Helielectric-ferroelectric transition mediated by a tilt-suppressing intermediate phase in liquid crystals

In the newly synthesized compound MORA12 [S-4-0-(6 methyl)octylresorcylidene-4’-dodecylaniline], we observed a novel intermediate phase (which we call $P^{\mathrm{*}}$) between two tilted smectic liquid-crystal phases, $C^{\mathrm{*}}$ and X. The $C^{\mathrm{*}}$ phase has a helicoidal structure whereas the X phase does not. In the $P^{\mathrm{*}}$ phase the layer spacing increases continuously from 32.3 Å to nearly 37 Å in a 10 °C temperature interval. This is a large temperature dependence of a lattice parameter which we attribute to the onset of hexagonal (as opposed to rectangular) in-plane ordering for which there is no easy axis for tilt: The tilt angle decreases as the degree of hexagonal in-plane order increases. The X phase is a true ferroelectric phase with a bistable electro-optic response whereas $P^{\mathrm{*}}$, being helielectric, is not bistable. Furthermore, for fields larger than 1 V/μm, the switching times in $C^{\mathrm{*}}$ and $P^{\mathrm{*}}$ increased with increasing voltages.