It is observed that the temperature dependence of the spontaneous polarization of FLCs is similar to that of other ferroelectric materials, varying as shown. One of the characteristics of ferroelectric materials is the initially rapid rise of their spontaneous polarization as their temperature drops below the critical value, Tcrit, as illustrated here. For FLCs, this is the temperature of the SmA* to SmC* transition. |
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In crystalline ferroelectrics, this initial rapid rise in spontaneous polarization is due to the elementary dipoles in the material interacting with each other, producing an internal field which lines up the dipoles, eventually yielding saturation of the spontaneous polarization. In FLCs, although the molecules are actually undergoing a rapid reorientation about their long axis, from symmetry considerations, a time averaging process results in a molecular dipole moment along a 2-fold symmetry axis. This is just the molecular polarization we have discussed earlier, lying in the smectic layer plane and perpendicular to the long axis of the molecule, as first noted by R. B. Meyer. The interaction of this ensemble of molecular polarizations then leads to the temperature dependence of the spontaneous polarization which could be observed in an SSFLC where the helielectric effect is avoided.
The molecular tilt angle, q, away from the normal to the SmC* layer plane follows a similar temperature dependence to the spontaneous polarization, growing rapidly at first, then asymptotically approaching a final low temperature value, before crystallization takes place.
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Electrooptical Effects |  |
Antiferroelectrics |  |