To Be or Not To Be Polar: The Ferroelectric and Antiferroelectric Nematic Phases

We report two new series of compounds that show the ferroelectric nematic, N-F, phase in which the terminal chain length is varied. The longer the terminal chain, the weaker the dipole-dipole interactions of the molecules are along the director and thus the lower the temperature at which the axially polar N-F phase is formed. For homologues of intermediate chain lengths, between the non-polar and ferroelectric nematic phases, a wide temperature range nematic phase emerges with antiferroelectric character. The size of the antiparallel ferroelectric domains critically increases upon transition to the N-F phase. In dielectric studies, both collective ("ferroelectric") and non-collective fluctuations are present, and the "ferroelectric" mode softens weakly at the N-N-X phase transition because the polar order in this phase is weak. The transition to the N-F phase is characterized by a much stronger lowering of the mode relaxation frequency and an increase in its strength, and a typical critical behavior is observed.