Abstract
Different liquid crystalline phases with long-range orientational but not positional order, so-called nematic phases, are scarce. New nematic phases are rarely discovered, and such an event inevitably generates much interest. Here, we describe a transition from a uniaxial to a novel nematic phase characterized by a periodic splay modulation of the director. In this new nematic phase, defect structures not present in the uniaxial nematic phase are observed, which indicates that the new phase has lower symmetry than the ordinary nematic phase. The phase transition is weakly first order, with a significant pretransitional behavior, which manifests as strong splay fluctuations. When approaching the phase transition, the splay nematic constant is unusually low and goes towards zero. Analogously to the transition from the uniaxial nematic to the twist-bend nematic phase, this transition is driven by instability towards splay orientational deformation, resulting in a periodically splayed structure. And, similarly, a Landau-de Gennes type of phenomenological theory can be used to describe the phase transition. The modulated splay phase is biaxial and antiferroelectric.
- Received 27 June 2018
- Revised 16 October 2018
DOI:https://doi.org/10.1103/PhysRevX.8.041025
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The behavior and properties of liquid crystals strongly depend on their molecular structure, so researchers are exploring new molecular architectures and shapes. The simplest of the liquid crystalline phases are the nematic phases, which can appear in materials made of elongated or disk-shaped molecules. The nematic phases are very similar to ordinary liquids, the only difference being that their molecules are, on average, oriented in the same homogeneous direction. Nematic phases are scarce, and new ones are rarely discovered. We report on a novel nematic phase made of wedge-shaped molecules, which organize in a manner that resembles an array of Japanese fans.
We use polarization microscopy, dynamic light scattering, and wide- or small-angle x-ray scattering to investigate the transition from an ordinary homogenous nematic phase to a new phase in a recently designed polar, rodlike, liquid crystalline material. X-ray scattering shows that the new phase is nematic, while optical observation of the transition reveals new defect structures, which appear at the transition and indicate the change of the phase symmetry. The transition shows distinct pretransitional behavior, evidenced by a strong tendency of the average molecular orientation towards fan-shaped deformation, which increases when approaching the transition. We conclude that this instability arises because the molecular shape provokes the transition.
The described phase is a major step forward towards the realization of an often-speculated polar nematic phase, which could lead to materials with optical and electrical behaviors desired for a wide range of applications. Thus, measurements of the phase periodicity and electrical properties are the next natural steps.
