Thermotropic reentrant isotropy and antiferroelectricity in the ferroelectric nematic material RM734

TL;DR

This study discovers a reentrant isotropic phase and an antiferroelectric phase in the ferroelectric nematic liquid crystal RM734, influenced by ionic doping and temperature cycling, revealing complex phase behavior.

Contribution

It reports the first observation of a reentrant isotropic phase and an antiferroelectric phase in RM734, including effects of ionic doping and temperature cycling.

Findings

Reentrant isotropic phase ($I_A$) observed in RM734 with ionic doping.

Antiferroelectric phase with lamellar structure identified.

Ionic doping and temperature cycling facilitate phase transitions.

Abstract

We report a transition from the ferroelectric nematic liquid crystal ($N_F$) phase to a lower-temperature, antiferroelectric fluid phase having reentrant isotropic symmetry ($I_A$), in the liquid crystal compound RM734 doped with small concentrations of the ionic liquids BMIM or EMIM. Even a trace amount of ionic liquid dopant facilitates the kinetic pathway for the transition from the $N_F$ to the $I_A$, enabling simple cooling to produce this isotropic fluid phase rather than resulting in crystallization. The $I_A$ was also obtained in the absence of specific ionic liquid doping by appropriate temperature cycling in three distinct, as-synthesized-and-purified batches of RM734, two commercial and one from our laboratory. An additional birefringent, lamellar-modulated, antiferroelectric phase with the director parallel to the layers, resembling the smectic $Z_A$, is found between the paraelectric and ferroelectric nematic phases in RM734/BMIM mixtures.