Crossover in the mechanism of ferroelectric phase transition of Pb[(Mg _1/3Nb_2/3)_1-xTi_x]O₃ single crystals studied by Brillouin light scattering

The inelastic light scattering spectrum of Pb [(Mg1/3 Nb2/3) 1-x Tix] O3 (PMN-xPT) single crystals with x=0-0.55 due to longitudinal-acoustic (LA) waves was investigated in a wide temperature range by using a Brillouin scattering technique. The acoustic frequency of LA modes of all PMN-xPT crystals showed a significant softening upon cooling in the high-temperature range where the deviation from the high-temperature linearity starts from ∼750 K. In addition, hypersonic damping exhibited a remarkable increase upon cooling toward the freezing or ferroelectric phase-transition temperature. These acoustic anomalies were attributed to the polarization fluctuations due to the displacive polarization component arising from the condensation of transverse-optic mode, flipping of polar nanoregions, and, for PMN-xPT with large x, correlated polar nanometer-sized regions (PNRs) as ordering units. The application of the modified superparaelectric model to the measured acoustic properties revealed that the activation energy for the flipping of PNRs displayed a sharp increase upon crossing the composition of morphotropic phase boundary (MPB) of PMN-xPT. This change was attributed to the appearance of order-disorder polarization component, i.e., polar clusters in PMN-xPT with large x beyond MPB, which indicated PNRs are correlated and oriented along the same direction. These results may suggest a crossover in the mechanism of phase transition with increasing x: disordered glass state with random directions of PNRs in PMN, comparatively abrupt growth of PNRs at the ferroelectric phase transition without changing the directions of their dipole moments at small x, and order-disorder type ferroelectric transition through alignment of PNRs into single direction via order-disorder mechanism at large x. These results are consistent with broadband dielectric and quasielastic scattering data that show critical slowing down for PMN-xPT with large x.