to E the bottom part of the curves has symmetric parabolic form [35] shown in Fig. 13.24b. Above the AF-F transition, the director acquires one of the two symmetric angular positions ( 9 on the conical surface) typical of the SmC phase. At these two extreme positions the transmission is maximum. With increasing temperature from T toTi the AF-F threshold decreases due to a decrease of the potential barrier separating structures with alternating and uniform tilt. It is natural because within the SmC A phase T1 is closer to the range of the SmC phase than T2 or T3. 

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. 

Orihara, H., Ishibashi, Y. Electro-optic effect and third rudernrailinear response in antiferroelectric liquid crystals. J. Phys. Soc. Jpn. 64, 3775-3786 (1995)... 

A. D.L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe, A. Fukuda Antiferroelectric Chiral Smectic Phases Responsible for the Tristable Switching in MHPOBC, Jpn. J. Appl. Phys. 28, L1265 (1989)... 

Figure 5. Response of polar dielectrics (containing local permanent dipoles) to an applied electric field from top to bottom paraelectric, ferroelectric, ferrielectric, antiferroelectric, and helielectric (helical anti-ferroelectric). A pyroelectric in the strict sense hardly responds to a field at all. A paraelectric, antiferro-electric, or helieletric phase shows normal, i.e., linear dielectric behavior and has only one stable, i.e., equilibrium, state for E=0. A ferroelectric as well as a ferrielectric (a subclass of ferroelectric) phase shows the peculiarity of two stable states. These states are polarized in opposite directions ( P) in the absence of an applied field ( =0). The property in a material of having two stable states is called bistability. A single substance may exhibit several of these phases, and temperature changes will provoke observable phase transitions between phases with different polar characteristics.

The response of an antiferroelectric is shown two diagrams below. The initial macroscopic polarization is zero, just as in a normal dielectric and the P- relation is linear at the beginning until, at a certain thresh-... 

At the limit where the hysteresis loops shrink to thin lines, as in the diagram at the bottom, we get the response from a material where the dipoles are ordered in a helical fashion. Thus this state is ordered but has no macroscopic polarization and therefore belongs to the category of antiferroelectrics. It is called helical antiferroelectric or heliec-tric for short. If an electric field is applied perpendicular to the helical axis, the helix will be deformed as dipoles with a direction almost along the field start to line up, and the response P-E is linear. As in the normal antiferroelectric case, the induced P value will be relatively modest until we approach a certain value of E at which complete unwinding of the helix takes place rather rapidly. Although the helielectric is a very special case, it shares the two characteristics of normal antiferroelectrics to have an ordered distribution of dipoles (in... 

In the middle diagram of Fig. 5 we have also traced the response for the modification of an antiferroelectric, which we get in the case where the two sublattices have a different polarization size. This phase is designated ferrielectric. Because we have, in this case, a macroscopic polarization, ferrielectrics are a subclass of ferroelectrics. It also has two stable states as it should, although the spontaneous macroscopic polarization is only a fraction of that which can be induced. If the polarization values of the sublattices are F, and P2saturation polarization after sublattice reversal is (F + P2). 

Chiral chains play a critical role in determining the stability of the antiferro-electricity. The variety of chains is rather limited. The most common chiral chain is - CH(CF3)C H2 +i, which stabilizes the antiferroelectricity much more than does - CH(CH3)C H2 +i, as illustrated in Figure 28 the odd-even effect is clearly observed in - CH(CH3)C H2 +i, while only the SmC phase occurs in - CH(CF3) C H2 +1. Trifluoromethylalk-oxyalkyl chains, - CH(CF3)Q H2mOC H2 +i, give a low threshold field and fast response... 

In polymer 76 (Fig. 90), which contains a chiral oxiranc ring in the spacer, a three-state switching was observ in the Sc phase [59,118]. In SSFLC ceUs thb polymer shows a characteristic stripe texture. The electrooptical response exhibits typical antiferroelectric properties, ix., two current response peaks, an optical and electrical double... 

The mesogens are either packed in closed cylinders that arrange in a hexagonal smectic layer (columnar hexagonal (Colh) liquid crystal phase), or they form a closed-sphere cubic (Cub) liquid crystal phase (Fig. 9.32a) [149, 150]. The Colh phase has a spontaneous polarization that is in the column axis direction and shows an antiferroelectric response. Dissipative structures appear specifically in systems without molecular asymmetry. That is, as described in Fig. 9.32b, when asymmetric molecules are packed in layers, a difference in packing density occurs in the lower and upper halves of the layer [150]. 

---