Modulated Anti-Ferroelectric Smectic Phases with Orthogonal and Tilted Structures

TL;DR

The study investigates anti-ferroelectric smectic phases, validating that SmA$_{AF}$ hosts a density modulation with a lateral length scale and reporting a newly identified SmC$_{AF}$ phase with a similar 1D modulation. By combining 1D/2D X-ray scattering, POM, DSC, current response, and SHG, the authors identify a 1D splay-like density modulation and index the SmAAF structure with a 2D rectangular lattice, while showing that SmCAF exhibits a 1D chevron-like modulation parallel to the tilt plane. In compound 1, the layer spacing $d_{01}$ shifts modestly with temperature while the lateral modulation length $d_{10}$ is ~8.4 nm; upon cooling into SmCAF, the tilt reaches ≈ $20^ obreak ext{o}$ within $4^ obreak ext{o}$C and $d_{10}$ increases, with satellites vanishing at the SmCAF→SmC$_ ext{P}^ ext{H}$ transition. Compound 2 shows a longer lateral modulation and similar progression, with both compounds maintaining the 1D modulation across the SmAAF→SmCAF transition, indicating a robust modulated state linked to strong flexoelectric coupling. Together, the results reveal a new class of polar smectic phases and a distinctive electric-field response arising from concurrent control of tilt and polarization, advancing the understanding of frustrated polar order in layered liquids.

Abstract

The discovery of the ferroelectric nematic phase has brought with it a plethora of new polar liquid crystalline phases. One in particular is the anti-ferroelectric smectic A SmA\textsubscript{AF} phase. In this letter we show via observation and analysis of satellite peaks in the X-ray scattering pattern that the structure of the SmA\textsubscript{AF} phase involves a density modulation of $\approx$10-20 nm lateral to the smectic layer normal. Further, we demonstrate a previously undiscovered phase where the anti-ferroelectric order is maintained into a tilted smectic phase demonstrating the robustness of the underlying frustration that leads to the modulated structure. We suggest that the modulations are only in a single dimension and appear parallel to the tilt plane. This new phase also shows a significantly different and complex response to an electric field from other discovered polar LC phases due to the ability to modulate both tilt and polarisation direction.

Figures (4)

• Figure 1: POM images of compound 1 in a) the SmA, b) the SmAAF, c) the SmCAF and d) the SmC$_\text{P}^\text{H}$ phases. POM images were taken in a 5 $\mu$m cell with syn-parallel rubbed planar aligned substrates. White arrows indicate polarisor and analyser orientations and black arrow the rubbing direction. Current response measurements for compound 1 in a 5 $\mu$m cell with no alignment layer taken with a 10 Hz, 20 VRMS triangular voltage profile e) over the SmA-SmAAF phases, f) the SmCAF and g) the SmC$_\text{P}^\text{H}$ phase.
• Figure 2: a) 1D X-ray scattering pattern for compound 1 in the SmAAF phase. b) Temperature dependence of the layer thickness with the inset showing a zoomed in region of the SmA and SmAAF phase. c) Temperature dependence of the lateral density modulation thickness. d) 1D X-ray scattering pattern with a different sample-detector distance showing the absence of a [1,0] peak for all phases. 1D X-ray scattering patterns for the e) SmCAF phase and the f) SmC$_\text{P}^\text{H}$ phase.
• Figure 3: Schematics of the proposed structure of the a) SmAAF and b) SmCAF phase. Regions of alternate polarisation are separated by Ising walls while in the SmCAF splay is limited to the regions mediating change of tilt sign. While the SmCAF schematic has been drawn fully chevron, in reality this structure is likely weaker and more sinusoidal. c) Current response measurement for compound 1 in the SmCAF phase in a 5 $\mu$m thick cell with no alignment layer. (1) and (4) show the saturated SmAF state, (2) shows the SmCP state and (3) shows the ground state SmCAF phase.
• Figure 4: a) 1D X-ray scattering pattern for compound 2 throughout both the SmAAF and SmCAF phase with the transition between the two at 90$^\circ$C. b) Temperature dependence of the smectic layer thickness,d01, with the inset showing the dependence of the lateral density modulation, d10.