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*) between two tilted smectic liquid-crystal phases, C* and X. The C* phase has a helicoidal structure whereas the X phase does not. In the P* phase the layer spacing increases continuously from 32.3 {} to nearly 37 {} 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*, being helielectric, is not bistable. Furthermore, for fields larger than 1 V/{μ}m, the switching times in C* and P* increased with increasing voltages.