S Okada

Kokushikan University, Edo, Tōkyō, Japan

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Publications (425)1365.09 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A novel layered cobalt oxyphosphide Sr2ScCoPO3 crystallizes in a layered structure which represents an alternative stack of anti-fluorite type Co2P2 layer and K2NiF4 type Sr4Sc2O6 layer. This material has a similar structure to that of Sr2ScFePO3 which exhibits superconductivity below 17 K The resistivity of Sr2ScCoPO3 is 4.5 mΩcm at room temperature and decreases with decreasing temperature. The thermoelectric power of Sr2ScCoPO3 is -12 μV/K at room temperature and it decreases with decreasing temperature and rapidly increases below 50 K Provided the generalized Hikes formula, the S of Sr2ScCoPO3 is estimated to be -60 μV/K at high temperature region. This relative high value suggests that Sr2ScCoPO3 is a promising high performance thermoelectric material.
    Journal of Physics Conference Series 12/2012; 400(2):2091-.
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    ABSTRACT: With a counterintuitive approach to crystal growth, single crystals of α-TmAlB4 were successfully grown, which were indicated from transmission electron microscopy (TEM) and x-ray diffraction (XRD) measurements and analysis to be virtually free from the ubiquitous building defects, and their properties investigated. Recent developments in rare earth aluminoboride compounds with two-dimensional boron layers have attracted interest due to the heavy fermion superconductivity in β-YbAlB4, multiple anomalies manifesting below the Néel temperatures in α-TmAlB4 attributed to intrinsic building defects, and field stable state in Tm2AlB6. The physical properties of the obtained α-TmAlB4 crystals show a striking difference from the properties of conventional α-TmAlB4 crystals containing building defects. TN = 6.8 K is revealed to be higher than the 5.8 K formerly thought, and the multiple anomalies below TN are replaced by a sharp λ-like peak in the specific heat C. The large effect of the building defects on the physical properties could be directly confirmed, and strikingly it is shown how the large peak structure in the specific heat of the high quality sample could be destroyed by the presence of a small amount (one or two percent) of building defects.
    Journal of Applied Physics 02/2012; 111(7). · 2.21 Impact Factor
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    ABSTRACT: α-HoAlB4 and α-ErAlB4 were synthesized, and their magnetic properties and specific heat investigated in comparison with other known rare-earth analogs. Recent developments in rare-earth aluminoboride compounds with two-dimensional boron layers have attracted interest due to the heavy fermion superconductivity in β-YbAlB4, multiple anomalies manifesting below the Néel temperatures in α-TmAlB4 attributed to intrinsic building defects, and field stable state in Tm2 AlB6. Strikingly, α-HoAlB4 and α-ErAlB4 were discovered to exhibit superparamagnetic or spin glass behavior in contrast to the magnetic ordering or nonordering observed for the other rare-earth element compounds. The magnetic field dependence of the irreversibility was consistent with the de Almeida Thouless (AT) line versus the quadratic suppression typically observed for antiferromagnetic systems. The specific heat exhibited behavior indicative of a multilevel Schottky anomaly and four states of the 5 I8 Hund’s rule multiplet of Ho3+ are indicated to lie below 20 K. While building defects are not evident, it is indicated that disorder is strong in α-HoAlB4 and α-ErAlB4 and possible ferromagnetic interactions can be giving rise to frustration.
    Journal of Applied Physics 03/2011; 109(7):07E111-07E111-3. · 2.21 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 30(1).
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    ABSTRACT: Single crystal diffraction measurements were successfully carried out for spherical fine grains grown as single crystals of 0.05–0.2 mm in diameter. Local modulations in the silicon layers were also observed by means of high-resolution electron microscopy. The metallic tin–flux technique was used for crystal growth. The Fourier synthesis and maximum entropy method (MEM) were applied to x-ray diffraction data to obtain electron density distribution maps. Mn4Si7 is one of the most promising p-type thermoelectrics useable from 400 K to 700 K. The crystal structure is described in terms of a chimney-ladder structure. The doping effect, by which the system becomes n-type and a structure modulation occurs, was reported by our group previously. The resultant electron density maps were compared with those from the band calculation. The MEM calculation shows the displacement of silicon positions.
    Journal of Electronic Materials 01/2010; 39(9):1482-1487. · 1.64 Impact Factor
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    ABSTRACT: The rare-earth aluminum boride TmAlB4 was investigated in detail. TmAlB4 has previously been observed to exhibit an antiferromagnetic transition at 5.8 K. In this work, an additional transition in the in-plane magnetic susceptibility χ⊥c was observed at TN1≈40 K indicated to have short-range order or low-dimensional character. More than 100 compounds are known to take the YCrB4 type of crystal structure. However, TmAlB4 is only the second system in which magnetic transitions have been observed, following the RRuB4 compounds (R=rare-earth metal). Experimental and calculated results reveal a metal-like behavior for TmAlB4. Advanced refinement of the crystal structure from single-crystal x-ray diffraction data revealed that the TmAlB4 crystals measured are basically of the YCrB4 crystal structure type (Pbma) but with interesting anomalies. These anomalies were indicated to arise from intrinsic building defects in the basal plane due to the presence of fragments of the closely related ThMoB4-type (Cmmm) structure. Additional features in TmAlB4 were observed at low temperatures in the specific heat C and in χ⊥c at T1,T2<TN1. They are indicated to pertain to the basal plane, which points to intrinsic variations occurring in the in-plane magnetic structure and arrangement. It is proposed that the origin of this behavior is due to the presence of these intrinsic building defects in the crystal structure of TmAlB4, which lead to different environments for the thulium atoms.
    Phys. Rev. B. 01/2007; 76(6).
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    ABSTRACT: The modulated crystal structure of Bi2.12Ba2.00Rh1.95Ox has been elucidated by electron diffraction measurements and high-resolution electron microscopy (HREM). The crystal structure consists of two interpenetrating subsystems of a CdI2-type RhO2 layer and a distorted four-layered rock-salt (RS)-type block. Both subsystems have common a- and c-axes and β-angle with a = 5.38 Å, c = 15.15 Å and β = 95.3°. On the other hand, the crystal structure is incommensurate along the b-axes; b1 = 3.05 Å for the RhO2 layer and b2 = 5.45 Å for the RS-type block. The misfit ratio and the chemical composition ratio give a chemical formula of [(Bi1−xRhx)2(Ba1−yBiy)2O4+δ]0.56RhO2 with x = 0.037, y = 0.047 and δ = 0.39. It is found that this compound has two modulation vectors, and and, thus, can be regarded as (3 + 2)-dimensional crystal. The vector q1 is due to the misfit between the rock salt and RhO2 layers parallel to the b-axis. The modulation vector q2 is unique for the Ba–Rh system relative to Bi1.91Sr2.0Rh1.77Ox and [Bi0.87SrO2]2[CoO2]1.82. An additional modulation vector, , is locally observed. High-resolution images, taken with the incident electron beam parallel to the a- and c-axes, clearly exhibit modulated atomic arrangements.
    Philosophical Magazine A 01/2007; 87(18-21):2641-2646.
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    ABSTRACT: Al1.1Be0.7B22, Al1.4Mg0.45B22, AlxCuyB105 (x=2.8–3.3, y=2.9–1.0), Al1.3Cu1.1B25 and CuB23 crystals were grown from high-temperature melts of the systems Al–M–B (M=Be, Mg, Cu) and Cu–B in an argon atmosphere. The maximum dimensions of Al1.1Be0.7B22, Al1.4Mg0.45B22, Al3.1Cu2.0B105 and Al1.3Cu1.1B25 crystals were obtained in the ranges of approximately 2.5–5.2mm. CuB23 crystals prepared had maximum dimensions of approximately 30μm. The Al1.1Be0.7B22 and Al1.4Mg0.45B22 crystals were obtained as plate-like crystals having well-developed {101} and {001} faces, and Al3.1Cu2.0B105 and Al1.3Cu1.1B25 crystals were obtained in the form of needle or rhombohedral plate as bounded by {001} faces and grew as prismatic shape extending in the c direction. The values of Vickers microhardness on relatively large {101}, {100}, {001} and {001} faces of Al1.1Be0.7B22, Al1.4Mg0.45B22, AlxCuyB105 (x=2.8–3.3, y=2.9–1.0) and Al1.3Cu1.1B25 crystals are in the ranges of 25.4 (0.5) to 27.8 (1.7)GPa. Anomalous magnetic behavior was not observed for any of the compounds, and the good agreement of the temperature independent susceptibility for the AlxCuyB105 compounds with different ratio of metal elements indicate the non-metallic nature of this phase.
    Journal of Alloys and Compounds 01/2007; 442(1):320-323. · 2.73 Impact Factor
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    ABSTRACT: First principles calculations have been performed on perovskite-type RRh3B and RRh3C (R = Sc, Y and La) compounds to obtain the equilibrium lattice constants and elastic properties such as the elastic constants, bulk moduli, shear moduli, and Young’s moduli. The calculated lattice constants are in excellent agreement with the experimental results. A new relationship between the bulk modulus and the nearest neighbor distance is proposed for the present systems.
    Computational Materials Science. 05/2006; 36:12-16.
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    ABSTRACT: The crystal structures of thermoelectric (TE) compounds Bi1.91Sr2.0Rh1.77Ox and Bi 2.12Ba2.00Rh1.95Ox are investigated by means of electron diffraction measurements and high-resolution electron microscopy. These compounds have layered structures and consist of two interpenetrating subsystems exhibiting incommensurated periodicities along b-axis. Both subsystems, CdI2 -type RhO2 sheets and a distorted four-layered rock-salt (RS)-type (Bi,(Sr,Ba))O blocks, have common a-, c-axes and beta;-angles. On the other hand, the ratios of two axis lengths, i.e., b 1 (RhO2 sheet) / b2 (RS-type block), are irrational. Based on the cation ratio and b1/b2 value, the structural formulae are expressed as [(Bi1-x>Rhx)2(Sr1-yBi y)2O4+delta]0.63RhO2 with x = 0.077, y = 0.016 and delta = 0.46 for the Sr-system and [(Bi1-x>Rhx)2(Sr1-yBi y)2O4+delta]0.56RhO2 with x = 0.037, y = 0.047 and delta = 0.39 for the Ba-system, respectively. Based on the electron diffraction measurements, it is found that these compounds have irrational modulation vectors, q1 = -a* + 0.63b1* and q2 = 0.17b1* + c for Bi-Sr-Rh-O, and q1 = -a* + 0.56b1* and q 2 = 0.11b1* + 0.35c* for Bi-Ba-Rh-O, respectively, being the indications of (3+2) dimensional structures. High-resolution images taken with the incident electron beam parallel to the a- and c-axes clearly exhibit modulated atomic arrangements. These compounds are characterized by the displacement modulations in both RhO2 layers and the RS-type blocks
    01/2006;
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    ABSTRACT: Rare earth ternary borides, RRh3Bx (R=La, Gd, Lu and Sc) have been synthesized by arc melting method. Borides RRh3Bx (R=La, Gd, Lu and Sc) have perovskite-type cubic structure: space group Pm3m; Z=1. The lattice parameters a of the stoichiometric RRh3B for R=La, Gd, Lu and Sc are 0.4251(1), 0.4183(1), 0.4126(1) and 0.4080(1) nm, respectively. LaRh3Bx does not have boron-nonstoichiometry as x=0. In GdRh3Bx and LuRh3Bx, boron- nonstoichiometry ranges between 0.55≦x≦1 and 0.30≦x≦1, respectively. The boron-nonstoichiometry range is the widest, 0≦x≦1, for R=Sc. Boron-nonstoichiometry increases with decreasing atomic radius of R. The microhardness of the stoichiometric RRh3B for R=La, Gd, Lu and Sc is 4.2±0.1, 6.8±0.1, 7.7±0.5 and 9.9±0.1GPa, respectively. As a result, microhardness increases with decreasing atomic size of R in RRh3B; R is positioned at the eight corners of the cube in the perovskite-type structure. Thus, hardness is strongly dependent on R element. The hardness changes almost linearly with boron concentration x for R=Gd and Lu in RRh3Bx, while no linear dependency is found for R=Sc. Ab initio calculations have been performed to obtain the equilibrium lattice constants and the bulk moduli. The calculated lattice constants are in excellent agreement with experimental results.
    Journal of Alloys and Compounds 01/2006; 408:379-383. · 2.73 Impact Factor
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    ABSTRACT: We report synthesis of perovskite-type boride CeRh3Bx using the arc melting method. The samples are characterized by powder X-ray diffraction which shows that the crystal structure of CeRh3Bx belongs to the cubic system (space group: Pm3m) in the range of 0
    Journal of Alloys and Compounds 01/2006; 426(1):304-307. · 2.73 Impact Factor
  • Computational materials science. 01/2006; 36(1):12-16.
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    ABSTRACT: Perovskite-type RRh3B and RRh3C (R=Y, Sc) form a continuous solid solution, RRh3BxC1−x, in the range of 0≦x≦1 with cubic structure (space group: Pm3m, Z=1). The values of the microhardness of YRh3BxC1−x for x=0, 0.25, 0.50, 0.75 and 1.00 are investigated as 4.4±0.1, 4.9±0.1, 5.5±0.2, 6.4±0.2 and 7.5±0.15GPa, respectively. On the other hand, the values of the microhardness of ScRh3BxC1−x for x=0, 0.25, 0.50, 0.75 and 1.00 are 4.5±0.2, 6.1±0.2, 7.4±0.2, 8.9±0.2 and 9.6±0.1GPa, respectively. Thus, the microhardness of RRh3BxC1−x continuously becomes larger with increasing boron content. The oxidation onset temperatures of YRh3BxC1−x for x=0, 0.25, 0.50, 0.75 and 1.00 are 604, 631, 655, 687 and 978K, respectively. On the other hand, the oxidation onset temperatures of ScRh3BxC1−x for x=0, 0.25, 0.50, 0.75 and 1.00 are 674, 675, 695, 725 and 753K, respectively. Thermogravimetric analysis of the phase indicates that the oxidation onset temperature also increases with boron content. Thus, it appears that both mechanical strength and chemical stability of the RRh3BxC1−x phase essentially depend on its boron content. Ab initio calculations have been performed to obtain the equilibrium lattice constants and the bulk moduli. The calculated lattice constants are in excellent agreement with experimental results.
    Journal of Alloys and Compounds 01/2006; 408:375-378. · 2.73 Impact Factor
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    ABSTRACT: First principles calculations have been performed on perovskite-type RRh3B and RRh3C (R=Sc, Y and La) compounds to obtain the equilibrium lattice constants and elastic properties such as the elastic constants, bulk moduli, shear moduli, and Young’s moduli. The calculated lattice constants are in excellent agreement with the experimental results. A new relationship between the bulk modulus and the nearest neighbor distance is proposed for the present systems.
    Computational Materials Science - COMPUT MATER SCI. 01/2005;
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    ABSTRACT: We have studied the crystal structure of the misfit-layered crystal Bi1.91Sr2Rh1.77Ox by electron diffraction and high-resolution electron microscopy. This compound consists of two interpenetrating subsystems of a RhO2 sheet and a distorted four-layered rock-salt-type (Bi,Sr)O block. Both subsystems have common a-axes, c-axes, and β-angles with a=5.28 Å, c=29.77 Å, and β=93.7°. On the other hand, the crystal structure is incommensurated parallel to the b-axis, b1=3.07 Å for the RhO2 sheet and b2=4.88 Å for the (Bi,Sr)O block. The axis ratio, b1/b2˜ 0.63, characterizes the structural analogue as [Bi1.79Sr1.98Oy]0.63[RhO2]. This compound has two modulation vectors, q1=a*+0.63b1* and q2=0.17b1*+c*, and the superspace group is assigned as the Cc(1β 0,0μ 1) type from electron diffraction patterns. High-resolution images taken with the incident electron beam parallel to the a- and c-axes clearly exhibit modulated displacive as well as compositional atomic arrangements. A tentative structure model has been proposed, and the calculated image reproduces observed characteristic features reasonably well.
    Japanese Journal of Applied Physics 01/2005; 44(12):8557-8561. · 1.07 Impact Factor
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    ABSTRACT: A search for perovskite-type new borides in the Sc-TM-B (TM = Ti, V, Cr, Mn, Fe, Co, and Ni) system has been carried out by the arc-melting synthetic method. As a result, the chemical formula of the compound ScNi3B0.5 was obtained. This compound has the cubic perovskite-type structure (space group: Pm3m) with the lattice parameter a = 0.37760(7) nm. ScNi3B0.5 exhibits metallic behavior down to 4.2 K. The magnetic susceptibility of ScNi3B0.5 shows Pauli paramagnetism. No superconductivity appears down to the lowest temperature by the electric and magnetic measurements. The micro-Vickers hardness and oxidation resistivity in air are also reported for ScNi3B0.5.
    Journal of Alloys and Compounds. 11/2004; 383(s 1–2):294–297.
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    ABSTRACT: The out-of-plane thermal conductivity (κc) of Bi2−xPbxSr2Co2Oy is measured and analyzed. A low value of 4–8 mW∕cm K is observed at 300 K, which is close to the minimum thermal conductivity. This implies the phonon mean-free path is of the order of the c-axis lattice parameter. The temperature dependence is also anomalous, and the positive temperature coefficient dκc∕dT>0 is observed. These features can be understood semiquantitatively by a phenomenological expression of κ including the contribution of optical phonons.
    Physical Review B 01/2004; 70(21). · 3.66 Impact Factor
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    ABSTRACT: Single crystals of CrSi2 were obtained in the form of hexagonal prisms by the solution growth method using molten tin as a flux. The maximum size of the crystal is about 0.3mm in diameter and 25mm in length. The crystal structure of CrSi2 has hexagonal symmetry with space group P6222 and the lattice parameters are a=0.425(2)nm and c=0.6375(1)nm, respectively. The crystals are semiconducting. The value of the micro-Vickers hardness for the {11̄00} face with hexagonal symmetry is 11.2±0.4GPa. Weight gain of the crystals heated up to 1473K in air is negligible.
    Journal of Alloys and Compounds 01/2004; 383(1):319-321. · 2.73 Impact Factor
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    ABSTRACT: We have studied the dielectric responses of the layered cobalt oxysulfide Sr2Cu2CoO2S2 with the CoO2 square planes. With decreasing temperature below the Néel temperature, the resistivity increases like a semiconductor, and the thermopower decreases like a metal. The dielectric constant is highly dependent on temperature, and the dielectric relaxation is systematically changed with temperature, which is strongly correlated to the magnetic states. These behaviors suggest that carriers distributed homogeneously in the paramagnetic state at high temperatures are expelled from the antiferromagnetically ordered spin domain below the Néel temperature.
    Journal of Applied Physics 01/2004; 95:6816-6818. · 2.21 Impact Factor

Publication Stats

6k Citations
1,365.09 Total Impact Points

Institutions

  • 2003–2012
    • Kokushikan University
      Edo, Tōkyō, Japan
  • 1997–2010
    • Tohoku University
      • Institute for Materials Research
      Japan
    • Kyoto Prefectural University
      Kioto, Kyōto, Japan
  • 2004
    • Waseda University
      • Department of Applied Physics
      Tokyo, Tokyo-to, Japan
  • 2002
    • University of Hamamatsu
      Hamamatu, Shizuoka, Japan
    • Keio University
      • Department of Applied Physics and Physico-Informatics
      Edo, Tōkyō, Japan
  • 1999–2002
    • Osaka City University
      • Department of Pediatrics
      Ōsaka, Ōsaka, Japan
    • Osaka Medical Center for Cancer and Cardiovascular Diseases
      Ōsaka, Ōsaka, Japan
    • Tottori University
      • Division of Pediatrics
      Tottori, Tottori-ken, Japan
  • 1993–2002
    • Osaka Medical Center and Research Institute for Maternal and Child Health
      Izumi, Ōsaka, Japan
  • 1991–2002
    • National Cancer Center
      • Center for Cancer Control and Information Services
      Edo, Tōkyō, Japan
  • 1978–2001
    • Osaka University
      • • Division of Pediatrics
      • • Department of Medical Genetics
      • • School of Medicine
      • • Radiation Oncology
      Ōsaka-shi, Osaka-fu, Japan
    • Hyogo College of Medicine
      • Department of Genetics
      Nishinomiya, Hyogo-ken, Japan
  • 1999–2000
    • Chiba University Hospital
      Tiba, Chiba, Japan
  • 1997–2000
    • Toyonaka Municipal Hospital
      Toyonaka, Ōsaka, Japan
  • 1998–1999
    • The University of Tokyo
      • • Center for Human Genome
      • • Institute of Medical Science
      Tokyo, Tokyo-to, Japan
  • 1992–1999
    • National Hospital Organization Kyushu Cancer Center
      Hukuoka, Fukuoka, Japan
  • 1992–1997
    • Kanagawa University
      • • Faculty of Engineering
      • • Department of Chemistry
      Yokohama, Kanagawa, Japan
  • 1990–1997
    • Chiba University
      Tiba, Chiba, Japan
    • Fukushima Medical University
      • Department of Pediatrics
      Hukusima, Fukushima, Japan
    • Kyushu University
      • Faculty of Medical Sciences
      Fukuoka-shi, Fukuoka-ken, Japan
  • 1984–1992
    • Kyoto University
      Kioto, Kyōto, Japan