To be or not to be polar: the ferroelectric and antiferroelectric nematic phases

TL;DR

This paper explores the properties of new compounds exhibiting ferroelectric and antiferroelectric nematic phases, highlighting how molecular chain length influences phase behavior and dielectric properties.

Contribution

It introduces two new series of compounds with tunable ferroelectric and antiferroelectric nematic phases based on terminal chain length variations.

Findings

Longer chains weaken dipole interactions and lower ferroelectric phase transition temperature.

Intermediate chain lengths exhibit a wide temperature range of antiferroelectric nematic phase.

Dielectric studies reveal weak softening of ferroelectric mode at the phase transition.

Abstract

We report the properties of two new series of compounds that show the ferroelectric nematic phase in which the length of a terminal chain 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 ferroelectric nematic phase is formed. For homologues of intermediate chain length, between the non-polar and ferroelectric nematic phases, there is a wide temperature range nematic phase with antiferroelectric character. The size of the antiparallel ferroelectric domains critically increases upon transition to the ferroelectric phase. In dielectric studies, both collective ("ferroelectric" and non-collective fluctuations are present, 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, typical critical behavior is observed.