Do Han Kim

Correlation between the dynamics of polar nanoregions and temperature evolution of central peaks in Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 ferroelectric relaxors

Acoustic properties of Pb[(Zn1∕3Nb2∕3)0.91Ti0.09]O3 have been investigated in a wide temperature range. The sound velocity of acoustic phonons showed a deviation from its high-temperature linear behavior at the Burns temperature TB∼730K. Upon cooling, acoustic properties exhibited significant changes at about 550K in polarization state and width of the central peak in addition to changes in frequency and damping of acoustic phonons. This finding suggests that, besides TB, another intermediate characteristic temperature exists in the evolution of relaxor dynamics, which might be related to the formation of long-lived polar nanoregions and associated local strain fields.

Hysteresis in acoustic properties of ferroelectric relaxor Pb[(Zn1/3Nb2/3)0.955Ti0.045]O3 single crystals studied by Brillouin and dielectric spectroscopies

Acoustic and dielectric properties of Pb[(Zn1∕3Nb2∕3)1−xTix]O3 (PZN‐xPT) single crystals with x=0.045 have been studied by the high-resolution micro-Brillouin scattering and dielectric spectroscopy in a wide temperature range. The softening of the Brillouin shift and the increase of dielectric relaxation time upon cooling indicated the formation of polar nanoregions (PNRs) and the slowing down of their dynamics. In contrast to the acoustic properties of typical model relaxors such as lead magnesioniobate, the change in the Brillouin shift near its minimum became sharper on heating compared to the change on cooling, pointing to the clear existence of hysteresis in the dynamics of the diffuse phase transition in PZN-4.5%PT. Since the number of PNRs will increase upon cooling, it may be expected that the kinetics of the phase transition would become slower, the lower the transition temperature resulting in the more sluggish, broad feature of the Brillouin shift observed during cooling. This result may indica...

Elastic responses to different electric-field directions in relaxor ferroelectric Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals

Micro-Brillouin scattering was applied to relaxor-based complex perovskite Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 (PZN–9% PT) single crystals during a polarization switching under two different electric-field directions along 〈111〉 and 〈101〉. Brillouin spectra consisted of one longitudinal and two transverse acoustic modes, and the Brillouin shift and the full width at half maximum of these modes showed clear changes near the coercive field (Ec) where the polarization reversal occurred. In particular, the lowest transverse mode was the most sensitive to Ec. In the case of 〈111〉-poled crystals the Brillouin spectrum exhibited an asymmetric shape in a narrow electric-field range near Ec due to a coupling between the lowest transverse mode and a central peak. In the case of 〈101〉-poled crystals, the lowest transverse mode showed an abrupt softening at Ec and a slow recovery to the initial value upon further increasing the electric field. This may indicate the appearance of the monoclinic symmetry where the polarizatio...

Micro-Brillouin scattering study of ferroelectric relaxor Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals under the electric field along the [001] direction

Electric-field effects on structural phase transitions have been studied in ferroelectric relaxor Pb[(Zn1∕3Nb2∕3)1−xTix]O3 single crystals with x=0.09 by the high-resolution micro-Brillouin scattering. Sharp phase transitions from cubic-to-tetragonal and then from tetragonal-to-rhombohedral phases have been observed under zero-field-cooling (ZFC) condition. For two phase-transition temperatures a noticeable thermal hysteresis was clearly observed, consistent with dielectric measurements. The temperature range of a tetragonal phase has been markedly extended under the electric field of E=6.7kV∕cm along the [001] direction. A large difference of the LA-mode frequency and damping between the ZFC and field-cooling processes indicated that the multidomain structure induces the increase of the acoustic damping due to the elastic scattering. In addition, it was found that the elastic stiffness coefficient c33 in the tetragonal coordinates shows a slight decrease. The first-order character of the cubic-to-tetrago...

Microheterogeneity and field-cooling effects on Pb[(Zn1∕3Nb2∕3)0.955Ti0.045]O3 single crystals probed by micro-Brillouin scattering

Microheterogeneity and field-cooling effects were investigated on Pb[(Zn1∕3Nb2∕3)0.955Ti0.045]O3 (PZN-4.5%PT) single crystals by using a high-resolution micro-Brillouin scattering. The temperature dependence of Brillouin shift showed a typical relaxor behavior with marked softening on approaching the diffuse phase transition, but also revealed a clear microareal variation in a rhombohedral phase below 150°C which means a heterogeneity exists over a length scale of at least a few microns in PZN-4.5%PT. These two features seem to correlate with the coexistence of both micronsized domains and irregular nanosized domains, recently confirmed by high-resolution domain studies. This complex domain structure may make each microdomain represent different relaxor behaviors due to its own polar nanoregions and their dynamics. When the crystal was cooled under the electric field along the [001] direction from a cubic phase, two field-induced changes were observed in the Brillouin shift at around 143°C and 106°C. This...

Acoustic anisotropy in uniaxial tungsten bronze ferroelectric single crystals studied by Brillouin light scattering

Acoustic properties were investigated for four tungsten bronze (TB) uniaxial ferroelectric crystals, i.e., (Sr0.61Ba0.39)5Nb10O30 (SBN61), Cu-doped (K0.5Na0.5)1.0(Sr0.75Ba0.25)4.5Nb10O30 (KNSBN:Cu), K5.80Li3.82Nb10.12O30, and K4.74Li3.07Nb10.44O30 of which the spontaneous polarization is directed along the polar c axis. Large acoustic anisotropy between the two elastic constants C11 and C33 have been observed from all samples. C33 exhibits a significant softening on approaching the diffuse phase transition temperature from high-temperature side while C11 does not show any substantial change in the same temperature range. This softening is accompanied by substantial growth of hypersonic damping, appearance and growth of central peak (CP), and slowing down of the relevant dynamics of CP represented by the reducing half width. All these results indicate that the lattice motions along the c axis couple strongly to the one-component order parameter of the polar nanoregions (PNRs) or precursor polar clusters wh...

Dielectric Anomalies of (K 0.5 Na 0.5 ) 0.2 (Sr 0.75 Ba 0.25 ) 0.9 Nb 2 O 6 Single Crystals with the Tungsten Bronze Structure

Tetragonal tungsten bronze (K 0.5 Na 0.5 ) 0.2 (Sr 0.75 Ba 0.25 ) 0.9 Nb 2 O 6 single crystals have been investigated by dielectric spectroscopy. Ferroelectric phase transition at about 420 K shows a thermal hysteresis with some diffuseness, implying a first-order nature of the phase transition. Below the transition temperature where domain wall contribution is expected to be dominant in the dielectric susceptibility, logarithmic dielectric dispersion in addition to the polydispersive freezing loss peak has been observed. The former seems to exist in a wide temperature range and may be understood within the framework of relaxations of elastic interfaces in disordered media. Upon further decreasing temperature, very active transverse dynamics were revealed along the directions perpendicular to the polar c axis by dielectric and pyroelectric measurements. The dielectric relaxation rate followed the Arrhenius law, and the distribution of the relevant activation energy has been obtained from the scaling of the temperature- and frequency-dependent dielectric losses.

Brillouin Scattering Study on Polycrystalline Relaxor Ferroelectrics

Brillouin spectra of Pb(Yb1/2Nb1/2)O3(PYN)-based relaxor ferroelectric ceramics were measured using a conventional tandem Fabry-Perot interferometer. The measured spectra were composed of a strong elastic peak and a broad doublet with a sharp cutoff at the high-frequency side. The broad doublet was attributed to the distributed Brillouin shift of the longitudinal acoustic mode owing to the scattering events at all scattering angles from 0° to 180° which is due to multiple reflections and refractions. From the sharp high-frequency cutoff of this band, which corresponds to the largest Brillouin shift of the backward scattering geometry, the average longitudinal sound velocities were estimated to be 4.22–4.33 km/s for PYN-based relaxor ceramics. The present results imply that Brillouin spectroscopy can be used to derive elastic properties of not only single crystals but also ceramic samples in the gigahertz range, which will be very important for high-frequency applications.

Low-Frequency Central Peaks In Relaxor Ferroelectric PZN-9%PT Single Crystals

The Brillouin spectra of lead-based complex perovskite relaxors Pb[(Zn 1/3 Nb 2/3 )1 − x Ti x ]O 3 (PZN-x%PT) single crystals with x = 9%, located near the morphotropic phase boundary, have been examined in detail. The longitudinal acoustic mode became softened markedly upon cooling from 523 K down to 430 K, which was accompanied by the significant increase of the acoustic damping. Below 523 K, a quasi-elastic central peak began to appear and grew. Relaxation time estimated from the half width of the central peak increased from approximately 3 to 15 ps in the same temperature range. This reflects the slowing down of the dynamics of relaxators which is responsible for the formation of the central peak, and precursor polar nano regions at high temperatures have been suggested as a possible origin.