Fig. 1. （a）Molecular arrangement in conventional nematic phase；（b）Molecular arrangement in ferroelectric nematic phase.

Fig. 2. （a）Molecular formula and phase transition temperature of DOBAMBC^［24］；（b）Molecular arrangement of chiral layered smectic phase^［54］.

Fig. 3. （a）Molecular structural formula of polar rod-like aromatic copolyesters of HBA and HNA^［56］；（b）Phase transition relationship among isotropic，nematic and ferroelectric nematic phases in disk-shaped molecules with permanent dipole moment^［59］；（c）Phase diagram of a hard sphere-column system with dipole interactions^［58］.

Fig. 4. （a）Molecular structural formula of DIO^［44］ and RM734^［67］；（b）Dielectric spectrum of DIO（In dielectric spectrum of DIO，the M1 phase exhibits a uniform orientation after surface alignment，and therefore the symbols ε∥' and ε⊥' are used；however，for the M2 and MP phases，the orientation is not perfect and ε(∥)' and ε(⊥)' are used.‍）^［44］；（c）Polarized light micrograph of DIO^［44］；（d）Freedericksz torsional transition in ferroelectric domains with opposite polarity orientations in RM734^［67］.

Fig. 5. Schematic diagram and phase diagram of the helical ferroelectric nematic phase^［68］

Fig. 6. Structural tunning of RM734^［69］.（a）Length of the terminal group；（b）Electron-withdrawing effect of the terminal group；（c）Side group；（d）Direction of the linking unit；（e）Molecular structural formula of liquid crystal molecule 1^［71］.

Fig. 7. （a）Synthetic molecular library；（b）~（h）Machine learning-driven analysis of molecular parameters toward stable N_F phases^［72］.

Fig. 8. （a）Chemical structures of main-chain LC oligomer/polymer；（b）Schematic diagram of asymmetric mesogens；（c）Corresponding phase diagrams^［74］.

Fig. 9. Schematic diagram of the splay nematic phase^［66］

Fig. 10. （a）3D model structure of N phase and N_F phase；（b）POM and simulation image of N_F phase fringe texture^［72］.

Fig. 11. Schematic diagrams of（a）N phase and（b）N_F phase liquid crystals in a circular rubbed cell（R and L denote right- and left-handed twist regions，respectively）；Disclination lines of RM734 in a circular rubbed cell in（c）N phase and（d）N_F phase^［78］.

Fig. 12. （a）Dielectric spectrum of RM734^［67］；（b）Road maps of both the dielectric constant versus the loss tangent of N_F in synthetic molecular library^［72］.

Fig. 13. Temperature dependence of（a）elastic constant K_i and（b）viscosity η_i of RM734 near N~N_F phase transition^［66］；（c）Opening elastic constant and（d）opening elastic constant of RM734 and RM734-CN in nematic phase viscosity^［90］.

Fig. 14. （a）P-E hysteresis loop of DIO and RM734^［92］；（b）P-E hysteresis measurement of Nt₂^［74］.

Fig. 15. （a）SHG curves of N and N_F phases；（b）Schematic diagram of dielectric constant and nonlinear coefficient of N_F in synthetic molecular library^［72］.

Fig. 16. SHG-I microscope system^［72］.（a）Schematic diagram of optical path；（b）SHG micrograph and interference curve.

Fig. 17. SHG-I microscopic detection of helielectric nematic phase（HN^*）‍^［68］.（a）Schematic diagram of optical path；（b）Imaging comparison of ordinary SHG and SHG-I；（c）Polarized light of HN^* and SHG- I micrograph；（d）Signal contrast curve of ordinary SHG and SHG-I.

Fig. 18. Electro-optic evidence of ferroelectricity in the N_F phase（3.2 μm thick sample）of planarly aligned Nt₂ observed by POM^［74］