Browsing by Author "Fujii, K"

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    High proton conduction in Ba2LuAlO5 with highly oxygen-deficient layers
    (Springer Nature, 2023-06-06) Morikawa, R; Murakami. T; Fujii, K; Avdeev, M; Ikeda, Y; Nambu, Y; Yashima, M
    Proton conductors have found diverse applications, such as electrolytes in proton ceramic fuel cells, which require high ionic conductivity at low temperatures and high chemical stability. Here, we report the oxide, Ba2LuAlO5, which exhibits proton conductivities of 10−2 S cm−1 at 487 °C and 1.5 × 10−3 S cm−1 at 232 °C, high diffusivity and high chemical stability without chemical doping. Ba2LuAlO5 is a hexagonal perovskite-related oxide with highly oxygen-deficient hexagonal close-packed h′ layers, which enables a large amount of water uptake x = 0.50 in Ba2LuAlO5·x H2O. Ab initio molecular dynamics simulations and neutron diffraction show the hydration in the h′ layer and proton migration mainly around cubic close-packed c layers existing at the interface of octahedral LuO6 layers. These results demonstrate that the high proton conduction allowed by the highly oxygen-deficient and cubic close-packed layers is a promising strategy for the development of high-performance proton conductors. © 2023 The Authors - Open Access CC BY
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    Hydrothermal synthesis, crystal structure, and superconductivity of a double-perovskite Bi oxide
    (American Chemical Society, 2015-12-23) Rubel, MHK; Takei, T; Kumada, N; Ali, MM; Miura, A; Tadanaga, K; Oka, K; Azuma, M; Yashima, M; Fujii, K; Magome, E; Moriyoshi, C; Kuroiwa, Y; Hester, JR; Avdeev, M
    Double-perovskite Bi oxides are a new series of superconducting materials, and their crystal structure and superconducting properties are under investigation. In this paper, we describe the synthesis and characterization of a new double-perovskite material that has an increased superconductive transition temperature of 31.5 K. The structure of the material was examined using powder neutron diffraction (ND), synchrotron X-ray diffraction (SXRD), and transmission electron microscopy (TEM). Rietveld refinement of the sample based on ND and SXRD data confirmed an A-site-ordered (K1.00)(Ba1.00)3(Bi0.89Na0.11)4O12 double-perovskite-type structure with the space group Im3̅m (No. 229). This structural analysis revealed the incorporation of Na with Bi in the structure and a bent bond between (Na, Bi)–O–(Na, Bi). TEM analyses also confirmed a cubic double-perovskite structure. This hydrothermally synthesized compound exhibited a large shielding volume fraction, exceeding 100%, with onset of superconductivity at ∼31.5 K. Its electrical resistivity dropped near onset at ∼28 K, and zero resistivity was confirmed below 13 K. The calculated band structure revealed that the metallicity of the compound and the flatness of the conduction bands near the Fermi level (EF) are important for the appearance of superconductivity. © 2015 American Chemical Society
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    Microscopic solvation structure and phase behavior of thermo-responsive polymers in ionic liquids
    (International Conference on Neutron Scattering, 2017-07-12) Hirosawa, K; Fujii, K; Ueki, T; Kitazawa, Y; Watanabe, M; Gilbert, EP; Shibayama, M
    Ionic liquids (ILs) are molten salts having their melting points near room temperature. ILs consist of only ion species, and thus they exhibit unique solvent properties such as high ion conductivity, negligible volatility and nonflammability. Recently,it was reported that poly(benzyl methacrylate) (PBnMA) and its derivatives show a lower critical solution temperature type phase separation in ILs. Interestingly, the phase separation temperature of the thermo-responsive polymers in IL systems strongly depends on both chemical structures of the polymer and the ILs. It indicates that macroscopic phase behavior of the systems is strongly affected by microscopic molecular interactions between polymers and ILs. In this study, we performed small-angle neutron scattering (SANS) experiments on various PBnMA derivatives in deuterated IL solutions. The interaction parameter, ? between the polymers and the ILs was estimated from the obtained SANS profiles. Here, enthalpic (?H) and entropic (?S) contributions to ? were obtained from temperature dependence of ?. As a result, it was found that ?H strongly depends on the chemical structure of the polymers and the ILs. Furthermore, microscopic solvation structure of the polymers in IL systems was investigated by high-energy X-ray total scattering measurement with the aid of molecular dynamics (MD) simulations. It was found that there is strong correlation between the value of ?H and the microscopic solvation structure.
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    New perovskite-related structure family of oxide-ion conducting materials NdBaInO4
    (ACS Publications, 2014-03-21) Fujii, K; Esaki, Y; Omoto, K; Yashima, M; Hoshikawa, A; Ishigaki, T; Hester, JR
    Oxide-ion conducting ceramic materials, which include pure oxide-ion conductors and mixed oxide-ion and electronic or hole conductors, have received considerable attention because of their potential application for oxygen separation membranes, oxygen sensors, and solid oxide fuel cells (SOFCs) electrolytes and cathodes.1 Perovskite-type and perovskite-related materials have been widely investigated as oxide-ion conductors.2 For example, the K2NiF4-type compounds are known to exhibit high oxide-ion conductivity.3 Because the oxide-ion conductivity is strongly dependent on crystal structure, it is necessary to design and synthesize novel materials belonging to a new structure family for further innovative developments of the oxide-ion conductors. Herein, we report a new perovskite-related structure family with AA′BO4 composition, which exhibits oxide-ion conduction. Here, A and A′ are relatively larger cations and B is a smaller cation. In this work we have succeeded in solving the crystal structure of NdBaInO4 and show oxide-ion conduction in NdBaInO4. © 2014, American Chemical Society.
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    Polymorphism and temperature-induced phase transitions of Na2CoP2O7
    (American Chemical Society, 2019-12-04) Avdeev, M; Wang, CW; Barpanda, P; Fujii, K; Yashima, M
    Polymorphism and temperature-induced phase transitions of Na2CoP2O7 were studied by in situ neutron powder diffraction and complemented by ab initio calculations to reconcile previous reports of its three polymorphs. We show that the “blue” form prepared at 873 K exists at room temperature in the orthorhombic Pna21 (= P21cn) phase, which transforms via a first-order transition to the tetragonal form at the temperature close to room temperature (∼335 K). Just above the transition, the tetragonal form is likely incommensurately modulated with the modulation vanishing at ∼423 K. Above that temperature the phase remains in the unmodulated tetragonal state (P42/mnm) until melting at ∼900 K. Upon cooling after melting, Na2CoP2O7 crystallizes into the “rose” triclinic P1 form which persists while it cools to room temperature, apparently stabilized by the barrier of the reconstructive “rose”–“blue” transition. We also discuss the relationship between the tetragonal and orthorhombic structures, the driving forces of the orthorhombic distortion, and similarity to Na2ZnP2O7 and the melilite-type structural family. © 2019 American Chemical Society
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    Structural origin of the anisotropic and isotropic thermal expansion of K2NiF4-type oxides
    (American Chemical Society, 2015-04-02) Kawamura, K; Yashima, M; Fujii, K; Omoto, K; Hibino, K; Yamada, S; Hester, JR; Avdeev, M; Miao, P; Torii, S; Kamiyama, T
    K2NiF4-type LaSrAlO4 and Sr2TiO4 exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO4 and Sr2TiO4 have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the c-axis (αc) being higher than that along the a-axis (αa) of LaSrAlO4 [αc = 1.882(4)αa] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α(Al–O2) being higher than that between Al and equatorial oxygen O1 α(Al–O1) [α(Al–O2) = 2.41(18)α(Al–O1)]. The higher α(Al–O2) is attributed to the Al–O2 bond being longer and weaker than the Al–O1 bond. Thus, the minimum electron density and bond valence of the Al–O2 bond are lower than those of the Al–O1 bond. For Sr2TiO4, the Ti–O2 interatomic distance, d(Ti–O2), is equal to that of Ti–O1, d(Ti–O1) [d(Ti–O2) = 1.0194(15)d(Ti–O1)], relative to LaSrAlO4 [d(Al–O2) = 1.0932(9)d(Al–O1)]. Therefore, the bond valence and minimum electron density of the Ti–O2 bond are nearly equal to those of the Ti–O1 bond, leading to isotropic thermal expansion of Sr2TiO4 than LaSrAlO4. These results indicate that the anisotropic thermal expansion of K2NiF4-type oxides, A2BO4, is strongly influenced by the anisotropy of B–O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance d(B–O2)/d(B–O1) of A22.5+B3+O4 compared with A22+B4+O4, A22.5+B3+O4 compounds have higher α(B–O2), and A22+B4+O4 materials exhibit smaller α(B–O2), leading to the anisotropic thermal expansion of A22.5+B3+O4 and isotropic thermal expansion of A22+B4+O4. The “true” thermal expansion without the chemical expansion of A2BO4 is higher than that of ABO3 with a similar composition. © 2015 American Chemical Society