Observation of a polar soft-phonon in hexagonal barium titanate

Abstract

Abstract By a hyper-Raman spectroscopy, it is found that a polar soft optic phonon with A2u symmetry exists in the hexagonal BaTiO3 polymorphic to perovskite BaTiO3, independently of another non-polar E2u optic phonon responsible for a phase transition at Ta = 222 K. The frequency is monotonously soft from 670 K to Ta, and thereby a virtual characteristic temperature of ferroelectric instability is deduced as 40K. Further, the magnitude of the TO-LO splitting of the relevant mode is found to be very large as is the case for perovskite BaTiO3 and SrTiO3. The remarkable temperature dependence of the dielectric constant εc can be well accounted for from the lattice-dynamical point of view, including an anomalous bump at Ta. Implication of the present findings is discussed.