The smectic Z _A phase: Antiferroelectric smectic order as a prelude to the ferroelectric nematic

Abstract

We have structurally characterized the liquid crystal (LC) phase that can appear as an intermediate state when a dielectric nematic, having polar disorder of its molecular dipoles, transitions to the almost perfectly polar-ordered ferroelectric nematic. This intermediate phase, which fills a 100-y-old void in the taxonomy of smectic LCs and which we term the “smectic Z A ,” is antiferroelectric, with the nematic director and polarization oriented parallel to smectic layer planes, and the polarization alternating in sign from layer to layer with a 180 Å period. A Landau free energy, originally derived from the Ising model of ferromagnetic ordering of spins in the presence of dipole–dipole interactions, and applied to model incommensurate antiferroelectricity in crystals, describes the key features of the nematic–SmZ A –ferroelectric nematic phase sequence.

Spontaneous mirror symmetry breaking and chiral segregation in the achiral ferronematic compound DIO

“…Homogeneous and uniform textures are observed for all temperature/phases as reported earlier in different studies 3,9–11 except that the N F phase shows a sharp texture transition at a temperature of ∼3 °C below the N X –N F transition temperature. This behavior is discussed in Section 2.3.…”

“…N X is found to have modulated structure at the nanoscale level, and it exhibits smectic-like SmZ A antiferroelectric behaviour. 9 The nomenclature for the phases of DIO adopted here follows from the works of Nishikawa et al 3 and of Chen et al 9 The N X phase is also termed as the splayed nematic (N S ) by Sebastian et al, 4,10 and as unknown N X by Gorecka et al 11 Hereafter we refer it to N X for the reasons given above. DIO was first synthesized by Nishikawa et al 3 but for the works reported here, it is resynthesized by the group of G. H. Mehl at the University of Hull, Hull, UK.…”

“…The temperature dependence of the birefringence indicates that the high-temperature nematic phase is a conventional nematic. 3,[9][10][11] However, one observes that this phase surprisingly is also dielectrically active, i.e. the phase shows up collective relaxation processes explained by the existence of some polar groups (clusters/domains).…”

“…The intermediate M2 phase shows field-induced polarization properties. This compound has recently been reexamined by X-ray diffraction 9 and it was observed that M2 is a density modulated phase, with a period of 8.6 nm with a slightly splayed director distribution, designated as antiferroelectric SmZ A . However, Sebastian et al 10 found the structure of SmZ A is identical to the splay N S phase, 4 thus belonging to the class of modulated nematic phases.…”

Achieving enhanced second-harmonic generation in ferroelectric nematics by doping D–π–A chromophores

“…2b-e). 16,45 When the concentration was further increased, phase separation occurred (Fig. S2, ESI †).…”