Yoshiki Kubota

RIKEN, Вако, Saitama, Japan

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Publications (88)593.02 Total impact

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    ABSTRACT: We prepared polycrystalline samples of GeBi6−x Inx Te10 (x = 0, 0.06, 0.18, 0.3, and 0.6) by melting a stoichiometric mixture of Ge, Bi, In, and Te, followed by quenching in water and annealing at 723 K for 1 week. Under these preparation conditions and irrespective of the degree of In-substitution, the samples contained two different periodic structures: GeBi6Te10 and GeBi4Te7 structures. The relationship between the Seebeck coefficient, electrical resistivity, and phase fraction of the two structures was investigated at room temperature. We concluded that changes in the transport properties for all samples may reflect a mixture effect of the phase fraction of the two structures and the amounts of In-substitution in the framework of the assumed structure model in this study. The thermoelectric properties of the samples with x = 0 and 0.18 were determined from 300 K to 723 K from the measured electrical properties and the reported lattice thermal conductivity. The sample with x = 0.18 had the highest dimensionless figure of merit: ZT max = 0.11 at 466 K, which was 1.8 times larger than that of the sample with x = 0.
    No preview · Article · Oct 2015 · Journal of Electronic Materials
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    Dataset: Supplement
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    ABSTRACT: This zipped folder contains the supplementaly informantion (PDF) and raw data of SR powder diffraction and Raman spectrum. The raw data are organised into a folder structure with README.txt files used to provide guidance on the contents.
    Preview · Dataset · Jul 2015
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    ABSTRACT: We demonstrate a novel, simple synthetic method for metal (Ni) NPs in a MOF using the partial thermal decomposition of nickel(ii) 2,5-dihydroxyterephthalate (Ni-MOF-74). The Ni NPs inside the Ni-MOF-74 are several nanometers in size, and the size can be precisely controlled by the heating conditions.
    Full-text · Article · Jul 2015 · Chemical Communications
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    ABSTRACT: The ability to design and control properties of nano-sized space in porous coordination polymers (PCPs) would provide us with an ideal stage for fascinating physical and chemical phenomena. We found an interconversion of nuclear-spin isomers for hydrogen molecule H2 adsorbed in a Hofmann-type PCP, {Fe(pz)[Pd(CN)4]} (pz=pyrazine), by the temperature dependence of Raman spectra. The ortho (o)-para (p) conversion process of H2 is forbidden for an isolated molecule. The charge density study using synchrotron radiation X-ray diffraction reveals the electric field generated in coordination nano-space. The present results corroborate similar findings observed on different systems and confirm that o-p conversion can occur on non-magnetic solids and that electric field can induce the catalytic hydrogen o-p conversion.
    Preview · Article · Jul 2015 · Royal Society Open Science
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    ABSTRACT: Crystal structure of double-perovskite type BaYMn2O5+δ was studied by high-temperature synchrotron X-ray diffraction (SXRD) under precisely controlled oxygen pressure to gain deeper understanding of the remarkable oxygen intake/release capability of this oxide. The in situ SXRD analysis at 750 °C revealed that this oxide undergoes a distinct structural change upon lowering oxygen pressure, from a slightly oxygen-deficient “δ = 1” phase (BaYMn2O5.89; P(O2) = 103 Pa) to an oxygen-vacancy ordered “δ = 0.5” phase (BaYMn2O5.51; P(O2) = 10 Pa). The BaYMn2O5.89 structure (orthorhombic Cmmm) involves statistical distribution of oxygen vacancies within the yttrium plane. Meanwhile, the BaYMn2O5.51 structure (orthorhombic Icma) contains arrays of pyramidal MnO5 and octahedral MnO6 forming an alternate ordering, which is stabilized by a particular Mn3+ orbital ordering with collective displacements of Y3+ arrays. Thus, the discontinuous change in the oxygen content can be attributed to the structural reconstruction with oxygen/vacancy redistribution accompanied by yttrium displacement organization.
    No preview · Article · Jan 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: We report the effects of spark plasma sintering (SPS) and subsequent annealing on the crystal structure, microstructure, and thermoelectric properties of polycrystalline GeSb6Te10. GeSb6Te10 consolidated using SPS consisted of a mixture of GeSb6Te10-type homologous and Sb2Te3-type tetradymite structures, whereas the sample prepared by melting had a single homologous structure. SPS produced small amounts of Ge-rich precipitates with a few micrometers in size. Even excluding these precipitates, the elemental compositional deviation from the nominal composition was wider than that for the sample prepared by melting. This implies that SPS promoted atomic diffusion and rearrangement of elements, leading to a substantial change in the crystal structure and elemental distribution of GeSb6Te10. SPS improved the power factor but also increased the thermal conductivity, as a result of the increased electrical conductivity, yielding a maximum dimensionless figure of merit, ZTmax, of 0.33 at 710 K, which is similar to the value for the sample prepared by melting (0.39 at 710 K).
    No preview · Article · Jan 2015 · Journal of Alloys and Compounds
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    ABSTRACT: We studied the effects of different preparation method and In-substitution on its crystal structure and thermoelectric (TE) properties of polycrystalline GeSb6Te10. Firstly, the effects of spark plasma sintering (SPS) on the crystal structure and elemental distribution of GeSb6Te10 were discussed. GeSb6Te10 consolidated using SPS consisted of a mixture of GeSb6Te10-type homologous and Sb2Te3-type tetradymite structures, whereas the sample prepared by melting had a single homologous structure. The elemental compositional deviation from the nominal composition of the sample prepared by SPS was wider than that prepared by melting. This implies that SPS promoted atomic diffusion and rearrangement of elements, leading to a substantial change in the crystal structure and elemental distribution of GeSb6Te10. Secondly, the effects of In-substitution on the crystal structure and TE properties of GeInxSb6−xTe10 (x=0, 0.18, 0.3, and 0.6) prepared by melting were discussed. Rietveld and Le Bail analyses showed that all compositions crystallized in trigonal structures with a 51-layer period. Substituting In decreased both the lattice and electronic thermal conductivity, as well as markedly increased the Seebeck coefficient. We ascribed this increase to increases in the effective mass of the carriers, likely caused by the formation of additional energy states near the Fermi level. In GeIn0.6Sb5.4Te10, we found a maximum dimensionless figure of merit: ZTmax of 0.75 at 710 K, 1.9 times higher than that of GeSb6Te10.
    Preview · Article · Jan 2015 · Journal of the Japan Institute of Metals
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    ABSTRACT: Presented here is the synthesis of an ordered bcc copper-palladium nanoalloy, via the decomposition of a Pd nanoparticle@metal-organic framework composite material. In situ XRD measurements were performed in order to understand the mechanism of the decomposition process. This result gives a further perspective into the synthesis of new nanomaterials via metal-organic framework decomposition.
    Full-text · Article · Sep 2014 · Chemical Communications
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    Masaki Takata · Eiji Nishibori · Yoshiki Kubota · Hiroshi Tanaka
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    ABSTRACT: Wide-spread functionalization research of Metal Organic Frameworks(MOFs) has brought rapid increase in variety of materials since the beginning of structural study in nanoporous of MOFs were made by SR(Synchrotron Radiation) powder diffraction using the MEM(Maximum Entropy Method)/Rietveld Method(Kitaura et al, 2002). The MEM/Rietveld method has successfully applied to refine the structural position of absorbed molecules and to investigate a bonding nature between the molecules and MOF's pore walls. Noise-resistance electron density mapping with incomplete data set was a key advantage of MEM to visualize unmodeled feature of molecules in nanoporous. Since then, the charge density studies by the MEM/Rietveld Method have uncovered various ordering structure of absorbed molecules into nanoporous more and more(Takata, 2008). Those findings ignited trends to design the nanoporous as the space to be functionalized. Recently, the MEM/Rietveld method has been further developed as the method to map an electrostatic potential and electric field(Tanaka 2006). This technique is making a progress in structural science of MOFs since the visualized electrostatic potential in the nanoporous ought to provide information of interplay between the molecule and the pore walls. The talk will present the recent progress and challenges of the MEM/Rietveld method to the structural science of the MOFs.
    Preview · Article · Aug 2014 · Acta Crystallographica Section A: Foundations and Advances
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    ABSTRACT: Vanadium spinel oxides AV 2 O 4 have attracted much attention for recent years because they show the peculiar physical properties which are caused by competition and cooperation of spin, orbital and lattice degrees of freedom. Among such compounds, FeV 2 O 4 is a unique compound showing successive phase transitions: cubic to tetragonal (c < a) at ~140 K, from tetragonal to orthorhombic accompanied by ferrimagnetic transition at ~110 K and from orthorhombic to tetragonal (c > a) at ~70 K with decreasing temperature. It is suggested that these phase transitions originate from the orbital degrees of freedom of both Fe 2 + ions at A-site (tetrahedral site) and V 3 + ones at B-site (octahedral site), however, the origin remains controversial. In the present study, we investigate the substitution effect of Fe 2 + with Co 2 + having no orbital degrees of freedom to clarify the role of the orbital degree of Fe 2 + at the A-site. We carried out magnetization and specific heat measurements and synchrotron powder diffraction experiments by the Debye-Scherrer camera at the beamline BL-8B at Photon Factory in KEK. For x ≤ 0.1, the successive structural transitions similar to that observed in FeV 2 O 4 occur although the transition temperature of cubic-to-tetraLT transition rapidly decreases with increasing x. For 0.2≤ x ≤ 0.6, the only structural transition from cubic to tetragonal (c < a) was observed, however, the transition temperatures were somewhat different from the ferrimagnetic transition ones. On the other hand, for x ≥ 0.7, the crystal structure remains cubic down to 10 K similar to that of CoV 2 O 4 . These structural properties are discussed in terms of the orbital states of Fe 2 + ions obtained by the normal mode analysis, and they are compared with the results of the specific heat and magnetization measurements.
    No preview · Article · Aug 2014 · Acta Crystallographica Section A: Foundations and Advances
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    ABSTRACT: Orbital degrees of freedom plays an important role in condensed matter physics because it is strongly related with phase transitions and induces the fascinating physical properties. A spinel oxide FeV 2 O 4 is one of the peculiar examples because this compound has double orbital degrees of freedom at both Fe 2 + and V 3 + ions. Furthermore, this material represents exotic physical properties [1,2], i.e.; multiferroic, large magnetostriction, and successive structural transitions with decreasing temperature: cubic - tetragonal (c < a: tetraHT, 138K) - orthorhombic (orthoHT, 108 K) - tetragonal (c > a: tetraLT, 68 K). However, the origin of structural transitions and physical properties is controversial until now. In order to clarify the origin, we have performed synchrotron x-ray diffraction experiments at low temperatures at beamline BL02B2 (for the powder samples) in SPring-8 and BL-4C (for the single crystal) of the Photon Factory, KEK. Furthermore, we have carried out the magnetization and the specific heat measurements using polycrystalline samples and single crystal of FeV 2 O 4 . We have firstly found another orthorhombic phase (orthoLT) below 30 K in the polycrystalline sample of FeV 2 O 4 , shown in figure 1. The Rietveld analysis was performed, and the overall qualities of fittings were fairly good. In order to investigate the details of the orbital state of Fe 2 + and V 3 + in FeV 2 O 4 , we have performed the normal mode analysis, which is based on static displacements of the tetrahedron of FeO 4 and octahedron of VO 6 . In the orthoLT phase, we found the orbital order of Fe 2 + ions, which is mixture of 3z 2 -r 2 and y 2 -z 2 orbitals, without change of orbital order of V 3 + ions. This result indicates that the origin of the orthoLT phase is derived from the competition between cooperative Jahn-Teller effect and relativistic spin-orbit coupling of Fe 2 + ions. We also discuss the origins of the other phase transitions considering the orbital state of V 3 + and Fe 2 + ions, and then the orbital dilution effect, where the structural and magnetic properties are investigated by using powder samples substituted for Fe 2 + and V 3 + ions by other ions (Mn 2 + and Fe 3 + ) with no orbital degrees of freedom.
    No preview · Article · Aug 2014 · Acta Crystallographica Section A: Foundations and Advances
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    ABSTRACT: We studied the crystal structure and thermoelectric properties of polycrystalline GeInxSb6-xTe10 (x = 0, 0.18, 0.3, and 0.6). Rietveld and Le Bail analyses showed that all compositions crystallized in trigonal structures with a 51-layer period. Substituting In decreased both the lattice and electronic thermal conductivity, as well as markedly increased the Seebeck coefficient. We ascribed this increase to increases in the effective mass of the carriers, likely caused by the formation of additional energy states near the Fermi level. In GeIn0.6Sb5.4Te10, we found a maximum ZT of 0.75 at 710 K, 1.9 times higher than that of GeSb6Te10. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
    Preview · Article · Aug 2014 · APL Materials
  • Shogo Kawaguchi · Hiroki Ishibashi · Yoshiki Kubota
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    ABSTRACT: We have investigated structural and magnetic properties at low temperature phases in spinel oxides Fe1+xV2-xO4 (0 <= x <= 0.4) using synchrotron powder diffraction, magnetization, and specific heat experiments. The only lattice distortion from a cubic to a tetragonal phase (c > a) occurs at ferrimagnetic order temperature for x >= 0.3, indicating that this tetragonal phase is dominantly affected by the relativistic spin-orbit coupling of Fe2+ ions. Whereas, the structural transition from a tetragonal (c > a) to an orthorhombic phase was not detected in the Fe1+xV2-xO4 system. Furthermore, we have performed the accurate structural analysis by Rietveld method with synchrotron diffraction data. The very small distortion of the VO6 octahedron derived from the orbital order of V3+ ions below the non-collinear ferrimagnetic transition temperature was successfully observed for x <= 0.2. This magnetic transition accompanied by the slight distortion disappears near x = 0.25. This result indicates that the orbital order of V3+ ions is strongly related with the non-collinear ferrimagnetic transition in FeV2O4.
    No preview · Article · Jul 2014 · Journal of the Physical Society of Japan
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    ABSTRACT: Pd octahedrons and cubes enclosed by {111} and {100} facets, respectively, have been synthesized for investigation of the shape effect on hydrogen-absorption properties. Hydrogen-storage properties were investigated using in situ powder X-ray diffraction, in situ solid-state (2)H NMR and hydrogen pressure-composition isotherm measurements. With these measurements, it was found that the exposed facets do not affect hydrogen-storage capacity; however, they significantly affect the absorption speed, with octahedral nanocrystals showing the faster response. The heat of adsorption of hydrogen and the hydrogen diffusion pathway were suggested to be dominant factors for hydrogen-absorption speed. Furthermore, in situ solid-state (2)H NMR detected for the first time the state of (2)H in a solid-solution (Pd + H) phase of Pd nanocrystals at rt.
    Full-text · Article · Jul 2014 · Journal of the American Chemical Society
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    ABSTRACT: Hydrogen is an essential component in many industrial processes. As a result of the recent increase in the development of shale gas, steam reforming of shale gas has received considerable attention as a major source of H2, and the more efficient use of hydrogen is strongly demanded. Palladium is well known as a hydrogen-storage metal and an effective catalyst for reactions related to hydrogen in a variety of industrial processes. Here, we present remarkably enhanced capacity and speed of hydrogen storage in Pd nanocrystals covered with the metal-organic framework (MOF) HKUST-1 (copper(II) 1,3,5-benzenetricarboxylate). The Pd nanocrystals covered with the MOF have twice the storage capacity of the bare Pd nanocrystals. The significantly enhanced hydrogen storage capacity was confirmed by hydrogen pressure-composition isotherms and solid-state deuterium nuclear magnetic resonance measurements. The speed of hydrogen absorption in the Pd nanocrystals is also enhanced by the MOF coating.
    Full-text · Article · Jul 2014 · Nature Materials
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    ABSTRACT: Microporous coordination polymers having various shaped apertures were synthesized and crystallographically characterized using substituted terephthalate ligands. The substituents exert a notable influence on the channel size and shape depending on the orientations and disordering of the phenylene units such as a fin. This is a new aspect of porous coordination polymers, which would provide important information for a precise design of the micropore.
    No preview · Article · May 2014 · Microporous and Mesoporous Materials
  • Shogo Kawaguchi · Hiroki Ishibashi · Fumiya Miki · Yoshiki Kubota

    No preview · Conference Paper · Mar 2014
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    ABSTRACT: Porous coordination polymers (PCPs), constructed from organic linkers and metal ions, can provide special pore environments for selective CO2 capture. Although many PCPs have been reported, a rational design for identifying PCPs that adsorb CO2 molecules with a low binding energy, high separation ability and high chemical stability remains a great challenge. Here, we propose and validate, experimentally and computationally, a new PCP, [La(BTN)DMF].guest (PCP-1 superset of guest), that has a large aromatic organic surface and a low binding energy for high CO2 separation from four-gas mixtures (CO2-N-2-O-2-CO) at ambient temperature. In addition, it shows good water and chemical stability; in particular, it is stable from pH = 2 to 12 at 100 degrees C, which is unprecedented for carboxylate-based PCPs.
    No preview · Article · Feb 2014 · Chemical Science
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    ABSTRACT: PdxRu1-x solid solution alloy nanoparticles were successfully synthesized over the whole composition range through a chemical reduction method, although Ru and Pd are immiscible at the atomic level in the bulk state. From the XRD measurement, it was found that the dominant structure of PdxRu1-x changes from fcc to hcp with increasing Ru content. The structures of PdxRu1-x nanoparticles in the Pd composition range of 30-70% consisted of both solid solution fcc and hcp structures, and both phases coexist in a single particle. In addition, the reaction of hydrogen with the PdxRu1-x nanoparticles changed from exothermic to endothermic as the Ru content increased. Furthermore, the prepared PdxRu1-x nanoparticles demonstrated enhanced CO-oxidizing catalytic activity; Pd0.5Ru0.5 nanoparticles exhibit the highest catalytic activity. This activity is much higher than that of the practically used CO-oxidizing catalyst Ru and that of the neighboring Rh, between Ru and Pd.
    No preview · Article · Jan 2014 · Journal of the American Chemical Society
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    ABSTRACT: We report the first in situ NMR observation of 2H2 adsorbed inside [Cu3(btc)2] (HKUST-1; btc: benzenetricarboxylate) at ambient temperature and pressure. Hydrogen adsorption behavior was investigated by hydrogen sorption analysis and in situ solid-state 2H NMR under controlled 2H2 gas pressure. Although HKUST-1 adsorbed very little amount of H2 at 303 K, we could successfully observe the 2H2 signal inside HKUST-1 in the NMR spectra. Furthermore, the NMR signal demonstrated that HKUST-1 interacts with H2 molecules even at ambient temperature and hydrogen pressure.
    No preview · Article · Jan 2014 · Chemistry Letters

Publication Stats

4k Citations
593.02 Total Impact Points

Institutions

  • 2015
    • RIKEN
      Вако, Saitama, Japan
  • 2005-2015
    • Osaka Prefecture University
      • Department of Physical Science
      Sakai, Ōsaka, Japan
  • 2003-2006
    • Osaka Women's University
      Sakai, Ōsaka, Japan
  • 2002
    • University of Sydney
      • School of Chemistry
      Sydney, New South Wales, Australia
  • 1999
    • Shimane University
      • Interdisciplinary Faculty of Sciences and Engineering
      Matsu, Shimane Prefecture, Japan
  • 1993
    • Nagoya University
      Nagoya, Aichi, Japan