The interplay between spatial and heliconical bond order in twist-bend nematic materials

TL;DR

This study investigates the nanostructure and phase behavior of novel sulfur-containing dimers in twist-bend nematic materials, revealing temperature-dependent changes in heliconical pitch and coexistence of smectic layering with heliconical order.

Contribution

It provides new experimental insights into the temperature dependence of heliconical structures and phase coexistence in twist-bend nematic materials, especially for sulfur-containing dimers.

Findings

Heliconical pitch varies strongly near the N-NTB transition.

Heliconical order persists into the smectic phase.

Coexistence of smectic layering and heliconical order was observed.

Abstract

The nanostructure of two novel sulfur containing dimer materials has been investigated experimentally by hard and by resonant tender X-ray scattering techniques. On cooling the dimers through the nematic to twist-bend nematic (N-NTB) phase transition, the correlation length associated with short-range positional order drops, while the heliconical orientational order becomes more correlated. The heliconical pitch shows a stronger temperature dependence near the N-NTB transition than observed in previously studied dimers, such as the CBnCB series of compounds. We explain both this strong variation and the dependence of the heliconical pitch on the length of the spacer connecting the monomer units by taking into account a temperature dependent molecular bend and intermolecular overlap. and. The heliconical structure is observed even in the upper 3-4{\deg}C range of the smectic phase that forms just below the NTB state. The coexistence of smectic layering and the heliconical order indicates a SmCTB -type phase where the rigid units of the dimers are tilted with respect to the layer normal in order to accommodate the bent conformation of the dimers, but the tilt direction rotates along the heliconical axis. This is potentially similar to the SmCTB phase reported by Abberley et al (Nat. Commun. 2018, 9, 228) below a SmA phase.