Observation of a uniaxial ferroelectric smectic A phase

TL;DR

This paper reports the discovery of a new uniaxial ferroelectric smectic A liquid crystal phase with polar order and specific phase sequences in binary mixtures, expanding understanding of ferroelectric liquid crystal phases.

Contribution

It introduces the smectic $A_F$ phase, a novel ferroelectric smectic phase, and details its properties, formation conditions, and phase sequences in specific binary mixtures.

Findings

Identification of the smectic $A_F$ phase with polar order.

Observation of hysteretic polarization reversal.

Phase sequences in binary mixtures involving $A_F$ phase.

Abstract

We report the smectic $A_F$, a new liquid crystal phase of the ferroelectric nematic realm. The smectic $A_F$ is a phase of small polar, rod-shaped molecules which form two-dimensional fluid layers spaced by approximately the mean molecular length. The phase is uniaxial, with the molecular director, the local average long-axis orientation, normal to the layer planes, and ferroelectric, with a spontaneous electric polarization parallel to the director. Polarization measurements indicate almost complete polar ordering of the $\sim 10$ Debye longitudinal molecular dipoles, and hysteretic polarization reversal with a coercive field of about $2 \times 10^5$ V/m is observed. The smectic $A_F$ phase appears upon cooling in two binary mixtures of partially fluorinated mesogens: 2N/DIO, exhibiting a nematic ($N$) -- smectic $Z_A$ (Sm$Z_A$) -- ferroelectric nematic ($N_F$) -- smectic $A_F$ (Sm$A_F$) phase sequence; and 7N/DIO, exhibiting an $N$ -- Sm$Z_A$ -- Sm$A_F$ phase sequence. The latter presents an opportunity to study a transition between two smectic phases having orthogonal systems of layers.