Dynamical mechanism of phase transitions in a-site ferroelectric relaxor (Na1/2Bi1/2)TiO3
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Date
2014-10-13
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Publisher
APS Physics
Abstract
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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Keywords
Phonons, Vapor condensation, Crystallography, Neutrons, Optical modes, Transition temperature
Citation
Deng, G., Danilkin, S., Zhang, H., Imperia, P., Li, X., Zhao, X. & Luo, H. (2014). Dynamical mechanism of phase transitions in a-site ferroelectric relaxor (Na1/2Bi1/2)TiO3. Physical Review B, 90(13), 134104. doi: 10.1103/PhysRevB.90.134104