Anwar Hushur

Crystallization of amorphous bismuth titanate

We studied the transformation of amorphous bismuth titanate by heat treatments. After an as-quenched amorphous sample was annealed at 500 °C, the lowest Raman peak became intense like a boson peak for glass. This fact indicates the formation of intermediate range order. The medium range correlation length of 7 nm calculated from the boson peak frequency is in agreement with the mean cluster size measured by atomic force microscopy (AFM). The differential thermal analysis (DTA) shows the two-step crystallization at 608 and 830 °C on heating. It suggests the existence of a metastable state. The samples annealed at 770 and 1000 °C are identified as the pyroclore and the layered perovskite structures, respectively. It strongly suggests that at first a three-dimensional (3D) crystalline state appears by nucleation process from a 3D amorphous state and secondly it transforms into the pseudo-2D layered perovskite structure.

Order-disorder nature of ferroelectric BaTi2O5

Ferroelectric single crystals of BaTi2O5 have been studied in detail by Raman and high-resolution micro-Brillouin scattering with a focus on the central component of the relaxation mode over a wide temperature range (20–600°C). A relaxation mode has been clearly observed in these single crystals. The temperature dependences of the relaxation time and integrated intensity of the relaxation mode are well reproduced by the extended semiclassical tunneling model. The ferroelectric phase transition shows, at least near the vicinity of Tc, an order-disorder nature. Our results indicate that the spontaneous polarization mainly resulted from the dynamic disorder of the Ti ions in the octahedra along the polar b axis of BaTi2O5.

Critical slowing down of relaxation dynamics near the Curie temperature in the relaxor Pb(Sc0.5Nb0.5)O3

Brillouin spectroscopy of Pb(Sc0.5Nb0.5)O3 single crystals reveals an order-disorder ferroelectric phase transition occurs at 110 °C upon cooling. A softening of the longitudinal acoustic (LA) mode is observed that can be attributed to the coupling between polar nanoregions and acoustic modes. A critical slowing down of the central peak, a feature of order-disorder ferroelectric phase transitions, is observed near Tc. The similarity in temperature dependences of the two kinds of relaxation times determined from the central peak and LA mode suggests that the changes in the central peak arise from local polarization fluctuations in the polar nanoregions.

Observation of Relaxation Mode in Ferroelectric Barium Dititanate by Micro-Brillouin Scattering

New ferroelectric single crystals of BaTi2O5 have been studied in detail by high-resolution micro-Brillouin scattering with focus on the central component of the relaxation mode over a wide temperature range (20–600°C). A relaxation mode has been clearly observed in these single crystals for the first time. In addition, the usual phonon features are also observed in the Brillouin shifts determined by selection rules for a monoclinic phase. The ferroelectric phase transition at Tc~470°C, sharp anomaly has been observed from both the frequency shift and hypersonic damping. The relaxation time slows down and intensity of central peak increases markedly when temperature approaches Tc from above and below. Furthermore, the relaxation mode has an A(y) symmetry which implies that the relaxation mode involves interwell motion of Ti ions parallel to the polar axis. The temperature dependence of the relaxation time and integrated intensity of the relaxation mode are well reproduced by the extended semiclassical tunneling model in which the off-center ion in the octahedra tunnels between two sites of a double-well potential.

Acoustic anomalies and central peak in SrBi2Ta2O9 single crystals studied by micro-Brillouin scattering

Acoustic properties of layered perovskite SrBi2Ta2O9 (SBT) single crystals were investigated by micro-Brillouin scattering. Two elastic stiffness coefficients, c33 and c44, defined within the pseudotetragonal crystallographic axes, and the corresponding sound velocities were determined in the temperature range between 290 and 773 K. The longitudinal sound velocity propagating along the pseudotetragonal c axis was found to be very small, which can be related to the weak interlayer bonding of SBT. The corresponding c33 shows marked softening on approaching the ferroelectric phase transition temperature Tc around 600 K from both high- and low-temperature sides. The hypersonic attenuation increases markedly around Tc, which seems to reflect not only the bilinear coupling between the order parameter and the strain in the ferroelectric phase, but also the electrostrictive coupling in the paraelectric phase. Furthermore, a quasielastic central component was observed near Tc, which is believed to be partly respon...

Elastic Anomaly of Glass Transitions in Lithium Silicate

Brillouin spectra of lithium silicate glasses with various lithium oxide concentrations have been investigated over a wide temperature range. The room-temperature elastic constants increase significantly with increasing lithium oxide concentration. This indicates that rigidity increases with lithium oxide concentration. From the temperature dependence of the longitudinal elastic modulus, it was found that the rigidity decreases on heating, faster with a high lithium content. The rate of decrease becomes significantly larger above the glass transition temperature Tg. This is related to the disruption of network bonds, suppressing the rigidity of the melt structures on heating. The increased lithium oxide concentration leads to a faster degradation in the network structure. The Tg of lithium silicate glasses was determined. The increased fragility with increased lithium oxide concentration reflects the significant decrease in longitudinal elastic modulus with temperatures above Tg. Crystallization temperature decreases with increasing lithium oxide concentration.

The phase transitions of ferroelectric Sr2Ta2O7 crystals by MDSC, Brillouin and dielectric spectroscopy

The structural phase transitions of Sr2Ta2O7 single crystals have been studied by the modulated temperature differential scanning calorimetry (MDSC), Brillouin scattering and dielectric spectroscopy. The specific heat (Cp) was measured over a wide temperature range from −150°C to 25°C and from 100°C to 210°C. The Cp curve showed an anomaly at To = 166.7°C, indicating the phase transition Cmcm → P21/m. The transition enthalpy ΔH, the transition entropy ΔS and specific heat jump ΔCp at To were estimated to be 0.465 J g−1, 1.01 mJ g−1 K−1 and 9.78 mJ g−1 K−1, respectively. The Cp anomaly associated with the ferroelectric phase transition at Tc = −107°C has not been detected. However, both Brillouin and dielectric data showed the anomalies corresponding to the ferroelectric phase transition from P21/m to P21.