Browsing by Author "Angel, R. J."

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    A-site doping-induced renormalization of structural transformations in the PbSc0.5Nb0.5O3 relaxor ferroelectric under high pressure
    Maier, B. J.; Welsch, A. M.; Angel, R. J.; Mihailova, B.; Zhao, J.; Engel, J. M.; Schmitt, L. A.; Paulmann, C.; Gospodinov, M.; Friedrich, A.; Bismayer, U. (American Physical Society, 2010-05-01)
    The effect of A-site incorporated Ba2+ and Bi3+ on the pressure-driven structural transformations in Pb-based perovskite-type relaxor ferroelectrics has been studied with in situ x-ray diffraction and Raman scattering of PbSc0.5Nb0.5O3, Pb0.93Ba0.07Sc0.5Nb0.5O3, and Pb0.98Bi0.02Sc0.51Nb0.49O3 in the range from ambient pressure to 9.8 GPa. The substitution of Ba2+ for Pb2+ represents the case in which A-site divalent cations with stereochemically active lone-pair electrons are replaced by isovalent cations with a larger ionic radius and no active lone pairs, leading to formation of strong local elastic fields. In contrast, substitution of Bi3+ for Pb2+ involves the replacement of divalent A-site cations with active lone-pair electrons by aliovalent cations with nearly the same ionic radius and active lone pairs so it induces local electric fields but not strong elastic fields. The two types of dopants have rather distinct effects on the changes in the atomic structure under pressure. The embedding of Ba2+ and associated elastic fields hinders the development of pressure-induced ferroic ordering and thus smears out the phase transition. The addition of Bi3+ enlarges the fraction of spatial regions with a pressure-induced ferroic distortion, resulting in a more pronounced phase transition of the average structure, i.e., the preserved lone-pair order and the absence of strong local elastic fields enhances the development of the ferroic phase at high pressure. For all compounds studied, the high-pressure structure exhibits glide-plane pseudosymmetry associated with a specific octahedral tilt configuration.
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    Effect of Ba incorporation on pressure-induced structural changes in the relaxor ferroelectric PbSc0.5Ta0.5O3
    Welsch, A. M.; Maier, B. J.; Engel, J. M.; Mihailova, B.; Angel, R. J.; Paulmann, C.; Gospodinov, M.; Friedrich, A.; Stosch, R.; Guttler, B.; Petrova, D.; Bismayer, U. (American Physical Society, 2009-09-28)
    Pressure-induced structural changes in the canonical relaxor Pb0.78Ba0.22Sc0.5Ta0.5O3 were studied with both in-house and synchrotron single-crystal x-ray diffraction as well as Raman spectroscopy at pressures up to 9.8 GPa. The results reveal that the substitution of Ba for Pb in ABO(3) perovskite-type structures, i.e., the substitution of a two-valent element with an isotropic electron shell for an isovalent element with a stereochemically active lone pair, leads to a "diffuse pressure-induced phase transition," a structural transformation over a broad pressure range, without a well-defined critical pressure point. The smeared phase transition of the average structure results from the existence of local structural deformations in the vicinity of A-positioned Ba cations.
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    Effect of La doping on the ferroic order in Pb-based perovskite-type relaxor ferroelectrics
    Maier, B. J.; Welsch, A. M.; Mihailova, B.; Angel, R. J.; Zhao, J.; Paulmann, C.; Engel, J. M.; Marshall, W. G.; Gospodinov, M.; Petrova, D.; Bismayer, U. (American Physical Society, 2011-04-07)
    The structural alteration induced by the substitution of three-valent cations with an isotropic electronic outermost shell for Pb2+ in perovskite-type relaxors was investigated in the solid solutions Pb1-xLaxSc(1+x)/2Ta(1-x)/2O3, x = 0.08 (PST-La) and Pb1-xLaxSc(1+x)/2Nb(1-x)/2O3, x = 0.23 (PSN-La). In order to distinguish the "charge" effects from "strain" effects associated with the incorporation of La3+ in the structure, Sr-containing PbSc0.5Nb0.5O3 was characterized as well. The structure of the compounds was analyzed by in situ Raman spectroscopy, single-crystal x-ray diffraction, and powder neutron diffraction at different temperatures or pressures. It is shown that the embedding of La3+ strongly affects the ferroic structural species due to strain effects through a disturbance of the system of lone-pair electrons associated with Pb2+ and a decrease in the tolerance factor. La doping suppresses the dynamical coupling between off-centered Pb and B-site cations and enhances antiphase BO6 octahedral tilting which, depending on the level of doping, may lead to long-range order of antiphase BO6 tilts at ambient conditions and frustrated antiferroelectric order of Pb ions at low temperatures.
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    Fluorinert as a pressure-transmitting medium for high-pressure diffraction studies
    Varga, T.; Wilkinson, A. P.; Angel, R. J. (AIP Publishing, 2003-10)
    Fluorinert is a liquid pressure-transmitting medium that is widely used in high-pressure diffraction work. A systematic study of five different fluorinerts was carried out using single-crystal x-ray diffraction in a diamond-anvil cell in order to determine the pressure range over which they provide a hydrostatic stress state to the sample. It was found that none of the fluorinerts studied can be considered hydrostatic above 1.2 GPa, a lower pressure than reported previously. (C) 2003 American Institute of Physics.
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    Pressure-induced phase transition in PbSc0.5Ta0.5O3 as a model Pb-based perovksite-type relaxor ferroelectric
    Mihailova, B.; Angel, R. J.; Welsch, A. M.; Zhao, J.; Engel, J.; Paulmann, C.; Gospodinov, M.; Ahsbahs, H.; Stosch, R.; Guttler, B.; Bismayer, U. (American Physical Society, 2008-07-04)
    We report pressure-induced structural changes in PbSc0.5Ta0.5O3 studied by single-crystal x-ray diffraction and Raman scattering. The appearance of a soft mode, a change in the volume compressibility, broadening of the diffraction peaks, and suppression of the x-ray diffuse scattering show that a phase transition occurs near p(c)similar to 1.9 GPa. The critical pressure is associated with a decoupling of the displacements of the B site and Pb cations in the existing polar nanoregions, leading to the suppression of B-cation off-center shifts and enhancement of the ferroic distortion in the Pb-O system.
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    Structural state of relaxor ferroelectrics PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3 at high pressures up to 30 GPa
    Maier, B. J.; Waeselmann, N.; Mihailova, B.; Angel, R. J.; Ederer, C.; Paulmann, C.; Gospodinov, M.; Friedrich, A.; Bismayer, U. (American Physical Society, 2011-11-07)
    The pressure-induced structural changes in perovskite-type (ABO(3)) Pb-based relaxor ferroelectrics are studied on the basis of in situ single-crystal synchrotron x-ray diffraction and Raman scattering experiments on PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3 conducted under hydrostatic conditions up to 30 GPa. Complementary density functional theory calculations have been performed to compare the stability of various atomic configurations for both compounds at high pressures. By combining the experimental and theoretical results, the following sequence of structural transformations is proposed. At a characteristic pressure p(1)* the mesoscopic polar order is violated and a local antipolar order of Pb atoms as well as quasidynamical long-range order of antiphase octahedral tilts is developed. These structural changes facilitate the occurrence of a continuous phase transition at p(c1) > p(1)* from cubic to a nonpolar rhombohedral structure comprising antiphase octahedral tilts of equal magnitude (a a a). At a characteristic pressure p(2)* > p(c1) the octahedral tilts around the cubic [100], [010], and [001] directions become different from each other on the mesoscopic scale. The latter precedes a second phase transition at p(c2), which involves long-range order of Pb antipolar displacements along cubic [uv0] directions and a compatible mixed tilt system (a(+)b(-)b(-)) or long-range ordered antiphase tilts with unequal magnitudes (a(0)b(-)b(-)) without Pb displacement ordering. The phase-transition pattern at p(c2) depends on the fine-scale degree of chemical B-site order in the structure.