Abstract
In relaxor ferroelectrics, the role of randomly orientated polar nanoregions (PNRs) with weak random fields (RFs) is one of the most puzzling issues of materials science. The relaxation time of polarization fluctuations of PNRs, which manifests themselves as a central peak (CP) in inelastic light scattering, is the important physical quantity to understand the dynamics of PNRs. Here, the angular and temperature dependences of depolarized and polarized CPs in 0.44Pb(Mg1/3Nb2/3)O3-0.56PbTiO3 single crystals with weak RFs have been studied by Raman and Brillouin scattering, respectively. The CPs observed in Raman scattering show the very clear angular dependence which is consistent with the local tetragonal symmetry. It is different from the well-known local rhombohedral symmetry with strong RFs for Pb(Mg1/3Nb2/3)O3. In Brillouin scattering, depolarized and polarized CPs show two relaxation processes corresponding to transverse and longitudinal fluctuations of PNRs. The remarkable slowing down towards the Curie temperature was observed for transverse fluctuations in local tetragonal symmetry.
Highlights
Burns temperature TB, when the lifetime of polar fluctuations becomes longer than the inverse optical phonon frequency; intermediate temperature T*, when permanent PNRs appear; and cubic-tetragonal phase transition temperature TC-T
Since the polarization fluctuations of PNRs manifest themselves as a central peak (CP) in a low-frequency inelastic light scattering (ILS) spectrum, a CP is important to understand the dynamics of PNRs
The local symmetry of CPs and some other Raman modes in a paraelectric cubic phase related to rhombohedral PNRs were identified by the angular dependence of Raman scattering in Li-doped KTN18 and PMN19 single crystals
Summary
Burns temperature TB, when the lifetime of polar fluctuations becomes longer than the inverse optical phonon frequency; intermediate temperature T*, when permanent PNRs appear; and cubic-tetragonal phase transition temperature TC-T. To identify the local structural symmetry of PNRs, the angular dependence of Raman scattering (one of ILS) combined with Raman tensor analysis can be a very powerful technique, because it measures the scattering intensity with respect to the polarization direction of an incident light. The local symmetry of CPs and some other Raman modes in a paraelectric cubic phase related to rhombohedral PNRs were identified by the angular dependence of Raman scattering in Li-doped KTN18 and PMN19 single crystals. D. La-Orauttapong et al measured the composition dependences of neutron scattering studies on PZN-xPT and suggested that with increasing the PT content, the local polarization direction remains unchanged along [111] direction, while the static PNRs grow preferentially very closer to [100] direction. The detailed polarization analysis on the VV- and VH-CPs in a cubic phase, which is directly related to PNRs, is important and necessary in Pb-based perovskites with weak RFs
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