Thermotropic reentrant isotropy and 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, antiferroelectric phase having reentrant isotropic symmetry (the $I_A$ phase), the formation of which is facilitated to a remarkable degree by doping with small (below $1\%$) amounts of the ionic liquid ${\mathrm{BMIM}-\mathrm{PF}}_6$. 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. The two $I_A$ phases exhibit similar, strongly peaked, diffuse x-ray scattering in the WAXS range $(1<q<2{\mathring{A}}^{-1})$ indicative of a distinctive mode of short-ranged, side-by-side molecular packing. The scattering of the $I_A$ phases at small $q$ is quite different in the two materials, however, with RM734 exhibiting a strong, single, diffuse peak at $q\sim 0.08{\mathring{A}}^{-1}$ indicating mesoscale modulation with $\sim 80\mathring{A}$ periodicity, and DIO a sharper diffuse peak at $q\sim 0.27{\mathring{A}}^{-1}\sim (2\pi /\mathrm{molecularlength})$, with second and third harmonics, indicating that in the $I_A$ phase of DIO, short-ranged molecular positional correlation is smectic layer-like.

main text 21 pages including 7 figures; supplement 9 pages including 6 figures. arXiv admin note: text overlap with arXiv:2309.04935

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Cited by in corpus (2)

• Twist, splay, and uniform domains in ferroelectric nematic liquid crystals
• Double Splay Nematic Order in Confined Polar Fluids