Landau model of ferroelectric plastic crystals

Ferroelectric plastic crystals (FEPCs) are a new class of caloric materials in which inversion and orientational symmetries are broken spontaneously and concomitantly. The polar and orientational orderings associated with such symmetry-breakings occur on a mechanically soft medium, leading to an interplay between electric polarization, molecular orientation, and lattice strain which has not been understood. In this talk, we present a minimal Landau model of polar and orientational phase transitions in FEPCs in which the key components are electric polarization, a quadrupole moment, elastic strain, and thermal expansion. We show that our model captures characteristic features of the phase transitions in FEPCs, namely, dielectric constant, volume, and entropy. A comparison with experimental data is presented.