Hyper-raman spectroscopy in hexagonal barium titanate: A model structural phase transition induced by a non-polar γ soft phonon

Abstract

Abstract We have performed hyper-Raman spectroscopy in the prototype phase of hexagonal barium titanate above a structural phase transition at Ta = 221.5 K. First we report, on the basis of detailed measurements on the profile, that a silent soft phonon with E2u symmetry responsible for the phase transition, behaves ideally concerning the frequency ω0 and the scattering efficiency S or the fluctuations of order parameter Q 2, according to classical Landau theory, with both ωo −2 and Q 2 changing as proportional to (T – T a)−1 with temperature. Thus, we have for the first time established that the phase transition is a most typical one induced by a silent soft phonon. Second, we report the characteristics of several other phonon modes in the hyper-Raman spectra, which resemble considerably those of other related crystals composed of TiO6 octahedra such as SrTiO3, cubic BaTiO3 and TiO2. In particular, the absolute scattering efficiencies of the lowest lying TO and the highest frequency LO phonons are estimated to be anomalously large, implication of which is discussed in relation to third-order optical nonlinear susceptibility.