PHASE-TRANSITION ANALYSIS THROUGH SOFT-PHONON POLARITON BEHAVIOR IN THE TEMPORAL DOMAIN - A1-SYMMETRY INVESTIGATION OF THE PBTIO3 PEROVSKITE

We have studied the A1-symmetry lowest-frequency phonon behavior of Pb TiO3 near the tetragonal-to-cubic phase transition, by a temporal-doma in experiment. Study of the soft-phonon-polariton dispersion as a func tion of wave-vector value, at different temperatures up to T(c) at 493 -degrees-C, allowed us to deduce its contribution to the static dielec tric constant. Good agreement with measured values of the dielectric c onstant, which is associated with the nonappearance of a relaxational mode in the temporal response, clearly demonstrate the displacive orig in of the transition.