Giant Permittivity in Ferroelectric and Superparaelectric LCs

The long-awaited ferroelectric nematic LCs (NF) predicted more than a century ago were finally discovered in 2017, independently in two different materials: RM 734 [1] and DIO [2]. The two common features of these materials are (a) extremely high molecular dipole moment (µ ~10 D) and (b) giant dielectric permittivity (ε΄ ~ 10,000) in the ferroelectric N_F phase (Fig.1(a)). Two new compounds, WJ-16 and WJ-18, based on the chemical structure of DIO, were synthesized for a further enhancement of ferroelectricity by increasing the molecular dipole. As expected, both compounds exhibit extremely high dipole moments (10.6 D and 13.6 D respectively). Surprisingly the increase in dipole moment completely suppresses the ferroelectricity. However, giant permittivity was observed not only in nematic phase, but also in the SmA phases (see Fig.1(b)). After intensive study it is concluded that both samples are paraelectric rather than ferroelectric exhibiting colossal permittivity i.e. they belong to the class of super-paraelectrics (SPE) observed before only in the solid state. This it opens a new range of applications of liquid crystals as the working media for supercapacitors with the potential of using them in energy storage devices.

Acknowledgements: We thank the Irish Research Council for awarding the Government of Ireland PDF 2021, GOIPD/2021/858; and the CSC, China for a PhD scholarship.

References:

[1] R. J. Mandle et al, Phys. Chem. Chem. Phys. 19, 11429 (2017).

[2] H. Nishikawa et al., Adv. Mater. 29, 1702354 (2017).

[3] Neelam Yadav et al, J. Mol. Liq., 378, 121570 (2023).