DISPLACIVE TO ORDER-DISORDER CROSSOVER IN THE CUBIC-TETRAGONAL PHASE-TRANSITION OF PBTIO3

Light scattering measurements extended down to 2 cm-' of the laser line were performed on a single-domain PbTi03 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 lGHz 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 PbTi03 which was up to now considered as a typical example of a displacive mechanism. © 1990 The Japan Society of Applied Physics.