Displacive to Order-Disorder Crossover in the Cubic-Tetragonal Phase Transition of PbTiO 3

Light scattering measurements extended down to 2 cm −1 of the laser line were performed on a single-domain PbTiO 3 crystal. Spectra reveal the appearance of a central peak below the soft-phonon line when the cubic-tetragonal phase transition is approached with decreasing and increasing temperature. The relaxation process associated with this central peak presents a characteristic frequency smaller than 1 GHz and is shown to possess the main contribution to the dielectric permittivity near the transition temperature. This feature constitutes the first direct evidence of some order-disorder mechanism in the ferroelectric transition of PbTiO 3 which was up to now considered as a typical example of a displacive mechanism.