Browsing by Author "Nambu, Y"

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    Block magnetism coupled with local distortion in the iron-based spin-ladder compound BaFe 2 Se 3
    (American Physical Society, 2012-02-22) Nambu, Y; Ohgushi, K; Suzuki, S; Du, F; Avdeev, M; Uwatoko, Y; Munakata, K; Fukazawa, H; Chi, S; Ueda, Y; Sato, TJ
    Magnetism in the insulating BaFe2Se3 was examined through susceptibility, specific heat, resistivity, and neutron diffraction measurements. After formation of a short-range magnetic correlation, a long-range ordering was observed below TN∼255 K. The transition is obscured by bulk properties. Magnetic moments (∥a) are arranged to form a Fe4 ferromagnetic unit, and each Fe4 stacks antiferromagnetically. This block magnetism is of the third type among magnetic structures of ferrous materials. The magnetic ordering drives unusually large distortion via magnetoelastic coupling. ©2012 American Physical Society
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    Dimer-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, M
    Oxide-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.
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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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    Magnetic and transport properties of AFe_2Se_3 (A= Ba and Cs)
    (The Physical Society of Japan, 2012-03-05) Du, F; Ohgushi, K; Sasaki, R; Matsubayashi, K; Uwatoko, Y; Kawakami, T; Nambu, Y; Avdeev, M; Sato, TJ; Ueda, Y
    The orthorhombic AFe2Se3 (A=Ba and Cs) have attracted much attention due to the possibility of superconducivity AFe2Se3 The crystal structure of 14Fe2Se3 can be described as double chains of [Fe2Se3】formed by the edge sharing FeSe4 tetrahedra. We synthesize AFezSe3 single crystalline samples by the self flux method firstly, then studies the magnetic and transport properties under ambient and high pressure. BaFe2Se3 shows a semiconducting behavior. With increasing pressure 8 GPa, the charge gap of BaFe2Se3 gradually decreases; however, the ground state is still insulating at 8 GPa. Interestingly, CBFe2Se3 also shows a Semiconducting behavior although the formally mixed valence state of Fe ions. Mossbauer spectra of CsFe2Se3 indicate that electronic state of Fe ions is unique, ruling out the possibility of charge ordering. In addition, a magnetic transition at 170K is suggested from the hyperfine filed splitting in the Mossbauer spectra and neutron diffraction analysis．
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    Nitrogen-rich molybdenum nitride synthesized in a crucible under air
    (American Chemical Society, 2024-03-05) Demura, M; Nagao, M; Lee, CH; Goto, Y; Nambu, Y; Avdeev, M; Masubuchi, Y; Mitsudome, T; Sun, W; Tadanaga, K; Miura, A
    The triple bond in N2 is significantly stronger than the double bond in O2, meaning that synthesizing nitrogen-rich nitrides typically requires activated nitrogen precursors, such as ammonia, plasma-cracked atomic nitrogen, or high-pressure N2. Here, we report a synthesis of nitrogen-rich nitrides under ambient pressure and atmosphere. Using Na2MoO4 and dicyandiamide precursors, we synthesized nitrogen-rich γ-Mo2N3 in an alumina crucible under an ambient atmosphere, heated in a box furnace between 500 and 600 °C. Byproducts of this metathesis reaction include volatile gases and solid Na(OCN), which can be washed away with water. X-ray diffraction and neutron diffraction showed Mo2N3 with a rock salt structure having cation vacancies, with no oxygen incorporation, in contrast to the more common nitrogen-poor rock salt Mo2N with anion vacancies. Moreover, an increase in temperature to 700 °C resulted in molybdenum oxynitride, Mo0.84N0.72O0.27. This work illustrates the potential for dicyandiamide as an ambient-temperature metathesis precursor for an increased effective nitrogen chemical potential under ambient conditions. The classical experimental setting often used for solid-state oxide synthesis, therefore, has the potential to expand the nitride chemistry. © 2024 American Chemical Society.
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    Pressure-induced superconductivity in the iron-based ladder material BaFe2S3
    (Springer Nature, 2015-07-20) Takahashi, H; Sugimoto, A; Nambu, Y; Yamauchi, T; Hirata, Y; Kawakami, T; Avdeev, M; Matsubayashi, K; Du, F; Kawashima, C; Soeda, H; Nakano, S; Uwatoko, Y; Ueda, Y; Sato, TJ; Ohgushi, K
    All the iron-based superconductors identified so far share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square-lattice structures but also in ladder structures. Yet iron-based superconductors without a square-lattice motif have not been found, despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below ∼120 K. On the application of pressure this compound exhibits a metal–insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity. © 2015, Nature Publishing Group
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    Stripelike magnetism in a mixed-valence insulating state of the Fe-based ladder compound CsFe2Se3
    (American Physical Society, 2012-06-28) Du, F; Ohgushi, K; Nambu, Y; Kawakami, T; Avdeev, M; Hirata, Y; Watanabe, Y; Sato, TJ; Ueda, Y
    Structural and electronic properties of the Fe-based spin-ladder compound CsFe2Se3 was investigated by means of resistivity, susceptibility, specific heat, Mossbauer, and neutron diffraction measurements. Despite the single-site nature in a mixed-valence state, the ground state is a magnetic insulator characterized by a charge gap similar to 0.34 eV and an antiferromagnetic transition temperature 175 K. The magnetic structure was stripelike, with magnetic moments of 1.77(6)mu(B) coupled ferromagnetically (antiferromagnetically) along the rung (leg) direction. Both the insulating behavior and stripelike ordering can be understood by assuming extra carriers delocalized on the rung. Our findings reveal that CsFe2Se3 is an appealing compound with the stripelike magnetic structure in an insulating state among Fe-based compounds, and provide significant supplemental insight into the magnetism of Fe-based superconductors. © 2012, American Physical Society.
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    Structural, electrical, magnetic, and optical properties of iron-based ladder compounds BaFe2(S1−xSex)3
    (American Physical Society, 2020-07-01) Imaizumi, S; Aoyama, T; Kimura, R; Sasaki, K; Nambu, Y; Avdeev, M; Hirata, Y; Ikemoto, Y; Moriwaki, T; Imai, Y; Ohgushi, K
    We performed a comprehensive study on structural, electrical, magnetic, and optical properties for iron-based ladder materials BaFe2(S1−xSex)3(0≤x≤1), which shows pressure-induced superconductivity in the vicinity of the Mott transition at x=0 and 1. We obtain a complete electronic phase diagram in a temperature-composition plane, which reveals that the magnetic ground state switches from the stripe-type to the block-type phase without any intermediate phase at x=0.23 with increasing x. This behavior is in sharp contrast to the filling controlled system Ba1−xCsxFe2Se3, in which a paramagnetic state down to the lowest temperature is realized between two magnetic ordered states. The structural transition, which is considered to be relevant to the orbital order, occurs far above the magnetic transition temperature. The magnetic and structural transition temperatures exhibit a similar composition dependence, indicating a close relationship between magnetic and orbital degrees of freedom. In addition, we found that charge dynamics are considerably influenced not only by the magnetic order but also by the structural change (orbital order) from the detailed measurements of electrical resistivity and optical conductivity spectra. We discuss the magnetism and orbital order by comparing the experimental results with the proposed theory based on the multiorbital Hubbard model. The relationship between the charge dynamics and the magnetic/orbital order is also discussed. ©2020 American Physical Society
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    Temperature and composition phase diagram in the iron-based ladder compounds Ba1−xCsxFe2Se3
    (American Physical Society, 2015-05-28) Hawai, T; Nambu, Y; Ohgushi, K; Du, F; Hirata, Y; Avdeev, M; Uwatoko, Y; Sekine, Y; Fukazawa, H; Ma, J; Chi, S; Ueda, Y; Yoshizawa, H; Sato, TJ
    We investigated the iron-based ladder compounds (Ba,Cs)Fe2Se3. Their parent compounds BaFe2Se3 and CsFe2Se3 have different space groups, formal valences of Fe, and magnetic structures. Electrical resistivity, specific heat, magnetic susceptibility, x-ray diffraction, and powder neutron diffraction measurements were conducted to obtain a temperature and composition phase diagram of this system. Block magnetism observed in BaFe2Se3 is drastically suppressed with Cs doping. In contrast, stripe magnetism observed in CsFe2Se3 is not so fragile against Ba doping. A new type of magnetic structure appears in intermediate compositions, which is similar to stripe magnetism of CsFe2Se3, but interladder spin configuration is different. Intermediate compounds show insulating behavior, nevertheless a finite T-linear contribution in specific heat was obtained at low temperatures. ©2015 American Physical Society