Modulated Hexatic-B* with giant electroclinic effect rather than anticlinic Hexatic-I _A * —A novel mechanism for stabilizing antiferroelectricity below Smectic-C _A *

Abstract

The antiferroelectric hexatic phase observed below antiferroelectric SmC * A has so far been considered to have the same anticlinic structure as in SmC *A . The orientational order parameter determined by infrared spectroscopy distinctively disproves the previous view and concludes that molecular long axes are not tilted from the SmA layer normal. The phase can be identified as non-tilted modulated HxB * and the electric-field-induced phase transition from this phase to tilted ferroelectric HxI * can be considered as the giant electroclinic effect due to the opening of closed modulation loops.

Molecular model for de Vries type smectic-A–smectic-Cphase transition in liquid crystals

“…We have selected the average < | cos γ| > as a simple surrogate of the layer spacing [21], i.e., we assume that the spacing can be approximated by the average projection of the molecules onto the layer normal. This assumption is supported by recent experimental data [22] which indicate that for several different compounds there exists a good correlation between the temperature vari- ation of the smectic period d and the order parameter S k . At the same time, < | cos γ| >≈ 1 − 0.5 < sin 2 γ > and S k = 1 − 1.5 < sin 2 γ >, which yields < | cos γ| >≈ (2 + S k )/3.…”

Renewed focus on the small temperature change of smectic layer spacing in ferroelectric and antiferroelectric LCs

“…Antiferroelectric hexatic phase does not always emerge below antiferroelectric SmC Ã A , but ferroelectric hexatic phase appears instead of antiferroelectric hexatic phase at around 50% MHOBMB. The phases they identified as HxI Ã A and HxI Ã must be antiferroelectric and ferroelectric modulated HxB's, respectively, for the smectic layer spacing changes with temperature as pointed out by Korlacki et al [18]. When both modulated HxB's emerge below SmC Ã A at about 63% MHOBMB, the antiferroelectric one always appears above the ferroelectric one; this is understandable as the formation of the closed contour loops needs an increase in entropy, although the emerging order is just opposite to the case of SmC Ã and SmC Ã A .…”

“…and then proceed to the unexpected conclusion made by Korlacki et al [18]: The 'antiferroelectric' phase that had been so far identified as 'anticlinic HxI Ã A ' is 'modulated HxB' and the electric-field-induced phase transition from this phase to tilted HxI Ã is the giant electroclinic effect due to the opening of closed modulation loops.…”

“…Actual modulation must be much more irregular and disordered, and the actual size, shape and ordering of concentric regions are interesting problems to be solved in the future, as pointed out by Sirota. [17] Using sophisticated IR spectroscopic technique, Korlacki et al [18] showed that the 'antiferroelectric' phase that has so far been identified as 'anticlinic HxI Ã A ' [11,40,41] should be better identified as 'modulated HxB' and that the electric-field-induced phase transition from this phase to ferroelectric synclinic HxI Ã can be considered as the giant electroclinic effect due to the opening of closed modulation loops. They prepared homeotropically aligned cells in SmA with the tilting directions in SmC Ã /SmC Ã A sufficiently random within the IR beam.…”

“…The wiggles of the contours are regular in (b-1), but can have a whole class of variants, not accessible in HxF, as illustrated in (b-2). (From Refs [17,18]…”