Electric Permittivity Behavior in Highly Polar Liquid Crystals: From Paraelectricity through Ferroelectricity to Superparaelectricity

The discovery of polar order in achiral liquid crystals has revolutionized the field of soft matter, offering properties comparable to solid-state ferroelectrics.

In this study, the dielectric response of several homologous series of rod-like compounds was analyzed, considering how molecular architecture determines polar properties. By analyzing several homologous series of rod-like compounds, we investigate how specific structural modifications, such as terminal groups and core substitutions, influence mesophase stability and electrical response. The primary focus is placed on the role of terminal groups, including nitro-, cyano-, and fluorine fragments. Additionally, we examine how minimal structural changes, such as the addition of a single methylene unit, can radically shift a material from a paraelectric to a ferroelectric state.

The research shows that the emergence of the ferroelectric order is driven by a specific collective mode. In this state, molecules within different domains vibrate in or out of phase depending on their polarization direction. We also analyzed two achiral smectic phases. In the ferroelectric smectic A phase, we observed a soft mode (amplitude mode) where the dielectric strength increases and the relaxation frequency decreases as the temperature decreases. By contrast, in the ferroelectric smectic C phase, a phason mode appears, similar to the Goldstone mode in chiral systems.

Furthermore, in a specially designed mixture, we observed very high values of electric permittivity. These values are comparable to those found in ferroelectric fluids, even though the mixture itself does not have full ferroelectric properties. Instead, it can be described as superparaelectric. Understanding the specific dielectric modes and the phenomenon of superparaelectricity allows us to design stable, high-performance materials that respond quickly to an electric field.

The work is supported by The National Science Centre grant 2025/57/B/ST5/00509.