Shinya Tsukada

Micro-Brillouin Scattering of Relaxor Ferroelectrics with Perovskite Structure

Hypersonic elastic anomalies are investigated in ferroelectrics with perovskite structure by micro-Brillouin scattering to make clear the role of polar nanoregions (PNRs) and static random fields. In order to clarify the role of dynamic PNRs, the nearly second order normal ferroelectric phase transition is studied in 12 mol% KF substituted BaTiO3 (KF-BT/0.12). The precursor elastic behaviors and the critical slowing down are clearly observed above the Curie temperature TC . While in relaxor ferroelectric 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 (PZN-0.07PT) with PNRs and static random fields, below the intermediate characteristic temperature T*, the suppression of slowing down of relaxation time is observed. The equation of stretched slowing down well reproduces the observed temperature dependence above TC . The growth of the characteristic localized length of dynamic PNRs is estimated as a function of temperature.

Relaxor-like dynamics of ferroelectric K(Ta1−xNbx)O3 crystals probed by inelastic light scattering

The relaxor-like dynamics of the cubic-tetragonal ferroelectric phase transition was studied by Brillouin and Raman scattering in K(Ta1−xNbx)O3 (KTN) crystals with x = 0.40 (KTN40). The local symmetry breaking by the polar nanoregions (PNRs) was observed in a paraelectric phase by Raman scattering on the A1(z) mode of the PNRs with R3m symmetry. Upon cooling from a high temperature, the remarkable increase in the LA phonon damping starts at 45 K above the cubic-tetragonal phase transition temperature of TC-T = 308 K, which is defined as the intermediate temperature, T* ∼ 353 K, indicating the start of the rapid growth of the PNRs. The coupling between the LA mode and fluctuation of the PNRs caused a remarkable elastic anomaly in the vicinity of TC-T. The analysis of the temperature dependent central peak shows a critical slowing down towards TC-T, which is the evidence for the order-disorder nature of a ferroelectric phase transition. The evolution of the dynamic PNRs is discussed by the estimation of the...

Role of dynamic polar nanoregions in heterovalent perovskite relaxor: Inelastic light scattering study of ferroelectric Ti rich Pb(Zn1/3Nb2/3)O3-PbTiO3

The role of dynamic polar nanoregions (PNRs) of (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-100xPT) single crystals with the PT content x = 0.15 (i.e., higher than above the morphotropic phase boundary composition x ∼ 0.09) has been investigated using inelastic light scattering. The remarkable anomaly of the longitudinal acoustic mode is clearly observed in the vicinity of the cubic-tetragonal phase transition temperature at the TC-T = 481 K. A broad central peak (CP) appears below the Burns temperature TB ∼ 700 K. Upon cooling, the relaxation time determined from the CP width clearly shows a critical slowing down when approaching TC-T in contrast with the suppressed slowing down previously observed in PZN-7PT [S. Tsukada and S. Kojima, Phys. Rev. B 78, 144106 (2008)]. These facts are due to the existence of dynamic PNRs and the disappearance of chemically ordered regions resulting from a higher PT content in the solid solution. In the cubic phase, the local symmetry breaking caused by the PNRs was observed by Ram...

Dielectric and Piezoelectric Properties of 10% KF-Doped BaTiO3 Ceramics

10% KF-doped barium titanate powders, Ba0.9K0.1TiO2.9F0.1, were synthesized through a sol–gel process. The powders, calcined at 650 °C, consist of cubic crystalline particles of ~70 nm in length; the particle size increases to ~200 nm as the firing temperature increases to ca. 800 °C, above which F2 begins to evaporate. Dense ceramics were fabricated by the spark plasma sintering (SPS) method; the average grain size is ~2 µm in lengths. The ceramics, well annealed at 1,000 °C in an O2 gas flow, have good dielectric and piezoelectric properties; the piezoelectric d33 value is 230 pC/N at room temperature. At the ferroelectric Curie temperature TC = 47 °C, the dielectric constant and loss tan δ are 10,000 and <5% at 10 kHz, respectively. The Curie–Weiss relation holds in the fully disordered cubic and ordered rhombohedral phases, showing the second order 1:2 relation. Below 10 kHz, large dielectric dispersion caused by a domain-wall motion appears at the temperature range of -50 to 107 °C. Some discussions are made for these dielectric properties of the ceramics.

Fano resonance of Li-doped KTa1−xNbxO3 single crystals studied by Raman scattering

The enhancement of functionality of perovskite ferroelectrics by local structure is one of current interests. By the Li-doping to KTa1−xNbxO3 (KTN), the large piezoelectric and electro-optic effects were reported. In order to give new insights into the mechanism of doping, the microscopic origin of the Fano resonance induced by the local structure was investigated in 5%Li-doped KTN single crystals by Raman scattering. The coupling between the continuum states and the transverse optical phonon near 196 cm−1 (Slater mode) caused a Fano resonance. In the vicinity of the cubic-tetragonal phase transition temperature, TC-T = 31 °C, the almost disappearance of the Fano resonance and the remarkable change of the central peak (CP) intensity were observed upon heating. The local symmetry of the polar nanoregions (PNRs), which was responsible for the symmetry breaking in the cubic phase, was determined to E(x, y) symmetry by the angular dependence of Raman scattering. The electric field induced the significant change in the intensity of both CP and Fano resonance. From these experimental results, it is concluded that the origin of the Fano resonance in Li-doped KTN crystals is the coupling between polarization fluctuations of PNRs and the Slater mode, both belong to the E(x, y) symmetry.

Dielectric properties of spark plasma sintered BaTi2O5 ceramics prepared through sol–gel process

Barium dititanate (BaTi2O5) ceramics with high density and phase purity were fabricated by spark plasma sintering (SPS) from the sol–gel derived powders. The fine grained ceramics exhibited a diffuse phase transition which is qualitatively the same as the conventional defects induced relaxor behavior but different in high transition temperature and weak diffuseness. Furthermore the dielectric constant of the present BaTi2O5 ceramics showed an abnormal enhancement under the dc bias. The tunability can reach 58%. This can be explained by the favor of b-axis during the cooling process with the help of the dc bias.

Large acoustic thermal hysteresis in relaxor ferroelectric Pb(Zn1/3Nb2/3)O3-PbTiO3

The diffuse phase transition in relaxor-based 0.93Pb(Zn1/3 Nb 2/3)O3–0.07PbTiO3 ferroelectric single crystals is studied by observing the large thermal hysteresis over 100 K of the longitudinal acoustic (LA) phonon. By observing this hysteresis in the LA phonon frequency with different temperature-cycles, it is concluded that nonequilibrium states are induced by supercooling. Relaxor ferroelectrics easily go supercooling, because they demonstrate structural hierarchy like glass-forming materials. The inhomogeneous structure disrupts sharp phase transitions, revealing diffuse phase transition on cooling. However, annealing at low temperatures reduces the inhomogeneity markedly. These interpretations are consistent with LA phonon behavior under electric fields, which reduce the inhomogeneity.

Brillouin Scattering Studies of Dynamics of Polar Nanoregions in Pb[(In1/2Nb1/2)0.65Ti0.35]O3 Single Crystals

The dynamical properties of polar nanoregions have been investigated in Pb[(In1/2Nb1/2)0.65Ti0.35]O3 single crystals by Brillouin scattering. Longitudinal acoustic phonons begin to show softening below the Burns temperature TB~700 K. The Brillouin shift in the longitudinal acoustic phonon mode exhibits clear anomalies at the cubic–tetragonal phase transition temperature of around Tct~550 K and at the tetragonal–rhombohedral one of around Ttr~460 K. Broad and narrow central peaks begin to appear below TB and the intermediate temperature T*~600 K, respectively. Relaxation rate was calculated in the longitudinal phonon mode and central peaks. Below T*, the relaxation rate calculated from the longitudinal phonon mode agrees well with that of the broad central peak, suggesting the coupling between local polarization and strain fluctuations due to the formation of long-lived polar nanoregions. The polarization of the central peaks in the cubic phase suggests that polar nanoregions have the tendency to form long-range polar ordering.

Role of polar nanoregions with weak random fields in Pb-based perovskite ferroelectrics

In relaxor ferroelectrics, the role of randomly orientated polar nanoregions (PNRs) with weak random fields (RFs) is one of the most puzzling issues of materials science. The relaxation time of polarization fluctuations of PNRs, which manifests themselves as a central peak (CP) in inelastic light scattering, is the important physical quantity to understand the dynamics of PNRs. Here, the angular and temperature dependences of depolarized and polarized CPs in 0.44Pb(Mg1/3Nb2/3)O3-0.56PbTiO3 single crystals with weak RFs have been studied by Raman and Brillouin scattering, respectively. The CPs observed in Raman scattering show the very clear angular dependence which is consistent with the local tetragonal symmetry. It is different from the well-known local rhombohedral symmetry with strong RFs for Pb(Mg1/3Nb2/3)O3. In Brillouin scattering, depolarized and polarized CPs show two relaxation processes corresponding to transverse and longitudinal fluctuations of PNRs. The remarkable slowing down towards the Curie temperature was observed for transverse fluctuations in local tetragonal symmetry.

Influence of KF substitution on electric field–temperature phase diagram in BaTiO3

The electric field–temperature (E–T) phase diagram of Ba1−xKxTiO3−xFx (x = 0.027) was established to investigate the relationship between critical phenomena and enhanced piezoelectric and dielectric properties after KF substitution. By measuring the dielectric constant under an electric field, the critical end point (CEP) was determined to be E = 4 kV/cm and T = 380 K. E at the CEP is markedly less than that of BaTiO3, indicating that the electric fields necessary for large polarization fluctuations decrease owing to KF substitution. We suggest that the CEP line ends at a tricritical point located at x = 0.10, T ~ 308 K, and E = 0 kV/cm in the x–E–T phase diagram.