New examples of ferroelectric nematic materials showing evidence for the antiferroelectric smectic-Z phase

We present a new ferroelectric nematic material, 4-((4′-((trans)-5-ethyloxan-2-yl)-2′,3,5,6′-tetrafluoro-[1,1′-biphenyl]-4-yl)difluoromethoxy)-2,6-difluorobenzonitrile (AUUQU-2-N) and its higher homologues, the molecular structures of which include fluorinated building blocks, an oxane ring, and a terminal cyano group, all contributing to a large molecular dipole moment of about 12.5 D. We observed that AUUQU-2-N has three distinct liquid crystal phases, two of which were found to be polar phases with a spontaneous electric polarization Ps of up to 6 µC cm–2. The highest temperature phase is a common enantiotropic nematic (N) exhibiting only field-induced polarization. The lowest-temperature, monotropic phase proved to be a new example of the ferroelectric nematic phase (NF), evidenced by a single-peak polarization reversal current response, a giant imaginary dielectric permittivity on the order of 103, and the absence of any smectic layer X-ray diffraction peaks. The ordinary nematic phase N and the ferroelectric nematic phase NF are separated by an antiferroelectric liquid crystal phase which has low permittivity and a polarization reversal current exhibiting a characteristic double-peak response. In the polarizing light microscope, this antiferroelectric phase shows characteristic zig-zag defects, evidence of a layered structure. These observations suggest that this is another example of the recently discovered smectic ZA (SmZA) phase, having smectic layers with the molecular director parallel to the layer planes. The diffraction peaks from the smectic layering have not been observed to date but detailed 2D X-ray studies indicate the presence of additional short-range structures including smectic C-type correlations in all three phases—N, SmZA and NF—which may shed new light on the understanding of polar and antipolar order in these phases.

Figure S1: Differential scanning calorimetry of AUUQU-2-N measured with cooling rates of (a) -10 K min -1 and (b) -50 K min -1 .The signal corresponding to the transition from N to SmZA only becomes observable after scanning with a high cooling rate.As a result of the monotropic nature of both the NF and SmZA phase, these phases only appear in the cooling process.Figure S7: Sector averages with limited integration over  of the X-ray data in Figure S6 for the homologous series AUUQUn-N (n = 2, 3, 5, 7).The integrations were carried out on the meridian and on the equator, to distinguish the side-to-side and end-to-end stacking of the mesogens.

Figure S2 :
Figure S2: Differential scanning calorimetry of AUUQU-n-N (n = 3, 5, 7; (a) -(c)) measured with a cooling rate of -10 K min -1 .In (a) AUUQU-3-N the transition into the antiferroelectric phase is not observable even with high cooling rates.For (b) AUUQU-5-N, a slight enthalpy change can be observed at around 35 °C.A transition into the SmZA phase is also observed for (c) AUUQU-7-N starting at around 27 °C.All of the transitions from the ordinary nematic to the antiferroelectric nematic phase are associated with only a miniscule change in enthalpy and are of monotropic nature.

Figure S3 :
Figure S3: Measurements of the dielectric susceptibility ′ of AUUQU-n-N (n = 2, 3, 5, 7; (a) -(d)) measured in 20 µm thick cells with polyimide and ITO coating.All measurements were done while cooling.For (a) AUUQU-2-N, there is an increase in ′ just before the transition into the SmZA phase, with a narrow notch separating the ferroelectric from the ordinary nematic phase.For the other homologues, ′ also drops rapidly once the antiferroelectric phase is reached, showing similar behavior to AUUQU-2-N.

Figure S5 :
Figure S5: Optical textures of AUUQU-2-N in the NF phase in 5 µm thick cells coated with polyimide and rubbed antiparallel on the two surfaces for planar alignment (all scale bars are 200 µm).In the NF phase, millimeter-sized chiral domains are formed (a), the twist sense of which can be determined by decrossing the polarizers ((b) and (c)).

Figure S6 :
Figure S6: Wide-angle 2D X-ray patterns of selected homologs in the AUUQU-n-N series (n = 2, 3, 5, 7, (a) -(d)) obtained with a Bruker AXS NanoSTAR system.The images were taken in (a) the NF and (b) -(d) the SmZA phase.The patterns are similar, but the small-angle signal is split to different extents.The grey cones represent different sector averages in  that focus on either the additional signals along the meridian (i) or the equatorial scattering arc (ii).

Figure S8 :
Figure S8: Sector averages (i) with limited integration over  of the X-ray data in Figure S6 for AUUQU-2-N over the full temperature range.

Figure S9 :
Figure S9: Synchrotron-based X-ray scattering of AUUQU-2-N in the N, SmZA and NF phases obtained on the SMI microbeam line at NSLS II.The upper row depicts a broader q range.A rotation of the diffraction pattern can be observed in the SmZA phase.