Comparison of the soft polar phonon in hexagonal barium titanate and ATiO 3 perovskite compounds

Abstract

A polar soft optic phonon recently found in hexagonal barium titanate has been studied in detail by hyper-Raman spectroscopy. The oscillator strength of this polar soft phonon, estimated from the observed hyper-Raman spectrum, is found to be 80% of the lattice part of the static dielectric constant along the c-axis. The existence of the polar soft phonon as well as its large oscillator strength are common to ATiO 3 perovskite compounds (A: Pb, Ba, Sr,...). We also find that an accidental coupling between this soft phonon and another optic phonon of different origin occurs below the hexagonal-to-orthorhombic-transition temperature T a.