Temperature dependence of acoustic behaviors and quasielastic central peaks ͑CPs͒ of Pb͓͑Zn 1/3 Nb 2/3 ͒ x Ti 1−x ͔O 3 ͑PZN-xPT͒ single crystals with x = 4.5% and 9% have been investigated in a temperature range of 300-900 K by using the Brillouin light scattering. The temperature dependence of the C 11 elastic constant of both crystals showed a deviation from normal lattice anharmonicity at the Burns temperature ͑T B ͒ of about 730 K upon cooling, indicating the onset of the electrostrictive coupling between the polar nanoregions ͑PNRs͒ and the longitudinal acoustic ͑LA͒ waves. Upon further cooling, depolarized CP began to appear at a certain temperature ͑T d ͒ located in 500-550 K, which was accompanied by substantial softening of the C 44 elastic constant below this temperature suggesting anisotropic electrostrictive coupling between PNRs and the two acoustic waves. In addition, the onset of significant increase in the acoustic damping of both acoustic waves at T d could be seen from the temperature dependence of the linewidth of Brillouin doublets. These results suggested that electrostrictive coupling of the strain to the square of the local polarization of PNRs and the resultant order-parameter fluctuations are enhanced not at T B but at a much lower temperature of T d , which may be ascribed to a local structural transformation occurring in PNRs at T d. This suggestion is consistent with recent studies on PZN-xPT single crystals reporting the appearance of strong acoustic emission signals ͓M. Roth et al., Phys. Rev. Lett. 98, 265701 ͑2007͔͒ and substantial changes in the Raman spectrum ͓O. Svitelskiy, Phys. Rev. B 72, 172106 ͑2005͔͒ at almost the same temperature. From the comparison of the present results to those of two typical relaxors, Pb͑Mg 1/3 Nb 2/3 ͒O 3 and Pb͑Mg 1/3 Ta 2/3 ͒O 3 , it was suggested that the existence of an intermediate characteristic temperature T d below T B may be considered as a more common characteristic in the temperature evolution of PNRs of relaxor ferroelectrics.

Acoustic properties of Pb͓͑Zn 1/3 Nb 2/3 ͒ 0.91 Ti 0.09 ͔O 3 have been investigated in a wide temperature range. The sound velocity of acoustic phonons showed a deviation from its high-temperature linear behavior at the Burns temperature T B ϳ 730 K. Upon cooling, acoustic properties exhibited significant changes at about 550 K in polarization state and width of the central peak in addition to changes in frequency and damping of acoustic phonons. This finding suggests that, besides T B , another intermediate characteristic temperature exists in the evolution of relaxor dynamics, which might be related to the formation of long-lived polar nanoregions and associated local strain fields.

The inelastic light scattering spectrum of Pb͓͑Mg 1/3 Nb 2/3 ͒ 1−x Ti x ͔O 3 ͑PMN-xPT͒ single crystals with x = 0-0.55 due to longitudinal-acoustic ͑LA͒ 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 ϳ750 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 ͑PNRs͒ 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 ͑MPB͒ 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.

Micro-Brillouin scattering was applied to relaxor-based complex perovskite Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 (PZN–9% PT) single crystals during a polarization switching under two different electric-field directions along 〈111〉 and 〈101〉. Brillouin spectra consisted of one longitudinal and two transverse acoustic modes, and the Brillouin shift and the full width at half maximum of these modes showed clear changes near the coercive field (Ec) where the polarization reversal occurred. In particular, the lowest transverse mode was the most sensitive to Ec. In the case of 〈111〉-poled crystals the Brillouin spectrum exhibited an asymmetric shape in a narrow electric-field range near Ec due to a coupling between the lowest transverse mode and a central peak. In the case of 〈101〉-poled crystals, the lowest transverse mode showed an abrupt softening at Ec and a slow recovery to the initial value upon further increasing the electric field. This may indicate the appearance of the monoclinic symmetry where the polarization can rotate freely toward the orthorhombic phase in the MB plane under the electric field along the 〈101〉 direction. The observation of no changes in the Brillouin spectra with decreasing the electric field seems to indicate that the PZN–9% PT crystal, poled under the field along 〈101〉, does not become rhombohedral, but remains in the induced orthorhombic phase upon the removal of the field.

Acoustic and dielectric properties of Pb͓͑Zn 1/3 Nb 2/3 ͒ 1−x Ti x ͔O 3 ͑PZN-xPT͒ single crystals with x = 0.045 have been studied by the high-resolution micro-Brillouin scattering and dielectric spectroscopy in a wide temperature range. The softening of the Brillouin shift and the increase of dielectric relaxation time upon cooling indicated the formation of polar nanoregions ͑PNRs͒ and the slowing down of their dynamics. In contrast to the acoustic properties of typical model relaxors such as lead magnesioniobate, the change in the Brillouin shift near its minimum became sharper on heating compared to the change on cooling, pointing to the clear existence of hysteresis in the dynamics of the diffuse phase transition in PZN-4.5%PT. Since the number of PNRs will increase upon cooling, it may be expected that the kinetics of the phase transition would become slower, the lower the transition temperature resulting in the more sluggish, broad feature of the Brillouin shift observed during cooling. This result may indicate that the number and size of polar nanoregions, which are dependent on temperature, play an important role in the development of the mesoscopic ferroelectric order in PZN-4.5%PT.