Browsing by Author "Yashima, M"
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- ItemCrystal structure, optical properties, and electronic structure of calcium strontium tungsten oxynitrides CaxSr1-xWO2N(American Chemical Society, 2013-09-12) Yashima, M; Fumi, U; Nakano, H; Omoto, K; Hester, JRNovel calcium strontium tungsten oxynitrides CaxSr(1-x)WO(2)N (x = 0.25 and 0.5) have been synthesized. The crystal and electronic structures, electron-density distribution, and optical properties of CaxSr(1-x)WO(2)N and CaxSr(1-x)WO(2)N (x = 0, 0.25, and 0.5) have been investigated by neutron, synchrotron, and X-ray powder diffraction; transmission electron microscopy energy-dispersive spectroscopy (TEMEDS); scanning electron microscopy; UV visible reflectance measurements; and ab initio density functional theory (DFT)based calculations. Precursor materials CaxSr(1-x)WO(2)N (x = 0, 0.25, 0.5, and 1) with a scheelite-type structure were prepared by solid-state reactions, and heated at 900 degrees C for 5 h under an ammonia flow. The main phase in the product for the composition x = 1 was metallic tungsten W, whereas cubic Pm3m perovskite-type oxynitrides CaxSr(1-x)WO(2)N were obtained for the compositions x = 0, 0.25, and 0.5. The unit-cell parameter a of the cubic perovskite-type CaxSr(1-x)WO(2)N obtained from the Rietveld analysis of synchrotron X-ray and neutron powder diffraction data decreases with an increase of Ca concentration x (0 < x < 0.5), which indicates the substitution of Ca for Sr. The existence of nitrogen in CaxSr(1-x)WO(2)N was confirmed by (I) the refined occupancy factor in the Rietveld analysis of neutron data and (2) EDS. The maximum-entropy-method electron-density analysis combined with the DFT calculations indicates W N and W-O covalent bonds in CaxSr1_xWO2N, which are formed by the overlapping of W 5d and anion 2p orbitals. The minimum electron density at the W N bond is higher than that at the W-0 one, which indicates that the W N bond is more covalent due to the smaller difference in the electronegativity between W and N atoms compared to the W and O ones: The oxidation number of W in CaxSr(1-x)WO(2)N was estimated to be 5.2 by bond valence sum, which indicates the W5+ ion with the 5di electron configuration. Precursor oxides Ca Sr,,WO, with W6* having the 5cl electron configuration are white and insulating, whereas the CaxSr(1-x)WO(2)N oxynitrides with the W5* ion having the 5di configuration are black and exhibit metallic character. These results indicate the insulator metal transition from the d oxide CaxSr(1-x)WO(2)N to the di oxynitride CaxSr(1-x)WO(2)N. © 2013, American Chemical Society.
- ItemDimer-mediated cooperative mechanism of ultrafast-ion conduction in hexagonal perovskite-related oxides(American Chemical Society, 2023-11-14) Sakuda, Y; Murakami, T; Avdeev, M; Fujii, K; Yasui, Y; Hester, JR; Hagihala, M; Ikeda, Y; Nambu, Y; Yashima, MOxide-ion and proton conductors have found diverse applications such as in electrolytes of solid-oxide, proton-conducting, and hybrid-ion fuel cells. Research of fuel cells with higher energy efficiency at lower operating temperature has stimulated the search for ion conductors and improved the understanding of the ion-diffusion mechanism. Ion conduction in hexagonal perovskite-related materials is rare, and the mechanism of ion diffusion is unclear. Herein, we report high oxide-ion and proton conductivity (bulk conductivities in wet air: 11 and 2.7 mS cm-1 at 537 and 326 °C, respectively), high chemical, and electrical stability in a new hexagonal perovskite-related oxide Ba7Nb3.8Mo1.2O20.1. Total direct current conductivity at 400 °C in wet air of Ba7Nb3.8Mo1.2O20.1 was 13 times higher than that of Ba7Nb4MoO20. We also report a unique dimer-mediated cooperative mechanism of the high oxide-ion conduction of Ba7Nb3.8Mo1.2O20.1 (bulk conductivities in dry air: 10 mS cm-1 at 593 °C and 1.1 mS cm-1 at 306 °C). Ab initio molecular dynamics (AIMD) simulations, neutron-diffraction experiments at 800 °C, and neutron scattering length density analyses of Ba7Nb3.8Mo1.2O20.1 indicated that the excess oxygen atoms are incorporated by the formation of both 5-fold coordinated (Nb/Mo)O5 monomer and its (Nb/Mo)2O9 dimer with a corner-sharing oxygen atom and that the breaking and reforming of the dimers lead to the high oxide-ion conduction in the oxygen-deficient BaO2.1 c′ layer. The long distance between Nb/Mo and Ba cations sandwiching the c′ layer of Ba7Nb3.8Mo1.2O20.1 was found to be responsible for its low activation energy for oxide-ion conduction, leading to high conductivity at low temperatures. AIMD simulations showed that high proton conduction can be attributed to proton migration in the hexagonal close-packed BaO3 layers of Ba7Nb3.8Mo1.2O20.1. The present findings hold a great promise for the development and design of ion conductors. Copyright © 2023 The Authors. Published by American Chemical Society.
- ItemHigh 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, MProton 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
- ItemHigh proton conductivity in Ba5Er2Al2ZrO13, a hexagonal perovskite-related oxide with intrinsically oxygen-deficient layers(American Chemical Society, 2020-05-15) Murakami, T; Hester, JR; Yashima, MFor the development of proton-based electrolytes, high proton conductivity at intermediate temperatures (300–600 °C) is crucial, but the available materials have been confined to a limited number of the structure families, such as cubic perovskites. Herein, we report Ba5Er2Al2ZrO13, a hexagonal perovskite-related oxide, as a new class of proton conductors exhibiting higher conductivities than 10–3 S cm–1 between 300 and 1200 °C. The protons as charge carriers are found to exist in the inherently oxygen-deficient h′ layer of Ba5Er2Al2ZrO13, which are supported by Rietveld analysis of neutron-diffraction data, bond-valence-based energy calculations, and thermogravimetric analysis. Our discovery of a new structure family of proton conductors with the inherently oxygen-deficient h′ layer offers a strategy in designing superior proton conductors based on hexagonal perovskite-related oxides. © 2020 American Chemical Society
- ItemHigh proton conductivity in β-Ba2ScAlO5 enabled by octahedral and intrinsically oxygen-deficient layers(John Wiley & Sons Inc., 2022-12-19) Murakami, T; Avdeev, M; Morikawa, R; Hester, JR; Yashima, MProton conductors are promising materials for clean energy, but most available materials exhibit sufficient conductivity only when chemically substituted to create oxygen vacancies, which often leads to difficulty in sample preparation and chemical instability. Recently, proton conductors based on hexagonal perovskite-related oxides have been attracting attention as they exhibit high proton conductivity even without the chemical substitutions. However, their conduction mechanism has been elusive so far. Herein, taking three types of oxides with different stacking patterns of oxygen-deficient layers (β-Ba2ScAlO5, α-Ba2Sc0.83Al1.17O5, and BaAl2O4) as examples, the roles of close-packed double-octahedral layers and oxygen-deficient layers in proton conduction are shown. It is found that “undoped” β-Ba2ScAlO5, which adopts a structure having alternating double-octahedral layer and double-tetrahedral layer with intrinsically oxygen-deficient hexagonal BaO (h') layer, shows high proton conductivity (≈10−3 S cm−1 above 300 °C), comparable to representative proton conductors. In contrast, the structurally related oxides α-Ba2Sc0.83Al1.17O5 and BaAl2O4 exhibit lower conductivity. Ab initio molecular dynamics simulations revealed that protons in β-Ba2ScAlO5 migrate through the double-octahedral layer, while the h′ layer plays the role of a “proton reservoir” that supplies proton carriers to the proton-conducting double-octahedral layers. The distinct roles of the two layers in proton conduction provide a strategy for developing high-performance proton conductors. © 2022 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH.
- ItemHydrothermal 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, MDouble-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
- ItemNew 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, JROxide-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.
- ItemPolymorphism and temperature-induced phase transitions of Na2CoP2O7(American Chemical Society, 2019-12-04) Avdeev, M; Wang, CW; Barpanda, P; Fujii, K; Yashima, MPolymorphism 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
- ItemStructural 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, TK2NiF4-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
- ItemStructural origin of the anisotropic thermal expansion of a K2NiF4-type oxide CaErAlO4 through interatomic distances(Chemistry Letters, 2013-12-11) Omoto, K; Yashima, M; Hester, JRThe anisotropic thermal expansion and crystal structure of K2NiF4-type CaErAlO4 have been investigated by neutron diffraction from 298 to 1473 K. The average thermal expansion coefficient (TEC) along the c axis αc is larger than that along the a axis, mainly due to the larger Al–(apical oxygen O2) TEC compared to the Al–(equatorial O1) TEC. The larger Al–O2 TEC is attributable to the weaker Al–O2 bond. Contrary to the literature, the mean TEC of K2NiF4-type CaErAlO4 is larger than that of perovskite-type ErAlO3.