Biferroelectricity of a homochiral organic molecule in both solid crystal and liquid crystal phases

Abstract

Ferroelectricity, existing in either solid crystals or liquid crystals, gained widespread attention from science and industry for over a century. However, ferroelectricity has never been observed in both solid and liquid crystal phases of a material simultaneously. Inorganic ferroelectrics that dominate the market do not have liquid crystal phases because of their completely rigid structure caused by intrinsic chemical bonds. We report a ferroelectric homochiral cholesterol derivative, β-sitosteryl 4-iodocinnamate, where both solid and liquid crystal phases can exhibit the behavior of polarization switching as determined by polarization-voltage hysteresis loops and piezoresponse force microscopy measurements. The unique long molecular chain, sterol structure, and homochirality of β-sitosteryl 4-iodocinnamate molecules enable the formation of polar crystal structures with point group 2 in solid crystal phases, and promote the layered and helical structure in the liquid crystal phase with vertical polarization. Our findings demonstrate a compound that can show the biferroelectricity in both solid and liquid crystal phases, which would inspire further exploration of the interplay between solid and liquid crystal ferroelectric phases.

Fig. 1. Guiding ideology for obtaining the ferroelectric 4I-CASS.

4I-CASS can show the biferroelectricity in both solid and liquid crystal phases. The light blue shapes represent the degree of molecular order in the solid, liquid crystal, and liquid phases. The inset molecule shows the molecular structure of 4I-CASS.

Fig. 2. Crystal structures of 4I-CASS.

a, b The asymmetric unit in phase I (a) and phase II (b), respectively. c Packing view in phase I along the [1 0 0] direction. d Diagram of the change in spatial symmetry operations from phase I to phase II. In phase I, the green symbols stand for the symmetry operation of two-fold screw axes. In phase II, the pink elliptical shape symbols denote the symmetry operation of two-fold rotation axes, while the other pink symbols represent the symmetry operation of two-fold screw axes. e Packing view in phase II along the [0 0 1] direction.

Fig. 3. Phase transitions of 4I-CASS.

a DSC curves of 4I-CASS. In the first heating and cooling runs, the phase below 342 K is phase I, the phase above 342 K but below 444 K is phase II, and the structural phase transition from phase I to phase II is completely reversible in the temperature range below the transition temperature of 444 K. In the second heating run, the phase between 444 and 490 K is the cholesteric LC phase (Ch), and the phase beyond 490 K is liquid phase (LP). In the second cooling run, an emerging LC phase between 389 and 431 K is SmC* phase, and a crystalline phase (phase III) is generated below 389 K. b Temperature-dependent PXRD patterns of 4I-CASS. c Polarized photomicrographs of 4I-CASS in phase I (300 K), phase II (353 K), Ch (453 K and 483 K), and LP (500 K) during the heating process, and Ch (473 K and 438 K), SmC* (423 K), and phase III (300 K) during the cooling process.

Fig. 4. Polarization switching for 4I-CASS.

a Scheme of the molecular arrangement in SmC* phase for 4I-CASS. The enlarged area of the red circle shows the molecular orientation between the adjacent three layers. The blue ellipses represent the 4I-CASS molecules, and the distance d between the adjacent yellow disks represents the layer spacing. The red, blue, green, and black arrows represent the molecular orientation vector, layer polarization direction, the supposed two-fold axis and normal direction, respectively. The distance between layers A and B with same molecular orientation vector is the helix pitch (HP). b Scheme of spontaneous polarization under applied electric field. c, d Polarization−voltage loops in the SmC* phase (c) and phase I (d) for 4I-CASS, respectively.

Fig. 5. PFM characterization of 4I-CASS in phase I.

a–c Topographic (a), lateral PFM amplitude (b), and phase (c) images mapped on the spin-coating thin film. d–f Topography (d), vertical PFM amplitude (e) and phase (f) images after applying a tip voltage of −100 V in the center area of a region with an initial state of a single domain state. g–i Topography (g), vertical PFM amplitude (h), and phase (i) images after applying +90 V tip voltage in the center region of the switched domain. The contrast of bright and dark in the topography images represents the height of the sample surface. The contrast of bright and dark in the amplitude images represents the magnitude of the piezoresponse. The violet and green regions in the phase images indicate the two different polarization-oriented states of ferroelectric domains. The white boxes indicate the area to which the tip voltage is applied.