Antiferroelectric phases

By our definition, the tilt plane is normal to the polarization in the ferroelectric state in the illustration in Figure 8.13 this is a vertical plane normal to the plane of the page. Since there is no tilt of the director projected onto this plane, the phase should be considered a type of SmA. We name this structure SmAPp (an untilted polar smectic the subscript F referring to a ferroelectric structure, in this case a ferroelectric state of an antiferroelectric phase). The antiferroelectric phase is therefore also an SmA denoted SmAPA (the subscript A for antiferroelectric). While this idea is certainly intriguing, no such antiferroelectric has yet been discovered. 

Apparently this switching mode is disfavored since, in fact, the chirality of the layers does not change upon switching to the ferroelectric state rather the layer interface clinicity changes. This occurs when the molecules in alternate layers simply precess about the tilt cone in a manner exactly analogous to antiferroelectric to ferroelectric switching in the chiral SmC phase. As shown in Figure 8.25, the ferroelectric state obtained from the ShiCsPa antiferroelectric phase is a ShiCaPf structure, an achiral macroscopic racemate with anticlinic layer interfaces. 

This interpretation of the properties exhibited by the major EO-active phases of NOBOW and the rest of the classic banana mesogens posits the existence of four supermolecular diastereomers, at least two of which, the antiferroelectric phases SmCsPA and ShiCaPa, must exist as separate wells on the... 

Squaric acid (H2SQ) has been chosen as a first test compound because it has a very simple molecular structure. Planar sheets of the squarate (C4O4) groups are linked to each other in a two-dimensional network through O - H...0 bonds (Fig. 1) with weak van der Waals forces [52,53]. The protons perform an order/disorder motion above the antiferroelectric phase transi-... 

Figure 2 shows the schematic structure in the paraelectric (T > Tn) and an-tiferroelectric (T < Tn) phases, hi the paraelectric phase the time-averaged position of the H atoms hes in the middle of an O - H...0 bond, whereas in the antiferroelectric phase, the protons locahze close to one or the other O atom. Prior to the recent NMR work [20-25], the largely accepted model of the phase transition was that the phase transition involved only the ordering of the H atoms in the O - H...0 bonds, and no changes in the electronic structure of the C4 moieties were considered to take place. The NMR results show that, in addition to the order/disorder motion of the H atoms, the transition also involves a change in the electronic charge distribution and symmetry of the C4 squares. 

Fig. 7 CP-MAS spectra of squaric acid, peak assignment and temperature variation in the close vicinity of its paraelectric-antiferroelectric phase transition [24], Note the pronounced coexistence of the spectra from the two phases...

Fig. 18 Temperature dependence of the CPMAS spectra of NH4H2ASO4 around the antiferroelectric phase transition temperature. The peaks corresponding to the paraelec-tric and antiferroelectric phases are labelled P and AF, respectively...

Fig. 19 Temperature evolution of the paraelectric and antiferroelectric phases around the phase transition temperature of NH4H2ASO4, as obtained from Fig. 18...

The temperature dependence of 1/Ti for is shown in Fig. 21. The discontinuity in the l/Ti data near Tn 216 K (highhghted by the arrow) appears at the onset of the antiferroelectric phase transition. Below Tn l/Ti increases abruptly from about 370 ms to 700 ms. 

Pociecha D, Gorecka E, Cepic M, Vaupotic N, Gomola K, Mieczkowski J (2005) Paraelectric-antiferroelectric phase transition in achiral liquid crystals. Phys Rev E 72 060701R... 

All of structures (3) relate to antiferroelectric phase that is in an agreement with the available measurements evidencing just the same character of low-temperature phase in the M3D(A04)2 crystals [6], It is also evident that the doubling of b-parameter of the paraelectric A2/a cell under transition to the low-temperature D-ordered phase directly follows from proposed scheme (3). Recently, such fe-doubling is observed experimentally in [7]. This scheme is also consistent with the doubling of c-parameter of A2/a cell found in the cited paper. In particular, such fe-doubling takes place when the layer sequence (3) takes the form ... 

On the other hand, the proton potential of the 5-bromo compound is exactly symmetrical with reference to the reaction coordinate of the tautomerization. Consequently, the proton transfer can proceed through the tunnelling mechanism. This is the reason why the paraelectric behaviour is maintained even at 4 K. The suppression of the antiferroelectric phase transition may be derived from a quantum tunnelling effect. Such paraelectric behaviour can be regarded as quantum paraelectricity , which is a notion to designate the phenomenon that (anti)ferroelectric phase transitions are suppressed even at cryogenic temperatures due to some quantum-mechanical stabilization, proton tunnelling in this case. 

Application of high external pressures influences the transition temperature to the antiferroelectric phase (Yasuda et al., 1979 Samara and Semmingsen, 1979). The Tq becomes lower as the applied pressure increases. Under an ultra-high pressure of about 3 GPa, the antiferroelectric transition itself disappears and the high dielectric constant of ca. 200 is maintained even at cryogenic temperatures (Moritomo et al., 1991). Since Raman diffraction measurements under 3-4.5 GPa revealed that squaric acid exists still as an alternating bond form, the tautomerization coupled with intermolecular proton transfer occurs even at low temperatures (Moritomo et al., 1990). 

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. 

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