Thermotropic reentrant isotropic symmetry and induced smectic antiferroelectricity in the ferroelectric nematic material RM734

We report a transition from the ferroelectric nematic liquid crystal (N_F) phase to a lower-temperature, apolar fluid phase having reentrant isotropic symmetry (I_R), in the liquid crystal compound RM734 doped with small concentrations of the ionic liquids 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆) or 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI). Even a trace amount of ionic liquid dopant facilitates the kinetic pathway for the transition from the N_F to the I_R, enabling simple cooling to produce this isotropic fluid phase rather than resulting in immediate crystallization. The I_R 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. Ionic liquid doping also stabilizes the smectic Z_A, an additional birefringent antiferroelectric phase having the director parallel to fluid smectic layers, significantly increasing its temperature range between the paraelectric and ferroelectric nematic phases with increasing BMIM concentration.