Silent soft mode in hexagonal barium titanate observed by hyper-Raman scattering.

Abstract

A soft optic phonon which is neither Raman nor infrared active has been directly observed for the first time in the higher-symmetry phase above ${T}_{a}$ (221.5 K) of hexagonal BaTi${\mathrm{O}}_{3}$ by an experiment of hyper-Raman scattering. The underdamped feature of the soft-phonon mode even close to ${T}_{a}$ allows one to determine unambiguously both the frequency and the damping constant as a function of temperature. It is found that the soft-mode frequency obeys the Landau-Cochran theory in contrast with the nonclassical behavior below ${T}_{a}$ which was previously found by a Raman scattering experiment. Furthermore, the second-harmonic-generation observation indicates that the crystal structure below ${T}_{a}$ should be of acentric symmetry, which is closely related with the ferroelastic nature of the phase transition at ${T}_{a}=74$ K.