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The lattice dynamical properties have been studied by Raman spectroscopy in bismuth layer-structure oxides Bi4Ti3O12, and ABi4Ti4O15 (A=Ba, Ca, Pb, Sr). The rigid-layer modes were observed in all compounds in the range 57–62.5 cm-1. The lowest mode of Bi4Ti3O12 shows marked softening, and the square frequency of the lowest mode is linearly related to the square of the spontaneous polarization. As for CaBi4Ti4O15 and SrBi4Ti4O15 the underdamped soft modes were also clearly observed. In contrast, in BaBi4Ti4O15 only low-frequency overdamped modes have been observed. In PbBi4Ti4O15 the low-frequency spectra have a complicated structure and the softening of the underdamped modes was not observed clearly.

The acoustic anomalies and precursor dynamics of high-quality barium titanate single crystals were investigated by Brillouin light scattering and the birefringence measurements in the paraelectric phase above the cubic-totetragonal ferroelectric phase transition temperature (T c ). Two elastic stiffness coefficients C 11 and C 44 , the related sound velocities, and their absorption coefficients were determined from T c to 400 • C for the first time. The longitudinal acoustic (LA) mode showed a substantial softening over a wide temperature range above T c which was accompanied by a remarkable increase in the acoustic damping as well as growth of central peaks. The broad central peak (CP) exhibited a two-mode and one-mode behavior in the paraelectric and ferroelectric phase, respectively, which was consistent with recent far-infrared reflectivity measurements and first-principle-based calculations [Ponomareva et al., Phys. Rev. B 77, 012102 (2008)]. The acoustic anomalies and CP behavior were correlated with the anomalous birefringence, piezoelectric effect, and the deviation of the Curie-Weiss law observed from the same crystal. This strongly indicates similarity between the dynamics of polar clusters in typical ferroelectrics and the dynamics of polar nanoregions in relaxors, consistent with recent acoustic emission measurements [Dul'kin et al., Appl. Phys. Lett. 97, 032903 (2010)]. The relaxation times estimated from the central peak and the LA mode anomalies exhibited similar temperature dependences with comparable orders of magnitude, indicating that the polarization fluctuations due to the precursor polar clusters couples to the LA mode through density fluctuations. All these anomalies share common microscopic origin, correlated Ti off-centered motions forming polar clusters having local symmetry breaking in the paraelectric phase. The existence of the polar clusters were directly evidenced by the temperature evolution of the precise birefringence map. The narrow central peak within ±5 GHz proposed before was not confirmed to exist in the present study.

Dynamical properties of relaxor ferroelectric 0.93Pb͑Zn 1/3 Nb 2/3 ͒O 3 -0.07PbTiO 3 single crystals have been studied by the broadband inelastic light scattering from gigahertz to terahertz frequency range. The longitudinal-and transverse-acoustic ͑LA and TA͒ phonon frequencies deviate below the Burns temperature T B = 736 K from a linear temperature dependence above T B , indicating the existence of polarization relaxations induced by the polar nanoregions ͑PNRs͒. On further cooling, a central peak ͑CP͒ which originates from the relaxations in the PNRs is observed clearly below 600 K Ͻ T B . The CP width decreases markedly down to T ‫ء‬ = 499 K and the change in the CP width becomes mild below T ‫ء‬ . The slower ͑ϳ10 −12 s͒ and faster ͑ϳ10 −13 s͒ relaxation times determined by the CP show good agreement with the relaxation times determined by TA and LA phonons, respectively. 180°and non-180°͑71°and 109°͒ polarization flippings are allowed in the PNRs with the polar rhombohedral symmetry. Considering the piezoelectric coupling in a PNR, it is suggested that 180°flipping is related to the relaxation observed in LA phonon, while non-180°flipping is related to that in TA phonon.

Precursor dynamics of a cubic to tetragonal ferroelectric phase transition in BaTiO3 is studied by the accurate measurement of the second harmonic generation (SHG) integral intensities. A finite signal holds for the SHG integrated intensity above the ferroelectric Curie temperature T(c)=403  K. Above the Burn's temperature T(d)≈580  K, the power law with the exponent γ=1 shows normal SHG nature originating from the hyper-Raman scattering by dynamical polar excitations, while, below T(d), a SHG signal from polar nanoregions becomes dominant with the larger exponent γ=2. Such a crossover of the power law exponent near T(d) is discussed on the basis of the effective Hamiltonian method and Monte Carlo simulation.