Soft Mode in cubic PbTiO3 by Hyper-Raman Scattering
  Hyper-Raman scattering experiments allowed collecting the spectra of the lowest F1u-symmetry mode of PbTiO3 crystal in the paraelectric phase up to 930K as well as down to about 1K above the phase transition. It is realized that this mode is fully responsible for the Curie-Weiss behavior of its dielectric permittivity above Tc. Near the phase transition, this phonon frequency softens down to 17 cm-1 and its spectrum can be well modeled as a response of a single damped harmonic oscillator. It is concluded that PbTiO3 constitutes a clean example of a soft mode-driven ferroelectric system. 
J. Hlinka, B. Hehlen, A. Kania, and I. Gregora
cond-mat.mtrl-sci cond-mat.other