Broadband dielectric measurements on the (R)-1-methylheptyl-6-(4'-decyloxybenzoyloxy)-2-naphthalene carboxylate antiferroelectric liquid crystal

Complex temperature dependence of the dielectric response in simple SmC* phases

The system under study has a SmA*↔SmC* phase sequence and is described by a basic phenomenological model. However, in contrast to a common approach, changes of the polarisation induced by the external electric field are considered independent of changes of the tilt. As a result, even in the basic model, which does not contain any additional terms usually used to obtain better fits to experimental observations, the frequencies of most of the relaxation modes depend on the temperature non-linearly and their dielectric response is not inversely proportional to the frequency of the mode. The behaviour is a direct consequence of chiral piezoelectric coupling, which influences the polar properties of the relaxation modes depending on the temperature.

Characteristics of the collective dielectric relaxation mode of the incommensurate SmC(α)(*) phase

Employing the dielectric relaxation spectroscopy technique, we have observed two different characteristics of the collective relaxation mode of an incommensurate SmC(α)(*) phase. Recent theoretical advances predict two different natures for the dielectric relaxation mode in the SmC(α)(*) phase. Our experimental results confirm two different behaviours of the relaxation mode in different temperature ranges for this phase for one unique compound having an exceptionally wide temperature range (∼9 °C) for the SmC(α)(*) phase.

Paraelectric-antiferroelectric phase transition in achiral liquid crystals

Critical freezing of molecular rotation in an achiral smectic phase, which leads to polar ordering through the second order paraelectric-antiferroelectric (Sm-A-->Sm-APA) phase transition is studied theoretically and experimentally. Strong softening of the polar mode in the Sm-A phase and highly intensive dielectric mode in the Sm-APA phase are observed due to weak antiferroelectric interactions in the system. In the Sm-APA phase the dielectric response behaves critically upon biasing by a dc electric field. Such a behavior is found general for the antiferroelectric smectic phase with significant quadrupolar interlayer coupling.

Enantiomeric excess dependence of the phase diagram of antiferroelectric liquid crystals

The phase diagram of the prototype antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl-4'-octyloxybiphengl-4-carboxylate (MHPOBC) in dependence of enantiomeric excess was measured. It was shown that the Sm-C*beta phase in very pure samples is the Sm-C*(FI2) phase with a four-layer structure, and only after small racemization it transforms into the ferroelectric Sm-C* phase. The phase diagram was theoretically explained by taking into account longer range bilinear and short range biquadratic interlayer interactions, that lead to the distorted clock structures and first-order transitions between them.

Twist modulated phases in chiral smectic liquid crystals

By considering short period helical planar modulations about the layer normal, we construct a model free energy for the ferriclinic phases observed in chiral smectic liquid crystals. We then use this free energy to construct the phase diagram for our model. The resulting phases are compared with the experimentally observed smectic-C* subphases (ferroclinic, antiferroclinic, and heliclinic). A strong coupling is found between the ferroclinic q=2pi/a and the heliclinic q=2pi/3a modes. This coupling was not considered in previous models. The resulting additional stability of this "locked in" phase is discussed.