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Now showing items 1 - 16 of 39

  • Pressure-induced anomalous valence crossover in cubic YbCu5-based compounds

    Yamaoka, Hitoshi   Tsujii, Naohito   Suzuki, Michi-To   Yamamoto, Yoshiya   Jarrige, Ignace   Sato, Hitoshi   Lin, Jung-Fu   Mito, Takeshi   Mizuki, Jun'ichiro   Sakurai, Hiroya   Sakai, Osamu   Hiraoka, Nozomu   Ishii, Hirofumi   Tsuei, Ku-Ding   Giovannini, Mauro   Bauer, Ernst  

    A pressure-induced anomalous valence crossover without structural phase transition is observed in archetypal cubic YbCu5 based heavy Fermion systems. The Yb valence is found to decrease with increasing pressure, indicating a pressure- induced crossover from a localized 4f(13) state to the valence fluctuation regime, which is not expected for Yb systems with conventional c-f hybridization. This result further highlights the remarkable singularity of the valence behavior in compressed YbCu(5)(-)based compounds. The intermetallics Yb2Pd2Sn, which shows two quantum critical points (QCP) under pressure and has been proposed as a potential candidate for a reentrant Yb2+ state at high pressure, was also studied for comparison. In this compound, the Yb valence monotonically increases with pressure, disproving a scenario of a reentrant non-magnetic Yb2+ state at the second QCP.
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  • Roles of transition metals interchanging with lithium in electrode materials

    Kawaguchi, Tomoya   Fukuda, Katsutoshi   Tokuda, Kazuya   Sakaida, Masashi   Ichitsubo, Tetsu   Oishi, Masatsugu   Mizuki, Jun'ichiro   Matsubara, Eiichiro  

    Roles of antisite transition metals interchanging with Li atoms in electrode materials of Li transition-metal complex oxides were clarified using a newly developed direct labeling method, termed powder diffraction anomalous fine structure (P-DAFS) near the Ni K-edge. We site-selectively investigated the valence states and local structures of Ni in Li0.89Ni1.11O2, where Ni atoms occupy mainly the NiO2 host-layer sites and partially the interlayer Li sites in-between the host layers, during electrochemical Li insertion/extraction in a lithium-ion battery (LIB). The site-selective X-ray near edge structure evaluated via the P-DAFS method revealed that the interlayer Ni atoms exhibited much lower electrochemical activity as compared to those at the host-layer site. Furthermore, the present analyses of site-selective extended X-ray absorption fine structure performed using the P-DAFS method indicates local structural changes around the residual Ni atoms at the interlayer space during the initial charge; it tends to gather to form rock-salt NiO-like domains around the interlayer Ni. The presence of the NiO-like domains in the interlayer space locally diminishes the interlayer distance and would yield strain energy because of the lattice mismatch, which retards the subsequent Li insertion both thermodynamically and kinetically. Such restrictions on the Li insertion inevitably make the NiO-like domains electrochemically inactive, resulting in an appreciable irreversible capacity after the initial charge but an achievement of robust linkage of neighboring NiO2 layers that tend to be dissociated without the Li occupation. The P-DAFS characterization of antisite transition metals interchanging with Li atoms complements the understanding of the detailed charge-compensation and degradation mechanisms in the electrode materials.
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  • Ruby pressure scale in a low-temperature diamond anvil cell

    Yamaoka, Hitoshi   Zekko, Yumiko   Jarrige, Ignace   Lin, Jung-Fu   Hiraoka, Nozomu   Ishii, Hirofumi   Tsuei, Ku-Ding   Mizuki, Jun'ichiro  

    Laser-excited N and R fluorescence lines of heavily doped ruby have been studied up to 26GPa at low temperatures. While the intensity of the R lines at ambient pressure significantly decreases with decreasing temperature, the intensity of N lines originating from exchange-coupled Cr ion pairs is enhanced at low temperatures. The pressure induced wavelength shift of the N lines at 19K is well fitted with an empirical formula similar to the equation for the R-1 line, showing that the intense N line could be used as an alternative pressure scale at low temperatures. We also observe continuous increase in non-hydrostaticity with increasing pressure at low temperatures when silicone oil and 4:1 mixture of methanol and ethanol are used as pressure media. (C) 2012 American Institute of Physics. []
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  • Study of Catalytic Reaction at Electrode-Electrolyte Interfaces by a CV-XAFS Method

    Kusano, Shogo   Matsumura, Daiju   Asazawa, Koichiro   Kishi, Hirofumi   Sakamoto, Tomokazu   Yamaguchi, Susumu   Tanaka, Hirohisa   Mizuki, Jun'ichiro  

    A method combining cyclic voltammetry (CV) with x-ray absorption fine structure (XAFS) spectroscopy, viz. CV-XAFS, has been developed to enable in situ real-time investigation of atomic and electronic structures related to electrochemical reactions. We use this method to study the reaction of a Pt/C cathode catalyst in the oxygen reduction reaction (ORR) in an alkaline electrolyte, using x-ray energies near the Pt L-III edge for XAFS measurements. It was found that the current induced by the ORR was first observed at approximately 0.08 V versus Hg/HgO, although the Pt valence, which is reflected in the oxidation states, remained almost unchanged. The electronic structure of the catalytic surface in the ORR was observed to be different in the negative and positive scan directions of CV measurements. Hydrogen adsorption is also discussed on the basis of the observation of this spectral change. We have demonstrated that CV-XAFS provides dynamical structural and electronic information related to electrochemical reactions and can be used for in situ real-time measurements of a catalyst.
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  • 2011 Annual Meeting of the Japanese Society for Synchrotron Radiation Research

    Mizuki, Jun'ichiro  

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  • Magnetic and dielectric properties of Tb(0.5)Ca(0.5)MnO(3)

    Hiramitsu, Yusuke   Yoshii, Kenji   Yoneda, Yasuhiro   Mizuki, Jun'ichiro   Nakamura, Akio   Shimojo, Yutaka   Ishii, Yoshinobu   Morii, Yukio   Ikeda, Naoshi  

    The magnetic and dielectric properties of a perovskite manganite Tb(0.5)Ca(0.5)MnO(3) were investigated. Magnetic measurements showed three characteristic temperatures, originating from charge, ordering (similar to 360K), antiferromagnetic ordering (similar to 120K), and a transition to a glassy state (similar to 50 K). The coexistence of the charge and antiferromagnetic orderings was confirmed by powder neutron diffraction. AC dielectric measurements showed large dielectric constants larger than similar to 1000 at approximately room temperature. The temperature dependence of the dielectric constant changed just below the charge-ordering temperature. From the analysis of tan delta, the activation energy for switching the directions of polar regions was calculated to be similar to 0.09 eV, which is close to the activation energy of the Mn 3d electron transfer. These results strongly suggest that the dielectric properties of this material are governed by the ordering and transfer of the Mn 3d charges.
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  • Modification of InAs quantum dot structure during annealing

    Kaizu, Toshiyuki   Takahasi, Masamitu   Yamaguchi, Koichi   Mizuki, Jun'ichiro  

    The structural modification of InAs/GaAs(0 0 1) quantum dots (QDs) grown with 2.0 or 2.7 monolayers (ML) of InAs during annealing was investigated by a combination of in situ X-ray diffraction (XRD), reflection high-energy electron-beam diffraction (RHEED) and ex situ atomic force microscopy (AFM). For 2.0 ML in coverage, QD size increased due to the incorporation of Ga atoms into the QDs, after which QD ripening occurred. For 2.7 ML in coverage, on the other hand, the change in the strain energy due to enhanced intermixing of In and Ga atoms induced a morphological transition from three dimension (3D) to two dimension (2D). These results yielded some useful information about control of the SK QD structure during growth interruption. (c) 2006 Elsevier B.V. All rights reserved.
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  • X-ray induced photoelectrochemistry on TiO2 RID B-1292-2008

    Tamura, Kazuhisa   Ohko, Yoshihisa   Kawamura, Hiroyuki   Yoshikawa, Hideki   Tatsuma, Tetsu   Fujishima, Akira   Mizuki, Jun'ichiro  

    The electrochemical behaviour of TiO2 under X-ray irradiation was studied. Under X-ray irradiation a current, a negative shift in the rest potential, and electrochemical oxidation and decomposition of [Fe-II(CN)(6)](4-) were clearly observed. The incident photon-current conversion efficiency and energy conversion efficiency was 400-2000% and 0.2-2%, respectively, depending on sample conditions. These results show that the photoelectrochemical reactions were promoted by X-rays with a high incident photon-current conversion efficiency. The photocurrent and photopotential were observed above 4.965 keV, which corresponds to the Ti-K edge, indicating that electron-hole pairs are formed during the relaxation process of the excited Ti atoms. (c) 2007 Elsevier Ltd. All rights reserved.
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  • Difference between local and average. structures of La(3)Ga(5)SiO(14) crystal

    Yoneda, Yasuhiro   Takeda, Hiroaki   Shiosaki, Tadashi   Mizuki, Jun'ichiro  

    The langasite (La(3)Ga(5)SiO(14), LGS) crystal grown by the Czochralski technique was investigated by X-ray diffraction measurements. The X-ray diffraction profiles of the LGS consisted of sharp Bragg peaks and large amounts of diffuse scatterings. The diffuse scatterings indicated the existence of a local structure, which is different from the average structure of LGS. The local structure was analyzed by the pair-distribution function method. The local structure fluctuated owing to the local disordering of La atoms.
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  • Fast and real-time observation of hydrogen absorption kinetics for palladium nanoparticles

    Matsumura, Daiju   Okajima, Yuka   Nishihata, Yasuo   Mizuki, Jun'ichiro  

    Structural change of Pd nanoparticles on aluminum oxide during hydrogen absorption reaction was directly observed by X-ray absorption fine structure with dispersive optics. Hydrogen pressure dependence of the expansion of the interatomic distance for Pd-Pd bonding in Pd nanoparticles was investigated by real-time-resolved and in situ observation with a rate of 50 Hz at room temperature. It has been revealed that the Pd nanoparticles show strong hydrogen pressure dependence of the reaction rate and the saturated interatomic distance for the hydrogen absorption. Determined reaction order implies that the rate of the hydrogen absorption reaction is limited by the surface dissociative adsorption step. (C) 2010 Elsevier B.V. All rights reserved.
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  • Tair-distribution function analysis of bismuth titanate

    Yoneda, Yasuhiro   Kohara, Shinji   Mizuki, Jun'ichiro  

    in ferroelectrics, it is sometimes observed that the local structure is different from the average structures. We observed the difference between the local and average structure of Bi4Ti3O12. The average and local structures were refined by Rietveld and Pair-distribution function (PDF) analysis, respectively. These were refined with the same data set collected by high-energy synchrotron X-ray diffraction. The crystal symmetry of both the local and average structures of Bi4Ti3O12 was orthorhombic. The TiO6 rotation angle around the b-axis of the local structure was larger than that of the average structure. The difference should originate from oxygen vacancies, which are generated in crystal synthesis.
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  • X-ray topography of piezoelectric La3Ta0.5Ga5.5O14 crystal grown by czochralski method

    Yoneda, Yasuhiro   Mizuki, Jun'ichiro   Takeda, Hiroaki   Shiosaki, Tadashi  

    We performed synchrotron X-ray topography on a La3Ta0.5Ga5.5O14 (LTG) crystal grown by the Czochralski method. Since a synchrotron X-ray source can provide high-energy X-rays, one can detect bulk structures by X-ray topography. LTG is one of the most attractive piezoelectric crystals along with La3Ga5SiO14 (LGS) because of its excellent acoustic properties (temperature compensation of acoustic losses). Since LTG single crystals can be grown from a stoichiometric melt, it was expected that single crystals with better quality than the LGS crystal, which cannot be grown from a stoichiometric system but only from a congruent melt, can be obtained. However, 60 keV X-ray topography revealed that the LTG crystal quality was not as high as the LGS crystal quality. The crystal quality of the central region was lower than that of the surrounding region.
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  • Application of Strain Scanning Method to Stress Measurement of Austenitic Stainless Steel

    SHOBU, Takahisa   MIZUKI, Jun'ichiro   SUZUKI, Kenji   SUZUKI, Hiroshi   AKINIWA, Yoshiaki   TANAKA, Keisuke  

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  • X-ray photon correlation spectroscopy of structural fluctuations in relaxor ferroelectrics PZN-9%PT

    Ohwada, Kenji   Shimomura, Susumu   Nakao, Hironori   Matsushita, Mitsuyoshi   Namikawa, Kazumichi   Mizuki, Jun'ichiro  

    We have performed an x-ray photon correlation spectroscopy measurement of 91%Pb(Zn(1/3)Nb(2/3))O(3)-9%PbTiO(3) and found a slow structural fluctuation in time near T(C). The slow fluctuation consists of long-term and short-term fluctuations. We have derived the short-term fluctuation from the intensity fluctuations and found out the relaxation time to be an order of 10 s. It is suggested that the slow fluctuation is owing to the competition between macroscopic tetragonal symmetry and the microscopic orthorhombic symmetry. The competition should be a main reason for the low-frequency dielectric permittivity.
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  • Growth of ferroelectric bismuth lanthanum nickel titanate thin films by rf magnetron sputtering

    Kobune, Masafumi   Fukushima, Koji   Yamaji, Toru   Tada, Hideto   Yazawa, Tetsuo   Fujisawa, Hironori   Shimizu, Masaru   Nishihata, Yasuo   Matsumura, Daiju   Mizuki, Jun'ichiro   Yamaguchi, Hideshi   Kotaka, Yasutoshi   Honda, Koichiro  

    The epitaxial growth, structural properties, and ferroelectric properties of bismuth lanthanum nickel titanate (Bi1-xLax) (Ni0.5Ti0.5)O-3 (BLNT) thin films deposited on Pt(100)/MgO(100) substrates by rf magnetron sputtering have been investigated using x-ray diffraction, transmission electron microscope, and polarization-electric field hysteresis loop measurements. The ferroelectric BLNT(00...) phase with c-axis orientation and a single-phase tetragonal perovskite structure appeared at x >= 0.3. The tetragonality (c/a) increased significantly from 1.004 to 1.028 with increasing La content. The fabricated BLNT films of x >= 0.3 indicated the apparent fourfold rotational symmetry observed for a MgO(202) substrate, a bottom Pt(202) electrode, and a BLNT(101) ferroelectric film, based on phi scan measurements. These results imply that the present La-substituted BLNT films are grown heteroepitaxially at x >= 0.3. It was confirmed that Bi in the BLNT films is in a trivalent state at A sites in the perovskite crystal, based on x-ray anomalous diffraction measurements and x-ray absorption near edge structure spectra. The sputtering technique using compacted powder targets designed by taking the bond dissociation energy of metal oxides into account provided epitaxial perovskite-structured BLNT thin films on Pt(100)/MgO(100) substrates. It is shown that the c-axis oriented epitaxial BLNT film exhibits a hysteresis loop shape with a P-r value of 12 mu C/cm(2) that is comparable to typical high-performance Bi3.25La0.75Ti3O12 (BLT) film. (c) 2007 American Institute of Physics.
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  • In situ determination of Sb distribution in Sb/GaAs(001) layer for high-density InAs quantum dot growth

    Kaizu, Toshiyuki   Takahasi, Masamitu   Yamaguchi, Koichi   Mizuki, Jun'ichiro  

    An Sb-adsorbed GaAs(0 0 1) substrate that serves as a template for high-density InAs quantum dot (QD) growth was investigated using in situ X-ray diffraction. The Sb distribution in the top eight layers from the surface was determined by crystal truncation rod scattering analysis. It was found that Sb atoms penetrated to the eighth layer when GaAs(0 0 1) came in contact with an Sb environment. The amounts of Sb in the first and second layers were, however, saturated at 1/3 atomic layer (AL) and 2/3 AL, respectively. A comparison between the X-ray results and atomic force microscopy observations of the QD density showed that the formation of high-density QDs is correlated with the total amount of Sb in the surface and subsurface layers. (c) 2008 Elsevier B.V. All rights reserved.
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