Kaoru Miura

Sequential Phase Transitions in Sm Substituted BiFeO3

The compositional and thermal evolution of the crystal structure of the solid solution Bi1-xSmxFeO3 synthesized at a high pressure of 4 GPa was investigated. It was found that Bi1-xSmxFeO3 with x = 0.10 and 0.15 had an antipolar PbZrO3-type structure with a √2a×2√2a×4a perovskite superstructure at room temperature, while that with x = 0.05 had the same structure as the parent BiFeO3 (x = 0). The x = 0.10 sample transforms from an antipolar PbZrO3-type orthorhombic structure to a polar BiFeO3-type rhombohedral structure and eventually to a nonpolar GdFeO3-type structure on heating as Zr-rich PZT.

Characterizations of epitaxial Bi(Mg1/2Ti1/2)O3–Bi(Zn1/2Ti1/2)O3 solid solution films grown by pulsed laser deposition

(111)-oriented epitaxial (1 − x)Bi(Mg1/2Ti1/2)O3–xBi(Zn1/2Ti1/2)O3 solid solution films were grown on (111)cSrRuO3/(111)SrTiO3 substrates by pulsed laser deposition, and their crystal structure and electrical properties were characterized. The solid solution films consisted of a perovskite single phase in the x range of 0–0.93. The coexistence region of rhombohedral and tetragonal phases was observed in the x range of 0.18–0.60, which differed from reported data for the powders synthesized at high temperatures and high pressures. The polarization–electric field hysteresis loops originating from ferroelectricity were observed at room temperature and 1 kHz in the x range of 0–0.33, and the remanent polarization monotonously decreased with increasing x. The relative dielectric constant measured at room temperature and 100 kHz and the apparent piezoelectric constant evaluated by piezoresponse force microscopy at room temperature and 5 Hz were maximum at approximately x = 0.14. These results suggest that these tendencies of the electrical and electromechanical properties were related to the volume fractions of the constituent phases in the solid solution films.

Platelet NaNbO3grown by single-step molten salt synthesis: Study on bismuth migration in topochemical conversion reaction

Platelet NaNbO3 grains were grown at 1150–1225 °C by single-step molten salt synthesis. The structural and compositional transformation from the precursor Aurivillius phase to perovskite NaNbO3 by the topochemical conversion reaction was studied. No compositional distribution was confirmed for the platelet grains grown at 1150 °C, whereas it was observed that the expulsion of bismuth and incorporation of sodium were simultaneously initiated in spots for the grains grown at 1170 °C. With increasing the growth temperature the topochemical conversion reaction was promoted, and single-phase NaNbO3 grains were eventually grown with heat treatment at 1225 °C for 6 h. In order to trace the structural transformation due to the topochemical conversion reaction, preconversion and postconversion platelet grains were chosen for characterizing the microstructure. It was found that the precursor Aurivillius phase is a mixed phase described as Bi2.5Nam−1.5NbmO3m+3 (m = 5, 6, and 8). In the interior of the platelet grains, migration paths vertically elongated to the principal surface are created, and bismuth is expelled via the vertical path as well as the horizontal path along the (Bi2O2)2+ layer. It was concluded that the distinctive migration network contributed to the structural transformation while maintaining the epitaxy.

Growth of (111) one-axis-oriented bi(Mg1/2Ti1/2) O3films on (100)Si substrates

Films of a high-pressure perovskite phase, Bi(Mg1/2Ti1/2)O3, were prepared on (111)c-oriented SuRuO3-coated (111)Pt/TiO2/SiO2/(100)Si substrates. The perovskite Bi(Mg1/2Ti1/2)O3 films had a (111) one-axis orientation, because their constituent grains were epitaxially grown on (111)c-oriented perovskite SrRuO3 ones. The remanent polarization and piezoelectric constant measured at an applied electric field of 600 kV/cm were about 30 µC/cm2 and 40 pm/V, respectively. A remarkable phase transition was not observed from room temperature to 350 °C in a (111) one-axis-oriented Bi(Mg1/2Ti1/2)O3 film, suggesting that the Curie temperature of this film is above 350 °C.

Growth of (1-x)NaNbO3–xBaTiO3 Single Crystals by Slow-Cooling and Flux Methods

(1-x)NaNbO3–xBaTiO3 single crystals were grown by slow-cooling and flux methods. In the slow-cooling method, 0.88NaNbO3–0.12BaTiO3 powder was melted in a platinum crucible at 1500 °C and cooled down to 1000 °C. Slow cooling at less than 21 °C/h led to a bulky single crystal, and the BaTiO3/NaNbO3 ratio was graded inside the single crystal as expected from the phase diagram. Further slow cooling at 5 °C/h with the aim of improving the compositional uniformity increased the level of contamination of platinum in the single crystals from the crucible. Cuboidal single crystals with a (100) facet were grown in Na2B4O7 flux through a cooling process from 1200 to 1000 °C at 1 °C/h. There was no compositional gradient inside the cuboidal crystals, and the BaTiO3 content of the single crystals remained within 2–3% irrespective of the raw powder/flux ratio and the BaTiO3 composition in the raw powder. The obtained single crystals were orthorhombic ferroelectrics with a phase transition temperature of 310–330 °C.

Piezoelectric enhancement of relaxor-based lead-free piezoelectric ceramics by nanodomain engineering

0.3BaTiO₃-0.1Bi(Mg_1/2Ti_1/2)O₃-0.6BiFeO₃ ceramics were either doped with vanadium or sintered in calcined powder with the same composition. Compared to an undoped ceramic sintered without the calcined powder, both ceramics showed reduced leakage current densities (lower than 1 × 10^-7 A/cm²) and absence of dielectric relaxation behaviors observed in frequency-and temperature-dependent dielectric measurements. The Curie temperatures of both samples were higher than 460 °C. The maximum field-induced strain over the applied field, S_max/E_max, of 366 pm/V of the undoped ceramic sintered without the calcined powder increased to 455 and 799 pm/V for the V-doped sample and the sample sintered with the calcined powder, respectively. The increase was related to a reduced concentration of bismuth vacancy - oxygen vacancy defect dipoles.