Browsing by Author "Zhang, HW"

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    Dynamical mechanism of phase transitions in a-site ferroelectric relaxor (Na1/2Bi1/2)TiO3
    (APS Physics, 2014-10-13) Deng, GC; Danilkin, SA; Zhang, HW; Imperia, P; Li, XB; Zhao, X; Luo, HQ
    The dynamical phase transition mechanism of (Na1/2Bi1/2)TiO3 (NBT) was studied using inelastic neutron scattering. Softening was observed of multiple phonon modes in the phase transition sequence of NBT. As usual, the softening of the zone center transverse optical modes Δ5 and Σ3 was observed in the (200) and (220) zones, showing the Ti vibration instabilities in TiO6 octahedra for both cubic-tetragonal (C-T) and tetragonal-rhombohedral (T-R) phase transitions. In these two phase transitions, however, Ti4+ has different preferential displacement directions. Surprisingly, the longitudinal optic mode also softens significantly toward the zone center in the range of the transition temperature, indicating the Na+/Bi3+ vibration instability against TiO6 octahedra during the T-R phase transition. Strong inelastic diffuse scattering shows up near M(1.5, 0.5, 0) and R(1.5, 1.5, 0.5) in the tetragonal and rhombohedral phases, respectively, indicating the condensations of the M3 and R25 optic modes for the corresponding transitions. This reveals the different rotation instabilities of TiO6 in the corresponding transition temperature range. Bottleneck or waterfall features were observed in the dispersion curves at certain temperatures but did not show close correlations to the formation of polar nanoregions. Additional instabilities could be the origin of the complexity of phase transitions and crystallographic structures in NBT. © 2014 American Physical Society.
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    Dynamical mechanism of phase transitions in a-site ferroelectric relaxor (Na1/2Bi1/2)TiO3
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Deng, GC; Danilkin, SA; Zhang, HW; Imperia, P; Li, XB; Zhao, X; Luo, HQ
    The dynamical phase transition mechanism of (Na1/2Bi1/2)TiO3 (NBT) was studied using inelastic neutron scattering. Softening of multiple phonon modes were observed to correlate with the phase transition sequence of NBT. As usual, the softening of the zone centre transverse optic (TO) modes Δ5 and Σ3 was observed in (200) and (220) zone, showing the Ti vibration instabilities in TiO6 octahera for both cubic-tetragonal (C-T) and tetragonal rhombohedral (T-R) phase transitions. In this two phase transitions, however, Ti4+ has different preferential displacement directions. Surprisingly, the longitudinal optic (LO) mode also soften significantly toward zone centre in the vicinity range of the transition temperature, indicating the Na+/Bi3+ vibration instability against TiO6 octahera during the R-T phase transition. Strong inelastic diffuse scattering shows up near M(1.5, 0.5, 0) and R(1.5, 1.5, 0.5) in the tetragonal and rhombohedral phases, respectively, indicating the condensations of the M3 and R25 optic modes for the corresponding phase transitions. This reveals the rotation instabilities of TiO6 in the corresponding phase transition temperature range. Bottleneck or waterfall features were observed in the dispersion curves at certain temperatures, but did not show the close correlations to the formation of polar nanoregions (PNRs). Additional instabilities are the origin of the complexity of phase transitions and crystallographic structures in NBT.
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    Neutron diffuse scattering of (1 − x)(Na0.5Bi0.5)TiO3–xBaTiO3 relaxor ferroelectric single crystals
    (Elsevier, 2014-09-01) Zhang, HW; Deng, GC; Studer, AJ; Li, XB; Zhao, X; Luo, HQ
    Peculiar L-shaped diffuse streaks along <0 0 1>pc were observed in (Na0.5Bi0.5)TiO3 (NBT) below the Burns temperature, revealing the existence of polar nanoregions (PNRs). The displacement of Bi3+ cations is ∼0.26 Å along <0 0 1>pc in the PNRs, with a size of ∼13 Å in NBT. The BaTiO3 doping drives the diffuse scattering patterns to evolve from the L-shape into an ellipsoidal along <0 1 1 >pc in (Na0.5Bi0.5)TiO3–xBaTiO3. An abnormally large correlation length was observed in the morphotropic phase boundary composition with x = 0.05. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.