Ferroelectric-Ferroelastic Phase Transition in a Nematic Liquid Crystal

Abstract

Ferroelectric ordering in liquids is a fundamental question of physics. Here, we show that ferroelectric ordering of the molecules causes formation of recently reported splay nematic liquidcrystalline phase. As shown by dielectric spectroscopy, the transition between the uniaxial and the splay nematic phase has the characteristics of a ferroelectric phase transition, which drives an orientational ferroelastic transition via flexoelectric coupling. The polarity of the splay phase was proven by second harmonic generation (SHG) imaging, which additionally allowed for determination of the splay modulation period to be of the order of 5 -10 microns, also confirmed by polarized optical microscopy. The observations can be quantitatively described by a Landau-de Gennes type of macroscopic theory. SI A. Material RM734 was synthesized according to Ref.[1] and additionally purified as described in Ref. [2]. As determined by means of differential scanning calorimetry phase transition temperatures have been determined to be: isotropic to nematic TIN= 187.9 °C, nematic to second nematic transition at TNNs = 132.7 °C, and a melting point at Tm = 139.8 °C. We used the Gaussian G09e01 suite of programs [3] to determine the B3LYP/6-31G(d) minimized geometry of RM734 shown in Fig.1. The molecular dipole moment calculated by the B3LYP/6-31G(d) level of DFT is of 11.3748D and oriented almost along the molecular long axis.

Chemically Induced Splay Nematic Phase with Micron Scale Periodicity

“…To a first approximation, the standard deviation in our measurement can be understood to result from two unrelated effects: a profile being taken at an angle other than 90 ° to the modulation period (giving larger values), and the plane of the splay modulation period not being perpendicular to the plane of the substrate (giving smaller values); in future the contribution of both of these effect could probably be minimised by some form of surface treatment of the substrate, however for the purpose of this communication the precision of these present measurements is sufficient. We are encouraged that our results agree with the measured splay periodicity recently reported by Sebastián et al 23 Typical splay-nematic materials, such as RM734 and the closely related compound 2, possess a minimal aliphatic content and a large transverse molecular electric dipole moment. The short chain means that the molecules are, to a first approximation, rod-or wedge-shaped.…”

Development of ferroelectric nematic fluids with giant-ε dielectricity and nonlinear optical properties

“…To date, the detailed structure of the N F phase remains unclear, and how the structure is developed from the high-temperature N phase with the uniform director field is unknown. Note that while earlier works (17,18) have already triggered intensive theoretical studies (20)(21)(22), the true structure still requires further confirmation. To systematically correlate between the structures and the emergence of polarity, we simultaneously measured POM, SHG, and dielectric measurements for our synthesized materials upon varying temperature.…”

“…1), exhibits, e.g.,  ~ 10 4 at 1 kHz characterized by undefined polar structures and inherent high fluidity (14). At almost the same period, Mandle et al (15)(16)(17)(18) reported an unknown liquid crystalline phase state in RM734 (1a in Fig. 1), which has been assigned to be splay nematic later.…”

First-principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics

“…In both cases, the molecules were rod-shaped, with several intramolecular dipoles distributed along their length whose projections onto the molecular long axis summed to a large overall axial dipole moment of ∼10 Debye. The high-temperature phase of both mesogens was reported to be a typical nematic, but they exhibited dramatic paraelectric (30,29) and ferroelastic (30) pretransitional effects, with a dielectric constant surpassing 1,000 as the transition to the lowtemperature phase was approached. The low-temperature phase exhibited enhanced dipolar molecular associations, reported to be antiparallel in the Mandle system (28) and suggested to be parallel in the Kikuchi system, the latter being termed "ferroelectric-like" (29), giving macroscopic polar ordering in response to an applied electric field.…”

“…Mandle et al subsequently synthesized a number of homologs of their molecule in an effort to develop structure-property relationships for this phase (31) and pursued, in collaboration with the Ljubljana group, a series of physical studies on one of these (RM734), shown in Fig. 1A, leading to the claim that this phase was locally polar, as evidenced by second harmonic generation, but, on some longer scale, an antiferroelecric splay nematic (32,33,30,34,35), a modulated phase stabilized by local director splay, of the type originally proposed by Hinshaw et al (36).…”