Thermotropic reentrant isotropy and induced smectic antiferroelectricity in the ferroelectric nematic realm: comparing RM734 and DIO

Abstract

The current intense study of ferroelectric nematic liquid crystals was initiated by the observation of the same ferroelectric nematic phase in two independently discovered organic, rod-shaped, mesogenic compounds, RM734 and DIO. We recently reported that the compound RM734 also exhibits a monotropic, low-temperature, apolar phase having reentrant isotropic symmetry (the I_R phase), the formation of which is facilitated to a remarkable degree by doping with small (below 1%) amounts of the ionic liquid BMIM-PF₆. Here we report similar phenomenology in DIO, showing that this reentrant isotropic behavior is not only a property of RM734 but is rather a more general, material-independent feature of ferroelectric nematic mesogens. We find that the reentrant isotropic phases observed in RM734 and DIO are similar but not identical, adding two new phases to the ferroelectric nematic realm. These phases exhibit similar, strongly peaked, diffuse X-ray scattering in the WAXS range (1 < q < 2 Å⁻¹) indicative of a distinctive mode of short-ranged, side-by-side molecular packing. The scattering at small q is, however, quite different in the two materials, with RM734 exhibiting a strong, single, diffuse peak at q ∼ 0.08 Å⁻¹ indicating mesoscale modulation with ∼80 Å periodicity, and DIO a sharper diffuse peak at q ∼ 0.27 Å⁻¹ ∼ (2π/molecular length), with second and third harmonics, indicating that in the reentrant isotropic phase of DIO (which we denote I_R^lam), short-ranged molecular positional correlation is smectic layer-like. Both reentrant isotropic phases are metastable, eventually generating birefringent crystals.