Central peaks, acoustic modes, and the dynamics of polar nanoregions in Pb[(Zn1/3Nb2/3)xTi1−x]O3 single crystals studied by Brillouin spectroscopy
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Temperature dependence of acoustic behaviors and quasielastic central peaks (CPs) of Pb[(Zn1/3Nb2/3)xTi1−x]O3 (PZN-xPT) 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 C11 elastic constant of both crystals showed a deviation from normal lattice anharmonicity at the Burns temperature (TB) of about 730 K upon cooling, indicating the onset of the electrostrictive coupling between the polar nanoregions (PNRs) and the longitudinal acoustic (LA) waves. Upon further cooling, depolarized CP began to appear at a certain temperature (Td) located in 500–550 K, which was accompanied by substantial softening of the C44 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 Td 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 TB but at a much lower temperature of Td, which may be ascribed to a local structural transformation occurring in PNRs at Td. 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 (2007)] and substantial changes in the Raman spectrum [O. Svitelskiy, Phys. Rev. B 72, 172106 (2005)] at almost the same temperature. From the comparison of the present results to those of two typical relaxors, Pb(Mg1/3Nb2/3)O3 and Pb(Mg1/3Ta2/3)O3, it was suggested that the existence of an intermediate characteristic temperature Td below TB may be considered as a more common characteristic in the temperature evolution of PNRs of relaxor ferroelectrics.
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