Electric Switching Behaviors and Dielectric Relaxation Properties in Ferroelectric, Antiferroelectric, and Paraelectric Smectic Phases of Bent-Shaped Dimeric Molecules

Abstract

This study reports the electric switching behaviors and dielectric properties of the ferroelectric smectic-A (SmAP_F), anti-ferroelectric smectic-A (SmAP_A), anti-ferroelectric SmC_AP_A, and smectic-A (SmA) phases formed by mixing the bent-shaped dimeric molecules, α,ω-bis(4-alkoxyanilinebenzylidene-4'-carbonyloxy)pentanes. These four phases each show characteristic features. The SmAP_F shows a low threshold electric field for ferroelectric switching and a large dielectric strength due to the collective fluctuation mode of dipoles at around 500 Hz. Both the threshold electric field and dielectric strength are strongly dependent on the cell thickness. The threshold field decreases to 0.1 V μm^-1, and the dielectric strength increases up to a huge value of 10,000 as the cell thickness increases up to 80 μm. The SmAP_A also shows a similar collective mode at around 2 kHz with a relatively small dielectric strength (around 200), which may be induced by the anti-phase rotation of dipoles in adjacent layers. In these collective modes, the dielectric strength is found to be inversely proportional to the switching threshold field. On the other hand, another anti-ferroelectric SmC_AP_A as well as the paraelectric SmA show only the non-collective mode (i.e., rotational relaxation of individual molecules around their short axes) at a high frequency of around 100 kHz.