Fig. 1.

(A) Representative members DIO [(2,3',4',5'-tetrafluoro-[1,1'-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxane-2-yl) benzoate] and RM734 [4-[(4-nitrophenoxy) carbonyl]phenyl2,4-dimethoxybenzoate] of nitro- and fluoro-based molecular families of molecules that exhibit the ferroelectric nematic phase. This behavior is attributable in part to their rod-like molecular shape, large longitudinal dipole moment, and a common tendency for head-to-tail molecular association. (B) Phase diagram of DIO and RM734 and their binary mixtures exhibiting: (C) Paraelectric nematic (N) phase, in which the molecular long axes are aligned in a common direction but with no net polarity, making a nonpolar phase with a large dielectric response. (D) Ferroelectric nematic (N_F) phase, in which the polar orientation of dipoles is unidirectional and long-ranged, with a large polar order parameter (~0.9). The N_F phase is uniaxial and spatially homogeneous, with a macroscopic polarization/director field P(r), shown here for antiparallel polar orienting surface alignment layers on the plates (9). The phase diagram shows that the transition temperature into the N_F depends nearly linearly on concentration c, indicating that these molecules exhibit near ideal mixing behavior of the transition into the ferroelectric nematic (N_F) phase. This transition is weakly first-order, with entropy ΔS ≈ 0.06R. (E) Intermediate phase observed in neat DIO and in DIO-rich mixtures. Originally called the M2 (6), we find this phase to be electron density-modulated into ~9-nm thick, polar layers, in which the director and polarization are parallel to the layer plane and alternate in direction from layer to layer, as sketched. We have termed this lamellar, antiferroelectric phase the smectic Z_A (SmZ_A). The SmZ_A can be prepared with the layers either normal to the plates (bookshelf geometry) or parallel to the plates, or with the chevron texture shown in Figs. 2 and  and SI Appendix Figs. 6-8.