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

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 Tₐ (221.5 K) of hexagonal BaTiO₃ by an experiment of hyper-Raman scattering. The underdamped feature of the soft-phonon mode even close to Tₐ 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 Tₐ which was previously found by a Raman scattering experiment. Furthermore, the second-harmonic-generation observation indicates that the crystal structure below Tₐ should be of acentric symmetry, which is closely related with the ferroelastic nature of the phase transition at Tₐ=74 K.