Hyper-Raman Scattering Study of Lattice Dynamics and Disorder in Perovskite Crystals

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Abstract
It is shown that the hyper-Raman scattering provides a powerful tool for studying the lattice dynamics and disorder in the high-symmetry phases of perovskite crystals at atmospheric pressure as well as high pressure. Detailed information about lattice-dynamical properties in both BaTiO3 and SrTiO3, with the mode-assignment in cubic BaTiO3 is given. For BaTiO3, a picture in terms of pseudo-spin for lattice-dynamical disorder can explain well the different aspects of the cubic-tetragonal phase transition, which has long been controversial. Even for SrTiO3, lattice-dynamical disorder is found to be involved at low temperatures, but nevertheless the soft-phonon picture may suffice to account for the incipient ferroelectric phase transition. A newly developed technique for observing Raman-inactive phonons at high pressure is also reported in SrTiO3.

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