Spectroscopic signature of sublattice polarization in the lattice dynamics of an antiferroelectric crystal

Spectroscopic measurements of the far-infrared phonon dynamics for a model antiferroelectric crystal are performed at low temperature. In agreement with the phenomenological expectations for a displacive antiferro-electric transition, a polar soft mode associated to a close-lying polar phase of the otherwise centrosymmetric structure is observed. Incomplete softening of the polar soft mode due to the antiferroelectric transition quantifies the energy barrier between the neighboring states. The dynamics are modeled with a biquadratic Landau potential incorporating a symmetry constraining effective interaction between displaced ions. A signature of sublattice polarization is observed in a scaling of the effective antipolar interaction in the antiferroelectric phase.