Gradual phase transition between the smectic- C* and smectic- CA* phases and the thresholdless antiferroelectricity

We have constructed the phase diagrams for a binary-mixture system of antiferroelectric and ferroelectric liquid-crystalline materials in both thick and thin cells. In the phase diagrams the boundary between the smectic- C* and smectic- CA* phases runs almost parallel to the temperature axis below from ca. 70 °C down to at least -25 °C. The Sm C* -Sm CA* phase transition for a thin cell shows a large supercooling, and a gradual transition occurs near the boundary. Moreover, the thin cell shows a continuous evolution from the antiferroelectric to the ferroelectric state by increasing the electric field applied across the cell. The continuous evolution seemingly reflects the phenomenon of thresholdless antiferroelectricity. In order to explain these phenomena and in clarifying the mechanism of the so-called frustration between ferroelectricity and antiferroelectricity, we have measured the interlayer interaction energy by varying the constituent concentrations in the binary-mixture system. The measured interlayer interaction close to the boundary indicates that the gradual phase transition and continuous evolution result from the suppression of the solitary-wave propagation by the effect of surfaces.