High-temperature polar clusters in perovskite-type relaxor ferroelectrics and their response to an electric field
Zusammenfassung der Projektergebnisse
Lead-based perovskite-type (ABO3) relaxor ferroelectrics are fascinating materials with exceptional dielectric, electromechanical and electrooptic responses due to the existence of dynamic nanoregions with correlated polar local structural distortions. Although the extensive studies by a wide spectrum of structure-sensitive methods, the structure and evolution of polar nanoregions is still not clarified, which requires the application of further advanced methods. The aim of this project was two-fold: (i) to analyze the electric-field-induced renormalization of the characteristic temperatures above the temperature of the dielectric permittivity maximum by polarized Raman spectroscopy and complementary acoustic emission and (ii) to explore the potential of resonance Raman spectroscopy to study the phonon modes of polar nanoregions and the influence of chemical variations on the intrinsic electron-phonon coupling. As model compounds stoiciometric and heavily doped PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3 single crystals were used. The complementary analysis of the results obtained by in situ polarized Raman-scattering and acousticemission experiments under a dc electric field clearly revealed that in the rexalor ergodic state there are coexisting mesoscopic ferroelectric coupling within the B-sitecation system and antiferroelectric coupling related mainly to the Pb system, which results in a ferrielectric order of the atoms in the polar nanoregions. Chemical variations may tune the ferroelectric atomic coupling, the antiferroelectric atomic coupling, as well as the coherence between the two types of order and in such a way may govern the size and abundance of polar nanoregions and the strength of local polarization. Under resonance conditions with an excitation wavelength λ = 325 nm (3.8 eV), first- and second-order Raman scattering of longitudinal optical phonon modes related to the antisymmetric intrinsic BO6 vibrations is enhanced. Group theory analysis revealed that such a simultaneous enhancement is allowed only in non-centrosymmetric structures, indicating that the resonance Raman spectra collected from compounds in a relaxor state is purely caused by the atomic dynamics of polar nanoregions. Isovalent as well as aliovalent A-site substitution of Pb2+ by cations with no lone-pair electrons reduces the strength of electric field associated with the mean polarization of polar nanoregions and consequently the electronphonon coupling. The chemically-induced mesoscopic BO6 tilt order due to the smaller ionic radius of the substituting A-site element impacts stronger on the average polarization of polar nanoregions than the local charge imbalance. However, A-site doping with aliovalent cations (Bi3+) having the same outermost electron shell and ionic radius as the host Pb2+ cations intensifies the resonance behaviour, indicating stronger electron-phonon coupling due to enhanced local random electric fields.
Projektbezogene Publikationen (Auswahl)
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(2010): Evidence of local anisotropic strains in relaxor ferroelectrics below the intermediate temperature T* detected by acoustic emission. J. Phys.: Condens. Matter, 22, 222201/1-4
Dul'kin, E., Mihailova, B., Gospodinov, M., Mojaev, M.E., Roth, M.
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(2010): Phase transformation above Tm in PbSc0.5Ta0.5O3 relaxor as seen via acoustic emission, Phys. Rev. B, 82, 180101(R)/1-4
Dul’kin, E., Mihailova, B., Catalan, M., Gospodinov, M., Roth, M.
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(2011): Electric field dependence of the characteristic temperatures in PbSc0.5Ta0.5O3 and Pb0.78Ba0.22Sc0.5Ta0.5O3 relaxors studied via acoustic emission. Eur. Phys. Lett. 94, 57002/1-5
Dul’kin, E., Mihailova, B., Gospodinov, M., Roth, M.
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(2012): Effect of A-site La and Ba doping on the structural state of PbSc0.5Ta0.5O3 relaxor near the dielectricpermittivity maximum studied by acoustic emission, J. Appl. Phys. 112, 064107/1-5
Dul'kin, E, Mihailova, B., Gospodinov, M., Roth, M.
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(2012): Influence of the electric field on local phase transformations in relaxor ferroelectrics PbSc0.5Ta0.5O3 and Pb0.78Ba0.22Sc0.5Ta0.5O3. J. Appl. Phys. 112, 124111/1-6
Maier, B. J., Steilmann, T., Gospodinov, M., Bismayer, U., Mihailova, B.
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(2013): Effect of A-site La, Ba and Sr doping on the threshold field and characteristic temperatures of PbSc0.5Nb0.5O3 relaxor studied by acoustic emission. J. Appl. Phys. 113, 054105/1-5
Dul'kin, E., Mihailova, B., Gospodinov, M., Roth, M.
