Sub-THz Raman response in BaTiO3 and link with structural phase transition.

The THz and sub-THz polarized Raman response was measured in tetragonal and cubic phases of single domain BaTiO3 crystal. A large peak was detected at very low wavenumber, within a scattering geometry in which all phonon lines are Raman inactive. It lies below 700 GHz in the whole temperature range of the tetragonal phase, and is clearly distinct from the soft phonon band. This peak has relaxational behavior with a slowing down on approaching the phase transition from above and from below.