Yuichi Shimakawa 교수

Crystal structures of ferroelectric materials of stoichiometric SrBi2Ta2O9 (TC=300 °C) and Sr-deficient-and-Bi-excess Sr0.8Bi2.2Ta2O9 (TC=400 °C) were refined by neutron powder diffraction. Bi2O2 layer and TaO6 octahedra are considerably distorted and atomic displacements along the a axis cause ferroelectric spontaneous polarization. In Sr0.8Bi2.2Ta2O9, both Bi substitution and cation vacancies at the Sr site were revealed and a chemical composition of (Sr0.82Bi0.12)Bi2Ta2O9.0 was obtained. The

Crystal structures and ferroelectric properties of a series of Bi-layered compounds, ${\mathrm{CaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9},$ ${\mathrm{SrBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9},$ and ${\mathrm{BaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9},$ were investigated. The structures of ${\mathrm{CaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ and ${\mathrm{SrBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ are orthorhombic, while that of ${\mathrm{BaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ is ps

The crystal structures of Bi4Ti3O12 and Bi3.25La0.75Ti3O12 were refined by neutron powder diffraction. Large structural distortions were revealed, and ferroelectric polarizations along the a and c axes were calculated from the displacements of the constituent ions. In Bi3.25La0.75Ti3O12, La atoms substitute for Bi atoms in a perovskite-type unit only, and the substitution causes less distortion of the structure, resulting in smaller spontaneous polarization and lower ferroelectric Curie temperat

${T}_{c}$ variations observed in some Tl-based superconductors were investigated. Clear correlations were found between ${T}_{c}$, carrier concentration, and c-axis length. In particular, for ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6}$, a decrease in oxygen content of about 0.1 per formula unit, which corresponded to a decrease in hole concentration of 0.2, increased ${T}_{c}$ up to about 80 K, and elongated the c axis by about 0.4%. The carrier concentration in that system could

Ferroelectric materials of the SrBi2(Ta1−xNbx)2O9 solid-solution system were synthesized, and their structural and ferroelectric properties were investigated. Atomic displacements of the ions in the (Ta,Nb)O6 octahedron significantly increase as x increases, which leads to more structural distortion of the perovskite-type unit. The Bi2O2 layer, in contrast, is less distorted in SrBi2Nb2O9 than in SrBi2Ta2O9. The contribution of the perovskite-type unit to total ferroelectric polarization is grea

Recent findings on intriguing physical properties of new A-site-ordered perovskite structure oxides are reviewed. High-pressure and high-temperature conditions stabilize the square-coordinated Jahn-Teller Cu (2+) ions in the original 12-fold-coordinated positions of the perovskite structure. The special-ordered arrangement of the square-coordinated A'O 4 units that align perpendicularly to each other sets a characteristic structural framework, and the presence of Cu (2+) ions at the A' site and

New multiferroic compounds with double-perovskite structures were synthesized. Bi₂NiMnO₆ was synthesized in bulk form by high-pressure synthesis and also in a thin-film form by epitaxial growth. The material showed both ferromagnetic and ferroelectric properties, i.e., the multiferroic property at low temperature. Bi₂FeCrO₆ was also fabricated in a (1 1 1) oriented BiFeO₃/BiCrO₃ artificial superlattice, with a 1/1 stacking period. The superlattice film showed ferromagnetic behavior and polarizat

We refined the crystal structures of four pseudotetragonal samples of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$ with ${\mathit{T}}_{\mathit{c}}$'s of 0 (metallic), 48, 58, and 73 K by Rietveld analysis of time-of-flight neutron-powder-diffraction data. The presence of excess oxygen atoms located at an interstitial site between double TlO layers was confirmed. The change in oxygen content and the corresponding one in hole carrier concentration in this

Variable-emittance radiators based on the metal–insulator transition of (La,Sr)MnO3 thin films have been developed. The emittance property of the films was evaluated from infrared reflectance spectra; that is, the (La,Sr)MnO3 thin films show low emittance at low temperature but high emittance at high temperature. Moreover, the emittance property significantly changes at the metal–insulator transition temperature, where the material changes from a highly reflective (i.e., low emissive) metal to a

Crystal structures, magnetic and transport properties, and electronic band structures for four compounds with the pyrochlore structure, ${\mathrm{Tl}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7},$ which exhibits colossal magnetoresistive behavior, and insulating ${A}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7} (A=\mathrm{Y},\mathrm{}\mathrm{In}$ and Lu), are discussed. The cubic unit cell dimensions increase with increasing ionic size of the ${A}^{3+}$ ions. The ferromagnetic behavior is consistent with a sup