The temperature-electric field phase diagram of a ferroelectric smectic C* liquid crystal.

摘要

We have studied the temperature-electric field phase diagram of a ferroelectric smectic C* liquid crystal. We used 4-(2′methyl butyl) phenyl 4′-n-octylbiphenyl-4-carboxylate (CE8) as our ferroelectric liquid crystal material. Using extensive electro-optical measurements which included the direct observation of the sample under a polarizing microscope and also laser light diffraction from the helicoidal structure of the unwound smectic-C* phase, the entire temperature-electric field phase space for the smectic-C* phase was explored thoroughly. We looked for all possible phase transitions in the smectic-C* region. The character of the phase transitions, that is, whether they are first order (discontinuous) or second order (continuous), and also whether they are the instability or nucleation type was determined. Our results shows that the experimental phase diagram is incompatible with the existing theoretically predicted phase diagrams. The discrepancy is in both the shape and the nature of the phase boundary lines. Based on our experimental results we have proposed a new temperature-electric field phase diagram. We have proposed that it consists of two unwound smectic-C* phases, C¯₁* (high temperature) and C¯₂* (low temperature, high electric field), and a helicoidally modulated C* (low temperature, low electric field) phase as well. There are three phase boundary lines, each one separating two of the above three phases. The phase boundary lines between C*-