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

Abstract

We report two new series of compounds that show the ferroelectric nematic, NF, 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 NF 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 NF phase. In dielectric studies, both collective (“ferroelectric”) and non-collective fluctuations are present, and the “ferroelectric” mode softens weakly at the N–NX phase transition because the polar order in this phase is weak. The transition to the NF 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.

Ferroelectric nematogens containing a methylthio group

“…This is followed by a small step-like increase at the transition to the N F phase, indicative of an increase of the order parameter, S . This observation is in agreement with the birefringence measurements that we have reported previously for the N X phase, 8,9 also referred to as the smectic Z A phase. 5,62 Based on both the textural observations and this change in birefringence, the N X phase in S1 may be assigned as being the same phase shown by DIO and some related compounds.…”

“…7 (c). The pattern seen for the N X phase, although not showing the long-range periodicity we have reported previously, 9 does support its assignment as a nematic phase.…”

“…In XRD studies (these will be discussed in more detail later) the N X phase shows only short-range correlations of the molecular positions which are characteristic for nematic phases and hence why the phase is named as such. However, in previous diffraction experiments performed at synchrotron facilities very weak Bragg-type diffraction signals were detected in the N X phase, 9 revealing a kind of long-range periodicity. This periodicity was attributed to existence of regular domains with antiparallel orientation of the polarization, and so this phase is referred to by other groups as the antiferroelectric SmZ A phase.…”

“…2EC6F exhibits a N F –I transition at 132 °C. 9 This implies that the decrease in T NI on replacing the methoxy group by an ethyl group is at least 25 K and for the methylthio group at least 11 K. By contrast, the value of T NFI is just 10 K lower than T NFN of O8 and 8 K higher than that of S8 . At first sight the reduction in T NI between S8 and 2EC6F is surprising given that the ethyl group is more compact than the methylthio unit.…”

“…2,5–8 The N X phase is thought to consist of regions of polar nematic phase arranged in a regular antiferroelectric structure. 5,7,9…”