Study of structural phase transitions by the hyper-Raman scattering

It is discussed that the hyper-Raman and Rayleigh scattering serves to study the structural phase transitions in solids including the case where the pseudo-spins in addition to or coupled to the phonons are involved, since both modes are usually hyper-Raman active in the prototype phase above Tc. Experimentally, rather anomalous aspects of the hyper-Raman spectra and their temperature dependences in cubic BaTiO3 are shown, and the overall results are compared with the ones in the incipient ferroelectric state of SrTiO3 showing rather simpler behaviour. As a result, a rather microscopic model that the phase transition of BaTiO3 is caused by the pseudospins coupled bilinearly with the lowest-frequency TO phonon is proposed for interpreting well the present results. This model is also shown to lead to the Curie-Weiss law of the dielectric constant.An enhancement of the intensity of a quasi-elastic hyper-Rayleigh component as T approaches Tc is also discussed.