On the temperature dependence of the tilt and spontaneous polarisation in high tilt antiferroelectric liquid crystals

The temperature-dependent physical properties of four related liquid crystalline materials are reported. The materials differ chemically only in the degree of fluorination of one of the core aromatic rings and all materials exhibit both chiral smectic-C (ferroelectric) and antiferroelectric liquid crystal phases. The steric tilt angles, calculated from the layer spacing, are relatively low, saturating at a maximum of 25°, while the optical tilt angles are much higher, achieving values of 42.5°. Most obviously, the tilt angle and spontaneous polarisation exhibit very different temperature dependencies, the former appearing to be discontinuous at the orthogonal to tilted transition, while the latter follows a simple power law. It is noted that this behaviour has been observed in other similar materials, but that it is not in agreement with the simple coupling between spontaneous polarisation and tilt that is often assumed in ferroelectric liquid crystals. A theoretical explanation is given for the different temperature dependence of these two physical quantities, suggesting that the biaxial order parameter, not directly determined by the tilt angle, is important in these systems.