Yasuhiro Yoneda

Solid solutions of Bi _0.5 Na _0.5 TiO ₃ (BNT) and Bi _0.5 K _0.5 TiO ₃ (BKT) with ABO ₃ -type perovskite system are known to possess good ferroelectric properties. A local structural analysis and an electronic structure analysis of high-quality BNT–BKT solid solutions were performed using synchrotron radiation X-rays to clarify the mechanism of enhanced ferroelectric properties. Owing to the large off-center shift of Bi in the BNT–BKT solid solution, the periodic structure is prone to disorder. By avoiding local structural disorder, we extracted the surrounding structure of Ti and compared it with changes in the electronic structure. The off-center shift in Bi and the reduction in the symmetry of the TiO ₆ octahedra were found to occur at different compositions.

The phase transition behavior of relaxor ferroelectric Bi1/2K1/2TiO3 (BKT) is investigated in ceramic form based on synchrotron radiation x-ray diffraction (SR-XRD) measurements and dielectric measurements as a function of temperature. The BKT ceramics show a spontaneous phase transition from the non-ergodic relaxor (NER) phase to the ferroelectric phase characterized by an abrupt change in relative permittivity around 310 °C. Analyses of the temperature dependence of dielectric properties reveal that, upon cooling from the high-temperature paraelectric phase, a phase transition to the ergodic relaxor phase associated with the formation of polar nanoregions (PNRs) occurs at the Burns temperature (TB) of 584 °C, and the PNRs are frozen at the freezing temperature (Tf) of 364 °C. SR-XRD measurements for the BKT powders show that the crystal structure transforms from the cubic (Pm3¯m) to the tetragonal (P4mm) in the temperature range of 327–347 °C, which is slightly higher than the temperature of the spontaneous relaxor-to-ferroelectric phase transition. These results indicate that the structural transformation from the cubic NER state to the tetragonal NER state may trigger the spontaneous relaxor-to-ferroelectric phase transition. Based on these experimental observations, a temperature phase diagram of BKT is established in terms of average crystal structure and polar order.

In order to enhance the ferroelectric properties, composite nanoparticles with a core–shell structure were prepared, and since BaTiO₃ (BT) core and KNbO₃ (KN) shell form a hierarchical structure, structural analysis was performed at various scales using a combination of synchrotron radiation and electron microscopy. The KN shell layer was epitaxially grown on the BT core and exhibited a distinct structure from the bulk. Additionally, the local structure and chemical bonding state of the KN shell layer exhibited discrepancies from those of the bulk, a phenomenon attributable to the two-dimensional growth and epitaxial effects analogous to those observed in thin films. There were significant changes in the electronic states not only at the interface with the core, but also at the surface of the shell layer. Composite nanoparticles can effectively modify the chemical bonding states of ferroelectrics through the effects of dimensionality and size.

NaNbO₃, AgNbO₃, and KNbO₃ with ABO₃-type perovskite systems are known to possess good ferroelectric properties. In their traditional average structure, a change in the A-site ion size changes the involved tolerance factor. Herein, we directly determined the rattling space of each atom through local structure analysis. By combining a pair distribution function and an extended X-ray absorption fine structure, this analysis revealed that the bonding sites with large fluctuations varied with varying ion sizes. Experimental evidence including soft X-ray absorption spectroscopy, indicates that the A-site ions are hybridized with oxygen.

Solid solutions of Bi_0.5Na_0.5TiO₃ (BNT) and BaTiO₃ (BT) with ABO₃-type perovskite system is known to possess good ferroelectric properties. A local structural analysis and an electronic structure analysis of high-quality BNT-BT solid solutions were performed using synchrotron radiation X-rays to clarify the mechanism of the enhanced ferroelectric properties. The BNT-BT solid solution undergoes a series of phase transitions to tetragonal structures with space groups P4bm and P4mm. The local structure analysis revealed reduced off-center shift of A-site atoms resulting of the BT substitution. The local structure obtained by high-energy X-ray total scattering is consistent with the electronic structure obtained by X-ray absorption spectroscopy.

The average and local structures of solid solutions of (1 – x )BiFeO ₃ – x BaTiO ₃ were investigated. At room temperature, pseudo-cubic phases appear in BaTiO ₃ compositions with x values ranging from 0.3 to 0.7. The ferroelectricity was not lost even in the cubic structure with the highest ferroelectricity observed at x = 0.3. We used a pair distribution function and an X-ray absorption fine structure to perform a structural analysis to determine the source of this ferroelectricity. These solid solutions’ local structures were consistent with the pure limits of BiFeO ₃ and BaTiO ₃ , respectively, but when the composition of BaTiO ₃ was 0.3, a structure peculiar to the structural boundary in the solid solutions appeared. We discovered that the structure had the greatest randomness around this composition by performing a model fit of this local structure. Local structure changes to improve nanoscale dipole interactions while maintaining the ferroelectric long-range order structure. As a result, the domain size grows and the ferroelectricity improves.

The layered perovskite LaSr ₃ Fe ₃ O ₁₀ (LSF) easily hydrates under ambient air. The hydrated LSF transports hydroxides in the middle‐temperature range of 100 °C–250 °C. This study used X‐ray absorption fine structure analysis on normal and hydrated LSF from room temperature to 350 °C to evaluate the relationship between local structures and ion transport characteristics. The H ₂ O molecules are mainly intercalated into rock‐salt interlayers, which are close to the La1 and O3 sites. The generated OH ⁻ ions are found at O3 sites, which are closer to Sr ions than La ions. The intercalated H ₂ O molecules and formed OH ⁻ ions expand the spaces of the rock‐salt interlayers. The temperature dependence of the local crystal structure was also studied in this study. According to the findings, the presence of OH ⁻ ions on the O3 sites, which are closer to Sr ions, plays a significant role in OH ⁻ transport in the medium temperature range.