Local phenomena in relaxor-ferroelectric PbSc0.5B '' O-0.5(3) (B '' = Nb, Ta) studied by Raman spectroscopy

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Erscheinungsjahr:
2003
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Text
Schlagworte:
  • Local structure
  • Normal mode calculations
  • Phonon anomalies
  • Raman spectroscopy
  • Relaxor-ferroelectrics
Beschreibung:
  • Raman spectroscopy was applied to study nano-scale structure and related phonon anomalies in relaxor-ferroelectric materials of type PbB′ 0.5B″0.5O3. By comparing the polarized spectra of single crystals, measured at temperature higher than the maximum of the dielectric permittivity, Tm, and by performing normal mode calculations the observed Raman peaks were assigned to definite atomic vibrations. Further, the temperature evolution of the phonon anomalies was investigated on the basis of calculated modes of representative structural units at various atomic positions. It is shown that within a few nm doubling of the unit cell should exist, independently of the degree of B-site ordering. The Raman activity of the silent F2u mode reveals the existence of dynamical off-centred structural fluctuations. The structural perturbation consists of a non-coplanarity of the Pb and O atoms in the planes perpendicular to the cubic body diagonal and non-centre-symmetrical deformations of the BO6 octahedra. On cooling, different structural changes are preferential: for B″=Nb the temperature decrease amplifies the deformation of the individual BO3-Pb species; for B″=Ta the temperature decrease favours the non-coplanarity of Pb and O atoms and enhances the correlation between the stereoactive electron lone pairs of Pb atoms. Besides, within nano-scale spatial regions, PbSc0.5Ta 0.5O3 undergoes an additional phase transition near 180 K that involves a reduction of the rotational symmetry. The comparison between the temperature dependence of the inelastic light scattering in stoichiometric and Nb-doped PbSc0.5Ta0.5O3 shows that the presence of Nb even in small concentrations affects the Pb-O sublattice and changes the direction of the local structural transformations. © 2003 Elsevier B.V. All rights reserved.
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Forschungsinformationssystem der UHH
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