Nanoscale clusters and phonon anomalies in relaxor-ferroelectric
Zusammenfassung der Projektergebnisse
Lead-based perovskite-type (ABO3) relaxor ferroelectrics are fascinating materials with outstanding dielectric, electroelastic and electrooptic properties related to their complex nanoscale structure. Relaxors and solid solutions of normal and relaxor ferroelectrics are of great technological importance, including for memory devices, and are also vital for the fundamental understanding of ferroic phase transitions in mesoscopically inhomogeneous solids. The structure of relaxors is characterised by polar nanoregions which nucleate well above the temperature of the dielectric permittivity maximum Tm and persist well below Tm. The origin of these structural peculiarities has been assumed to be mainly related to charge imbalance caused by the existence chemically ordered/disordered regions, although the existence of local strains in the vicinity of substitutional defects is also considered as plausible explanation. To gain further insights into the atomistic mechanism of formation of relaxor state we analysed in detail the temperature-driven transformation processes in single crystals of PbSc0.5Ta0.5O3 (PST) and PbSc0.5Ta0.5O3 (PST), PST with enhanced an suppressed oxygen deficiency and different degree of chemical B-site order, B-site doped PST, and A-site doped PST and PSN. We applied simultaneously Raman scattering and x-ray diffraction, which probe the structure on different length- and time-scales. Our results reveal that correlated off-centred Pb atoms exist at very high temperatures and they govern the formation and coupling of off-centred B cations. The development of polar nanoregions is a two-step process: first adjacent off-centred B-cations couple to form small polat clusters and, then, offcentred B-cations from adjacent clusters couple to form larger polar clusters. At low temperatures, long range ferroelectric ordering occurs or polar nanoregions are preserved, depending on the size of incipient ferroic Pb-O species at high temperatures. The size of these ferroic Pb-O species and, consequently, the lowtemperature structural state are not directly related to the size of chemically B-site ordered nanoregions. Our results highlight the local elastic fields as a primary factor for the occurrence of relaxor ferroelectric state.
Projektbezogene Publikationen (Auswahl)
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(2006): Crystal structure of Ba-doped PST relaxor ferroelectric. Annual report 2006 DESY.
Paulmann, C., Malcherek, T., Mihailova, B., Gospodinov, M. and Bismayer, U.
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(2006): Structural, optical and dielectric properties of relaxor-ferroelectric Pb0.78Ba0.22Sc0.5Ta0.5O3. Journal of Physics: Condensed Matter 18, L385-L393.
Marinova, V., Mihailova, B., Malcherek, T., Paulmann, C., Lengyel, K., Kovacs, L., Veleva, M., Gospodinov, M., Güttler, B., Stosch, R. and Bismayer, U.
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(2007): Ba-induced changes in the structure of PbSc0.5Ta0.5O3, Annual report 2007 DESY.
Paulmann, C., Malcherek, T., Mihailova, B., Gospodinov, M. and Bismayer, U.
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(2007): Ferroic clustering and phonon anomalies in Pb-based perovksite-type relaxors. Journal of Physics: Condensed Matter, 19, 275205/1-10.
Mihailova, B., Gospodinov, M., Güttler, B., Stosch, R. and Bismayer, U.
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(2007): Ferroic nanoclusters in relaxors: the effect of oxygen vacancies. Journal of Physics: Condensed Matter, 19, 246220/1-10.
Mihailova, B., Gospodinov, M., Güttler, B., Petrova, D., Stosch, R. and Bismayer. U.
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(2007): Resonance Raman scattering of relaxor-ferroelectric PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3. Applied Physics Letters 90, 042907/1-042907/3.
Mihailova, B., Bastjan, M., Schulz, B., Rübhausen, M., Gospodinov, M., Malcherek, T., Stosch, R., Güttler, B. and Bismayer, U.
