Updated on 2022/05/19

写真a

 
YAMAMOTO, Tomoyuki
 
Affiliation
Faculty of Science and Engineering, School of Fundamental Science and Engineering
Job title
Professor

Concurrent Post

  • Faculty of Science and Engineering   Graduate School of Fundamental Science and Engineering

  • Faculty of Science and Engineering   Graduate School of Advanced Science and Engineering

Research Institute

  • 2020
    -
    2022

    理工学術院総合研究所   兼任研究員

Education

  • 1995.04
    -
    1998.03

    Waseda University   Graduate School, Division of Science and Engineering   Department of Materials Science and Engineering  

    PhD. Course

  • 1993.04
    -
    1995.03

    Waseda University   Graduate School, Division of Science and Engineering   Department of Materials Science and Engineering  

    Master Cource

  • 1989.04
    -
    1993.03

    Waseda University   Faculty of Science and Engineering   Department of Materials Science and Engineering  

Degree

  • Waseda University   Doctor of Engineering

Research Experience

  • 2010
    -
    Now

    Waseda University   Faculty of Science and Engineering   Professor

  • 2005
    -
    2010

    Waseda University   Faculty of Science and Engineering   Associate Professor

  • 2002
    -
    2005

    Kyoto University, Researcher

  • 1999
    -
    2002

    RIKEN, Researcher

  • 1997
    -
    1999

    Waseda University, Research Associate

Professional Memberships

  •  
     
     

    The Ceramic Society of Japan

  •  
     
     

    The Materials Research Society of Japan

  •  
     
     

    The Physical Society of Japan

  •  
     
     

    The Japan Institute of Metals and Materials

 

Research Areas

  • Inorganic materials and properties

  • Metallic material properties

  • Semiconductors, optical properties of condensed matter and atomic physics

  • Magnetism, superconductivity and strongly correlated systems

  • Quantum beam science

Papers

  • Influence of Isostatic Pressure on the Elastic and Electronic Properties of K2SiF6:Mn4+

    Mekhrdod Subhoni, Umar Zafari, Chong-Geng Ma, Alok M. Srivastava, William W. Beers, William E. Cohen, Mikhail G. Brik, Michal Piasecki, Tomoyuki Yamamoto

    Materials   15 ( 2 ) 613 - 613  2022.01  [Refereed]

    Authorship:Last author

     View Summary

    Isostatic pressure effects on the elastic and electronic properties of non-doped and Mn4+-doped K2SiF6 (KSF) have been investigated by first-principles calculations within density functional theory (DFT). Bulk modulus was obtained by the Murnaghan’s equation of states (EOS) using the relationship between volume and pressures at pressures between 0 and 40 GPa, and elastic constants were calculated by the stress–strain relationship giving small distortions at each pressure point. The other elastic parameters such as shear modulus, sound velocity and Debye temperature, which can be obtained from the elastic constants, were also estimated. The influence of external isostatic pressure on the electronic properties, such as crystal field strength 10Dq and emission energy of 2E → 4A2 transition (Eem), of KSF:Mn4+ was also studied. The results suggest that 10Dq and Eem linearly increase and decrease, respectively, with increasing pressure.

    DOI

  • Origin of room-temperature ferromagnetism in Co-doped CeO2

    Kenji Tarui, Tomohiro Oomori, Yuya Ito, Tomoyuki Yamamoto

    Physica B: Condensed Matter   619   413158 - 413158  2021.10  [Refereed]

    Authorship:Last author, Corresponding author

    DOI

  • Theoretical and Experimental Investigations of Mn4+ Site Occupation in CaAl12O19

    Musashi Sagayama, Umar Zafari, Mekhrdod Subhoni, Alok M. Srivastava, William W. Beers, William E. Cohen, Mikhail G. Brik, Tomoyuki Yamamoto

    ECS Journal of Solid State Science and Technology   10 ( 7 ) 076004 - 076004  2021.07  [Refereed]

    Authorship:Last author, Corresponding author

    DOI

  • Evaluation of Damage Coefficient for Minority-Carrier Diffusion Length of Triple-Cation Perovskite Solar Cells under 1 MeV Electron Irradiation for Space Applications

    Yu Miyazawa, Gyu Min Kim, Ayumi Ishii, Masashi Ikegami, Tsutomu Miyasaka, Yudai Suzuki, Tomoyuki Yamamoto, Takeshi Ohshima, Shusaku Kanaya, Hiroyuki Toyota, Kazuyuki Hirose

    The Journal of Physical Chemistry C   125 ( 24 ) 13131 - 13137  2021.06  [Refereed]

    DOI

  • First Principles Analysis of Crystal Structure, Electronic and Optical Properties of CsSnI3–xBrx Perovskite for Photoelectric Applications

    D. D. Nematov, A. S. Burhonzoda, M. A. Khusenov, Kh. T. Kholmurodov, T. Yamamoto

    Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques   15 ( 3 ) 532 - 536  2021.05  [Refereed]

    Authorship:Last author

    DOI

  • Unusual hydrogen implanted gold with lattice contraction at increased hydrogen content

    Khac Thuan Nguyen, Van Hiep Vuong, The Nghia Nguyen, Trong Tinh Nguyen, Tomoyuki Yamamoto, Nam Nhat Hoang

    Nature Communications   12 ( 1 )  2021.03  [Refereed]

     View Summary

    <title>Abstract</title>The experimental evidence for the contraction of volume of gold implanted with hydrogen at low doses is presented. The contraction of lattice upon the addition of other elements is very rare and extraordinary in the solid-state, not only for gold but also for many other solids. To explain the underlying physics, the pure kinetic theory of absorption is not adequate and the detailed interaction of hydrogen in the lattice needs to be clarified. Our analysis points to the importance of the formation of hydride bonds in a dynamic manner and explains why these bonds become weak at higher doses, leading to the inverse process of volume expansion frequently seen in metallic hydrogen containers.

    DOI

  • First-principles investigations of geometrical and electronic structures of Mn4+ doped A2SiF6 (A= K, Rb, Cs) red phosphors

    Mekhrdod Subhoni, Umar Zafari, Alok M. Srivastava, William W. Beers, William Cohen, Mikhail G. Brik, Tomoyuki Yamamoto

    Optical Materials   115   110986 - 110986  2021.03  [Refereed]

    Authorship:Last author, Corresponding author

    DOI

  • Recent Progress in Nanostructured Functional Materials and Their Applications

    Tomoyuki Yamamoto, Masato Yoshiya, Hoang Nam Nhat

    MATERIALS TRANSACTIONS   61 ( 12 ) 2435 - 2441  2020.12  [Refereed]  [Invited]

    Authorship:Lead author, Corresponding author

    DOI

  • Microstructure and diffusion behavior in the multilayered oxides formed on a Co–W electroplated ferritic stainless steel followed by oxidation treatment

    Lu Gan, Tomoyuki Yamamoto, Hideyuki Murakami

    Acta Materialia   194   295 - 304  2020.08  [Refereed]

    DOI

  • Proton Irradiation Tolerance of High-Efficiency Perovskite Absorbers for Space Applications

    Shusaku Kanaya, Gyu Min Kim, Masashi Ikegami, Tsutomu Miyasaka, Kohtaku Suzuki, Yu Miyazawa, Hiroyuki Toyota, Kanta Osonoe, Tomoyuki Yamamoto, Kazuyuki Hirose

    The Journal of Physical Chemistry Letters   10 ( 22 ) 6990 - 6995  2019.11  [Refereed]

    DOI

  • Universal topological representation of geometric patterns

    Shousuke Ohmori, Yoshihiro Yamazaki, Tomoyuki Yamamoto, Akihiko Kitada

    Physica Scripta   94 ( 10 ) 105213 - 105213  2019.08  [Refereed]

    DOI

  • Theoretical investigation of the breakdown electric field of SiC polymorphs

    Kikou Yamaguchi, Daisuke Kobayashi, Tomoyuki Yamamoto, Kazuyuki Hirose

    Physica B: Condensed Matter   532   99 - 102  2018.03  [Refereed]

    Authorship:Last author, Corresponding author

     View Summary

    The breakdown electric field of several SiC polymorphs has been investigated theoretically using a concept of “recovery rate,” which is obtained by first principles calculations. A good relationship between the experimental breakdown electric fields and the calculated recovery rate of 4H-, 6H-, and 3C-SiC was obtained. In order to examine the stability of SiC polymorphs, the total electronic energies of various types of SiC crystal structures were calculated. Here, two candidates of polymorphs—GeS-type- and 2H-SiC—with energies comparable to those of experimentally well-established structures, have been obtained. The breakdown electric fields of these two polymorphs were estimated using a relationship obtained from the results of 4H-, 6H-, and 3C-SiC. This indicates that one of these polymorphs, GeS-type-SiC, has higher breakdown electric field than any other SiC polymorphs. In addition to the investigation with the recovery rate, relationship between experimental breakdown electric field and calculated band gap with recently developed accurate electron-correlation potential has been also discussed.

    DOI

  • Special issue on “Frontiers in Materials Science: Condensed matters”

    Nam-Nhat Hoang, Tomoyuki Yamamoto, Duc-Thang Pham

    Physica B: Condensed Matter   532   1  2018.03  [Refereed]

    DOI

  • Degradation of CH3NH3PbI3 perovskite due to soft x-ray irradiation as analyzed by an x-ray photoelectron spectroscopy time-dependent measurement method

    Keisuke Motoki, Yu Miyazawa, Daisuke Kobayashi, Masashi Ikegami, Tsutomu Miyasaka, Tomoyuki Yamamoto, Kazuyuki Hirose

    Journal of Applied Physics   121 ( 8 ) 085501 - 085501  2017.02  [Refereed]

     View Summary

    The effects of soft X-ray exposure on structures of CH3NH3PbI3 perovskite were investigated using an X-ray photoelectron spectroscopy (XPS) time-dependent measurement method. A crystalline sample was fabricated with the inverse-temperature crystallization method. The time evolutions of the core-level and valence-band spectra were recorded to determine the compositional ratios and valence band electronic structure of the sample, respectively. In addition, first-principles calculations were conducted to evaluate the valence band XPS spectra. The in situ XPS analysis combined with theoretical calculations demonstrated a degradation of the surface of CH3NH3PbI3 perovskite into PbI2 owing to the evaporation of methylammonium iodide.

    DOI

  • A valence state evaluation of a positive electrode material in an Li-ion battery with first-principles K- and L-edge XANES spectral simulations and resonance photoelectron spectroscopy

    Kei Kubobuchi, Masato Mogi, Masashi Matsumoto, Teruhisa Baba, Chihiro Yogi, Chikai Sato, Tomoyuki Yamamoto, Teruyasu Mizoguchi, Hideto Imai

    Journal of Applied Physics   120 ( 14 ) 142125 - 142125  2016.10  [Refereed]

     View Summary

    X-ray absorption near edge structure (XANES) analysis is an element-specific method for proving electronic state mostly in the field of applied physics, such as battery and catalysis reactions, where the valence change plays an important role. In particular, many results have been reported for the analysis of positive electrode materials of Li-ion batteries, where multiple transition materials contribute to the reactions. However, XANES analysis has been limited to identifying the valence state simply in comparison with reference materials. When the shape of XANES spectra shows complicated changes, we were not able to identify the valence states or estimate the valence quantitatively, resulting in insufficient reaction analysis. To overcome such issues, we propose a valence state evaluation method using K-and L-edge XANES analysis with first-principles simulations. By using this method, we demonstrated that the complicated reaction mechanism of Li(Ni1/3Co1/3Mn1/3)O-2 can be successfully analyzed for distinguishing each contribution of Ni, Co, Mn, and O to the redox reactions during charge operation. In addition to the XANES analysis, we applied resonant photoelectron spectroscopy (RPES) and diffraction anomalous fine structure spectroscopy (DAFS) with first-principles calculations to the reaction analysis of Co and Mn, which shows no or very little contribution to the redox. The combination of RPES and first-principles calculations successfully enables us to confirm the contribution of Co at high potential regions by electively observing Co 3d orbitals. Through the DAFS analysis, we deeply analyzed the spectral features of Mn K-edges and concluded that the observed spectral shape change for Mn does not originate from the valence change but from the change in distribution of wave functions around Mn upon Li extraction. Published by AIP Publishing.

    DOI

  • Preface to Special Topic: Cutting Edge Physics in Functional Materials

    Nam Nhat Hoang, Tomoyuki Yamamoto, Masato Yoshiya, Markus Muenzenberg

    JOURNAL OF APPLIED PHYSICS   120 ( 14 )  2016.10  [Refereed]

    DOI

  • Local environment analysis of Fe ions in BaMgSiO<inf>4</inf>

    Junya Kase, Yoshihiro Shingaki, Yuta Inaba, Kazune Meguro, Hidenobu Murata, Toshihiro Okajima, Tomoyuki Yamamoto

    AIP Conference Proceedings   1763  2016.08

     View Summary

    © 2016 Author(s). Polycrystalline Fe-doped BaMgSiO4 is synthesized by the conventional solid state reaction method, which shows strong photochromism. Photochromic property of the synthesized specimens is investigated by measuring the diffuse reflectance spectrum. Local environment of doped Fe ions in BaMgSiO4 has been studied by the analysis of the X-ray absorption near-edge structure (XANES) spectrum with the aid of the first-principles calculations.

    DOI

  • Existence of a Polycrystal Filled with an Arbitrary Finite Number of Self-Similar Crystals

    Akihiko Kitada, Shousuke Ohmori, Tomoyuki Yamamoto

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   85 ( 4 )  2016.04  [Refereed]

    Authorship:Last author, Corresponding author

     View Summary

    We discuss here a self-similar structure of an aggregate of crystals or noncrystalline grains. In particular, the sufficient condition for a polycrystal to be filled with an arbitrary finite number of self-similar crystals is investigated using a topological concept.

    DOI CiNii

  • Soft X-ray Irradiation Effect on Surface Structure of CH3NH3PbI3 Perovskite in Multi-Film Stack Device

    Keisuke Motoki, Yu Miyazawa, Daisuke Kobayashi, Masashi Ikegami, Tsutomu Miyasaka, Tomoyuki Yamamoto, Kazuyuki Hirose

    2016 IEEE 43RD PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC)   2016-November   1657 - 1660  2016  [Refereed]

     View Summary

    Soft X-ray exposure effects on CH3NH3PbI3 perovskite in a patterned device sample with a similar structure to solar cells, a promising candidate for X-ray detectors, have been investigated with an X-ray Photoelectron Spectroscopy (XPS) time-dependent measurement method. Our experimental analyses demonstrate compositional change from CH3NH3PbI3 to PbI2 due to evaporation of methylammonium iodide.

    DOI

  • In Situ Measurements of Work Function of Indium Tin Oxide after UV/Ozone Treatment

    Daisuke Yamashita, Atsushi Ishizaki, Tomoyuki Yamamoto

    MATERIALS TRANSACTIONS   56 ( 9 ) 1445 - 1447  2015.09  [Refereed]

    Authorship:Last author

     View Summary

    In situ measurements of work function of indium tin oxide after UV/ozone treatment were carried out using a photoemission yield spectrometer with an open counter. Although the work function increased just after UV/ozone treatment, it decreased as time passed and finally returned to the initial value. The continuous change in work function with exposure to air was observed under dry atmosphere and at various temperatures. The returning process at higher temperature proceeded faster than at lower temperature. By contrast, humidity has no influence on the work function recovery. The exponential decay of work function was consistent with the first-order reaction rate equation. The rate constant obeyed Arrhenius' equation, and the activation energy was estimated to be 22 kJ/mol.

    DOI CiNii

  • Quantum Chemical Study of the 1:1 Complex between Ferrocene and Zinc Porphyrin, a Building-Block of Charge-Transfer Molecular Materials

    Fabio Pichierri, Akiko Sekine, Tomoyuki Yamamoto

    MATERIALS TRANSACTIONS   56 ( 9 ) 1425 - 1427  2015.09  [Refereed]

    Authorship:Last author

     View Summary

    The molecular and electronic structure of the 1:1 charge-transfer complex between ferrocene (Fc) and zinc porphyrin (ZnP) are investigated with the aid of dispersion-corrected density functional theory (DFT) calculations. Four stable configurations were obtained, two with the Fe molecule laying on the ZnP plane and the other two where Fe interacts with the porphyrin's perimeter. The dipole moment vectors of these Fc:ZnP complexes indicate that they are stabilized by the transfer of electronic charge density from Fe to ZnP or vice versa.

    DOI CiNii

  • Local Environment Analysis of Na Ions in beta-Tricalcium Phosphate by X-ray Absorption Near-Edge Structure Measurements and First-Principles Calculations

    Kazuhiko Kawabata, Tomoyuki Yamamoto, Akihiko Kitada

    MATERIALS TRANSACTIONS   56 ( 9 ) 1457 - 1460  2015.09  [Refereed]

    Authorship:Corresponding author

     View Summary

    Na-incorporated beta-tricalcium phosphate (beta-TCP) was synthesized via the solid-state reaction method and a substitution mechanism for Na ions in the synthesized materials has been investigated by X-ray absorption near-edge structure (XANES) analysis. In addition, total electronic energy calculations within density functional theory were also carried out to obtain the most energetically favorable substitution site for Na ions in beta-TCP. Both the spectroscopic and computational analysis indicate that substituted Na ions are likely to occupy Ca(4) site in beta-TCP.

    DOI CiNii

  • Substitution Mechanism of Mn and Fe Ions in Bi4Ti3O12

    Kazuma Nishimura, Tsuyoshi Yoshioka, Tomoyuki Yamamoto

    IEEE TRANSACTIONS ON MAGNETICS   50 ( 6 ) 2502306  2014.06  [Refereed]

    Authorship:Last author, Corresponding author

     View Summary

    Single-phased polycrystalline Mn- and Fe-doped Bi4Ti3O12 were fabricated using a solid-state reaction technique, doping with various concentrations of Mn and Fe ions. Substitution mechanism of Mn and Fe ions in Bi4Ti3O12 were investigated with X-ray absorption near-edge structure (XANES) measurements and first-principles calculations. The valence states of the Mn and Fe ions are 4+ and 3+, respectively, inferred from the L-2,L-3-edge XANES profiles. From the K-edge XANES analysis, it is determined that Mn and Fe ions are substituted at one of the Ti sites, i.e., Ti(2a) or Ti(4e) sites. Our first-principles total electronic energy calculations suggest that Mn ions are likely to substitute at Ti(2a) sites rather than at Ti(4e) sites, whereas the opposite is true for Fe substitution. Taken together, these results give a clear description of the locations and charge states of the Mn and Fe dopants in Bi4Ti3O12.

    DOI

  • In situ Measurements of the Desorption of Water from a TiO2 Surface under Dry Air by Collecting the Photoemission Yield with an Open Counter

    Daisuke Yamashita, Atsushi Ishizaki, Tomoyuki Yamamoto

    ANALYTICAL SCIENCES   30 ( 5 ) 575 - 579  2014.05  [Refereed]

    Authorship:Last author

     View Summary

    We investigated the desorption of water from a TiO2 surface under a dry atmosphere by collecting the photoemission yield spectra with an open counter. For this purpose, a new attachment for the photoemission yield measurement was prepared. This apparatus is capable of detecting, in the open air, low-energy electrons excited by photons under dried atmospheres; the dew point is below -35 degrees C. A significant change in the photoemission yield spectra due to exposure to a dry atmosphere was observed. To gain a better understanding of these results, observations of the change in the photoemission yield spectra caused by the thermal desorption of adsorbed water were also carried out. The results are consistent with those obtained by exposure to a dry atmosphere. Based on the relationship between the photoemission yield and the thickness of the water layer, the time dependence of the change in the thickness was explained by the second-order reaction rate equation.

    CiNii

  • Local environment of silicon in cubic boron nitride

    Hidenobu Murata, Takashi Taniguchi, Shunichi Hishita, Tomoyuki Yamamoto, Fumiyasu Oba, Isao Tanaka

    JOURNAL OF APPLIED PHYSICS   114 ( 23 ) 23502  2013.12  [Refereed]

     View Summary

    Si-doped cubic boron nitride (c-BN) is synthesized at high pressure and high temperature, and the local environment of Si is investigated using X-ray absorption near edge structure (XANES) and first-principles calculations. Si-K XANES indicates that Si in c-BN is surrounded by four nitrogen atoms. According to first-principles calculations, the model for substitutional Si at the B site well reproduces experimental Si-K XANES, and it is energetically more favorable than substitutional Si at the N site. Both the present experimental and theoretical results indicate that Si in c-BN prefers the B site to the N site. (C) 2013 AIP Publishing LLC.

    DOI

  • Electronic structure analysis of Mn- and Fe-codoped In2O3 by photoemission yield measurements

    Daisuke Yamashita, Tsuyoshi Yoshioka, So Nishida, Tomoyuki Yamamoto

    PHYSICA B-CONDENSED MATTER   407 ( 22 ) 4485 - 4488  2012.11  [Refereed]

    Authorship:Last author

     View Summary

    The valence band electronic structures of Mn- and/or Fe-doped In2O3, i.e., In2O3:Mn, In2O3:Fe, and In2O3:(Mn, Fe), are investigated by photoemission yield measurements. Significant changes are observed in the threshold energy of photoemission, depending on the doped magnetic ions, which indicates that an additional occupied band appears above the top of the valence band of In2O3 owing to doping with Mn and/or Fe ions. It is confirmed that the order of the threshold energies of photoemission, E-PET is E-PET(In2O3:Mn)&lt;E-PET(In2O3:(Mn, Fe))&lt;E-PET(In2O3:Fe)&lt;E-PET(In2O3). To gain a better understanding of these results, first-principles molecular orbital calculations are also carried out, which successfully explain the observed changes in the photoemission threshold energies. (C) 2012 Elsevier B.V. All rights reserved.

    DOI

  • Co-L-3 X-ray absorption near-edge structure analysis of Pr1-xCaxCoO3-delta and Pr1-xSrxCoO3-delta

    Tsuyoshi Yoshioka, Tomoyuki Yamamoto, Akihiko Kitada

    PHYSICA B-CONDENSED MATTER   407 ( 21 ) 4114 - 4116  2012.11  [Refereed]

    Authorship:Last author

     View Summary

    The valence state of Co ions in Pr1-xCaxCoO3-delta and Pr1-xSrxCoO3-delta has been investigated by an analysis of the Co-L-3 X-ray absorption near-edge structure (XANES) profile. The observed intensity distributions of Co-L-3 XANES change continuously with increasing concentration of alkaline-earth ions. To investigate the origin of this change in the XANES profile, charge transfer multiplet calculations were carried out, which could successfully explain the change in the spectral profile; they also suggest that the valence state of Co ions in Pr1-xCaxCoO3-delta and Pr1-xSrxCoO3-delta is between 3+ and 4+ and increases gradually with the concentration of alkaline-earth ions. (c) 2012 Elsevier B.V. All rights reserved.

    DOI

  • Determination of valence state of Mn ions in Pr(1-x)A(x)MnO(3-delta) (A = Ca, Sr) by Mn-L-3 X-ray absorption near-edge structure analysis

    Haruto Kanamori, Tsuyoshi Yoshioka, Kazuyuki Hirose, Tomoyuki Yamamoto

    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA   185 ( 5-7 ) 129 - 132  2012.08  [Refereed]

    Authorship:Last author, Corresponding author

     View Summary

    The valence states of the Mn ions in Pr(1-x)A(x)MnO(3-delta) (A = Ca, Sr) are investigated by Mn-L-3 X-ray absorption near-edge structure (XANES) analysis. The spectral fine structures in the Mn-L-3 XANES analysis show a significant difference between Pr0.5Ca0.5MnO3-delta and Pr0.5Sr0.5MnO3-delta, a paramagnetic insulator and a ferromagnetic metal, respectively, at room temperature, whereas the spectral structures of Pr0.7Ca0.3MnO3-delta and Pr0.7Sr0.3MnO3-delta, paramagnetic insulators, are almost identical. These results indicate that the valence states of the Mn ions in these materials are highly correlated with their magnetic and electrical properties. A significant difference was also found between the Mn-L-3 XANES profiles of Pr1-xCaxMnO3-delta and the profiles formed by the linear combination of the Mn-L-3 XANES spectra of PrMnO3 (Mn3+) and CaMnO3 (Mn4+). This difference indicates that the Mn ions in these materials do not have a mixed-valence state of 3+ and 4+, but have an intermediate valence state o 3+ and 4+. (c) 2012 Elsevier B.V. All rights reserved.

    DOI

  • Analysis of Charge Compensation Mechanisms in Pr(1-x)A(x)CoO(3-delta) (A = Ca, Sr) by X-ray Absorption Near-Edge Structure

    Tsuyoshi Yoshioka, Tomoyuki Yamamoto, Akihiko Kitada

    JAPANESE JOURNAL OF APPLIED PHYSICS   51 ( 7 ) 073201  2012.07  [Refereed]

    Authorship:Corresponding author

     View Summary

    The charge compensation mechanisms in polycrystalline Pr1-xCaxCoO3-delta and Pr1-xCaxCoO3-delta here synthesized by the solid-state reaction method have been investigated by analyzing the Pr-L-3 and Co-K X-ray absorption near-edge structure (XANES) with the aid of first-principles calculations. The valence states of Pr ions in these materials were determined to be trivalent (Pr3+) and independent of the concentration of alkaline-earth ions observed in the Pr-L-3 XANES profile. The Co-K XANES spectra shifted to the higher-energy side with an increase in alkaline-earth concentration and were examined in detail by first-principles calculations. Using calculated XANES results, we successfully determined that the valence state of Co ions is an intermediate value between 3+ and 4+ and increases with an increase in the concentration of alkaline-earth ions in Pr1-xCaxCoO3-delta (A = Ca, Sr) accompanying oxygen vacancy. (C) 2012 The Japan Society of Applied Physics

    DOI

  • Valence state analysis of Mn and Fe ions in room-temperature ferromagnetic Mn- and Fe-codoped In2O3

    Teruki Okazaki, Tsuyoshi Yoshioka, Yasuyuki Kusakabe, Tomoyuki Yamamoto, Akihiko Kitada

    SOLID STATE COMMUNICATIONS   151 ( 23 ) 1749 - 1752  2011.12  [Refereed]

     View Summary

    The detailed valence states of Mn and Fe ions in room-temperature ferromagnetic In2O3 doped with Mn and Fe ions were investigated by the near-edge X-ray absorption fine structure (NEXAFS) measurements. It was confirmed that Mn2+ and Mn3+ coexist in In2O3. In addition to these two magnetic ions, Sn4+ was incorporated to control the charge states of doped Mn and/or Fe ions, which makes the Mn2+/Mn3+ ratio larger with an increase in the doped-Sn4+ concentration. It was found that the magnetic susceptibility decreased with an increase in Mn2+ because of the Sn4+-doping, which indicated that the coexistence of Mn2+ and Mn3+ is mandatory for room-temperature ferromagnetism in Mn- and Fe-codoped In2O3. (C) 2011 Elsevier Ltd. All rights reserved.

    DOI

  • Valence state analysis of Mn and Fe ions in room-temperature ferromagnetic Mn- and Fe-codoped In2O3

    Teruki Okazaki, Tsuyoshi Yoshioka, Yasuyuki Kusakabe, Tomoyuki Yamamoto, Akihiko Kitada

    SOLID STATE COMMUNICATIONS   151 ( 23 ) 1749 - 1752  2011.12  [Refereed]

     View Summary

    The detailed valence states of Mn and Fe ions in room-temperature ferromagnetic In2O3 doped with Mn and Fe ions were investigated by the near-edge X-ray absorption fine structure (NEXAFS) measurements. It was confirmed that Mn2+ and Mn3+ coexist in In2O3. In addition to these two magnetic ions, Sn4+ was incorporated to control the charge states of doped Mn and/or Fe ions, which makes the Mn2+/Mn3+ ratio larger with an increase in the doped-Sn4+ concentration. It was found that the magnetic susceptibility decreased with an increase in Mn2+ because of the Sn4+-doping, which indicated that the coexistence of Mn2+ and Mn3+ is mandatory for room-temperature ferromagnetism in Mn- and Fe-codoped In2O3. (C) 2011 Elsevier Ltd. All rights reserved.

    DOI

  • Substitution mechanism of Zn ions in beta-tricalcium phosphate

    Kazuhiko Kawabata, Tomoyuki Yamamoto, Akihiko Kitada

    PHYSICA B-CONDENSED MATTER   406 ( 4 ) 890 - 894  2011.02  [Refereed]

     View Summary

    Zn-doped beta-tricalcium phosphate (beta-TCP) is synthesized by the solid-state reaction method. The substitution mechanism of Zn ions in beta-TCP synthesized here is investigated by carrying out a combination of near-edge X-ray absorption fine structure (NEXAFS) measurements and first-principles calculations. From the results of the present study, the substitution site for Zn ions in beta-TCP is successfully determined. (C) 2010 Elsevier B.V. All rights reserved.

    DOI

  • Substitution mechanism of Zn ions in beta-tricalcium phosphate

    Kazuhiko Kawabata, Tomoyuki Yamamoto, Akihiko Kitada

    PHYSICA B-CONDENSED MATTER   406 ( 4 ) 890 - 894  2011.02  [Refereed]

     View Summary

    Zn-doped beta-tricalcium phosphate (beta-TCP) is synthesized by the solid-state reaction method. The substitution mechanism of Zn ions in beta-TCP synthesized here is investigated by carrying out a combination of near-edge X-ray absorption fine structure (NEXAFS) measurements and first-principles calculations. From the results of the present study, the substitution site for Zn ions in beta-TCP is successfully determined. (C) 2010 Elsevier B.V. All rights reserved.

    DOI

  • Electronic structure analysis of Sb-doped BaSnO3

    Daisuke Yamashita, Satoru Takefuji, Masato Tsubomoto, Tomoyuki Yamamoto

    MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS   173 ( 1-3 ) 33 - 36  2010.10  [Refereed]

     View Summary

    Sb-doped BaSnO3 is synthesized by the conventional solid-state reaction method changing a concentration of Sb. Electric resistivity of synthesized specimens decreases with increment of doped Sb concentration. In order to investigate an influence of Sb-doping on the electronic structure of BaSnO3, its valence band electronic structure is examined by the photoemission yield spectroscopy (PYS), which shows additional occupied band above the top of the valence band of BaSnO3 due to the Sb-doping. First-principles calculations are also carried out to obtain change in electronic structures of BaSnO3 by Sb-doping, which supports the PYS results. (C) 2009 Elsevier B.V. All rights reserved.

    DOI

  • Electronic structure analysis of Sb-doped BaSnO3

    Daisuke Yamashita, Satoru Takefuji, Masato Tsubomoto, Tomoyuki Yamamoto

    MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS   173 ( 1-3 ) 33 - 36  2010.10  [Refereed]

     View Summary

    Sb-doped BaSnO3 is synthesized by the conventional solid-state reaction method changing a concentration of Sb. Electric resistivity of synthesized specimens decreases with increment of doped Sb concentration. In order to investigate an influence of Sb-doping on the electronic structure of BaSnO3, its valence band electronic structure is examined by the photoemission yield spectroscopy (PYS), which shows additional occupied band above the top of the valence band of BaSnO3 due to the Sb-doping. First-principles calculations are also carried out to obtain change in electronic structures of BaSnO3 by Sb-doping, which supports the PYS results. (C) 2009 Elsevier B.V. All rights reserved.

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  • First-principles calculations of the elastic properties of hydroxyapatite doped with divalent ions

    Kazuhiko Kawabata, Tomoyuki Yamamoto

    JOURNAL OF THE CERAMIC SOCIETY OF JAPAN   118 ( 1378 ) 548 - 549  2010.06  [Refereed]

     View Summary

    The first-principles calculations are carried out to investigate the influence of the presence of divalent ions, Mg2+ and Sr2+, on the elastic properties of hydroxyapatite. The calculated elastic moduli for pure hydroxyapatite are in good agreement with the experimental values. After examination of the geometrical structures of hydroxyapatite doped with the divalent ions, the elastic properties of the models are investigated. (C) 2010 The Ceramic Society of Japan. All rights reserved.

    DOI

  • First-principles calculations of the elastic properties of hydroxyapatite doped with divalent ions

    Kazuhiko Kawabata, Tomoyuki Yamamoto

    JOURNAL OF THE CERAMIC SOCIETY OF JAPAN   118 ( 1378 ) 548 - 549  2010.06  [Refereed]

     View Summary

    The first-principles calculations are carried out to investigate the influence of the presence of divalent ions, Mg2+ and Sr2+, on the elastic properties of hydroxyapatite. The calculated elastic moduli for pure hydroxyapatite are in good agreement with the experimental values. After examination of the geometrical structures of hydroxyapatite doped with the divalent ions, the elastic properties of the models are investigated. (C) 2010 The Ceramic Society of Japan. All rights reserved.

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  • Analysis of Local Environment of Fe Ions in Hexagonal BaTiO3

    Shunsuke Chikada, Kazuyuki Hirose, Tomoyuki Yamamoto

    JAPANESE JOURNAL OF APPLIED PHYSICS   49 ( 9 )  2010  [Refereed]

     View Summary

    Fe-doped hexagonal-structured BaTiO3 is synthesized by the conventional solid-state reaction method. The local environment of the Fe ions doped in the synthesized hexagonal BaTiO3 is then investigated by X-ray absorption near edge structure (XANES) measurements and first-principles calculations. It is confirmed from the Fe L-2,L-3-edge XANES spectrum that the charge state of the Fe ions in the synthesized specimen is trivalent. By analyzing the Fe K-edge XANES spectrum of the present specimen with the aid of the first-principles calculations, it is found that doped Fe ions are substituted at the Ti site in Ti2O9 groups of face-sharing octahedra in hexagonal BaTiO3. (C) 2010 The Japan Society of Applied Physics

    DOI

  • Analysis of local environment of Fe ions in hexagonal BaTiO3

    Jpn. J. Appl. Phys.   49   091502  2010

  • Lattice dynamics and thermal properties of SrHfO3 by first-principles calculations

    Hidenobu Murata, Tomoyuki Yamamoto, Hiroki Moriwake, Isao Tanaka

    PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS   246 ( 7 ) 1628 - 1633  2009.07  [Refereed]

     View Summary

    First-principles lattice dynamics calculations are systematically made on four polymorphs of SrHfO3. The hierarchy of the total energy at the ground state is Pm (3) over barm &gt; I4/mcm &gt; Cmcm &gt; Pnma, which agrees with the sequence of iterative transitions with temperature as reported by experiments. Three structures, Pm (3) over barm, I4/mcm and Cmcm, are found to be dynamically unstable at the ground state. A soft mode of phonons appears in I4/mcm and Cmcm at the Gamma-point. The atomic displacements for the soft mode are analogous to those of the R-25 mode of Pm (3) over barm, which is related to the rotation of the HfO6 octahedron. Pnma phase shows lowest energy and it is the only dynamically stable structure among the four phases. The volume expansion coefficient, bulk modulus and heat capacity are computed within quasi-harmonic approximations using the vibrational density of states. They are compared to experimental data. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

    DOI

  • First Principles Lattice Dynamics Calculations of Ag+ Doped KX (X = Cl, Br and I)

    Hidenobu Murata, Tomoyuki Yamamoto, Isao Tanaka

    MATERIALS TRANSACTIONS   50 ( 5 ) 999 - 1003  2009.05  [Refereed]

     View Summary

    Phonon states of Ag doped potassium halides, KX:Ag+ (X = Cl, Br and 1), are computed by a first principles lattice dynamic method using 64-atoms supercells. Results are compared to experimental data in literature. Phonon density of states of host KCl and KI crystals satisfactorily agree to the experimental inelastic neutron scattering data. Experimental frequencies of the impurity-induced infra-red (IR) and Raman active modes in the low frequency region are reasonably well reproduced because the vibrations are localized within the first nearest neighbour anions of the Ag+-ion. On the other hand, limitations of present calculations to reproduce the high frequency impurity-induced modes are pointed out. They are less localized to the Ag+-ion. [doi: 10.2320/matertrans.MC200815]

    DOI

  • Geometric ferroelectricity in rare-earth compounds RGaO3 and RInO3

    Tetsuya Tohei, Hiroki Moriwake, Hidenobu Murata, Akihide Kuwabara, Ryo Hashimoto, Tomoyuki Yamamoto, Isao Tanaka

    PHYSICAL REVIEW B   79 ( 14 ) 144125  2009.04  [Refereed]

     View Summary

    We have studied the stability and ferroelectric properties of hexagonal RGaO3 and RInO3 (R: rare-earth elements) by first-principles calculations. Computed spontaneous polarization in the series shows a systematic increase with the rare-earth elements, with values being larger in RInO3 than in the corresponding RGaO3. The largest polarization found is about 10 mu C/cm(2) for ErInO3, which is about twice as large as those observed in hexagonal RMnO3. The polarization can be further increased by applying in-plane compressive stress. The Born effective charges of constituent ions in the compounds are found to be similar to their formal values, implying that the ferroelectric displacements are merely driven by the ionic size effect. A transition to the high-symmetry phase at around 1500 K was confirmed in GdInO3 and DyInO3 by in situ high-temperature powder x-ray diffractometry. The present systems should belong to the family of geometric ferroelectrics.

    DOI

  • Local environment analysis of dopants in ceramics by x-ray absorption near-edge structure with the aid of first-principles calculations

    Tomoyuki Yamamoto, Yoshitada Kawashima, Yasuyuki Kusakabe, Shigeru Matsuda, Yutaka Mizuoka, Yuki Nakade, Toshihiro Okajima

    JOURNAL OF PHYSICS-CONDENSED MATTER   21 ( 10 ) 104211  2009.03  [Refereed]

     View Summary

    Three types of functional ceramic materials, (1) dilute magnetic semiconductor, (2) phosphor and (3) electrolyte of a solid fuel cell, are fabricated by the conventional solid state reaction method. Local environments of dopants in these ceramic materials here synthesized are systematically investigated by using the x-ray absorption near-edge structure (XANES) with the aid of first-principles calculations. Our present analytical method by combined use of XANES and first principles calculations has successfully explained the local environment of dopants in the above ceramic materials.

    DOI

  • Local environment analysis of Mn ions in beta-tricalcium phosphate

    Kazuhiko Kawabata, Hironobu Sato, Tomoyuki Yamamoto

    JOURNAL OF THE CERAMIC SOCIETY OF JAPAN   116 ( 1349 ) 108 - 110  2008.01  [Refereed]

     View Summary

    Mn-doped beta-tricalcium phosphate (beta-TCP) is synthesized by the solid-state reaction method. The local environment of doped Mn ions in beta-TCP is investigated by the near-edge X-ray absorption fine structure (NEXAFS) measurements. The first principles calculations are also carried out to obtain the theoretical NEXAFS spectra within the density functional theory (DFT). Observed NEXAFS spectra from the Mn ions in beta-TCP are quantitatively well reproduced by the present theoretical calculations, which show Mn ions are substituted for Ca2+ site in beta-TCP.

  • Atomic structures of supersaturated ZnO-AL(2)O(3) solid solutions

    Satoru Yoshioka, Fumiyasu Oba, Rong Huang, Isao Tanaka, Teruyasu Mizoguchi, Tomoyuki Yamamoto

    JOURNAL OF APPLIED PHYSICS   103 ( 1 ) 014309  2008.01  [Refereed]

     View Summary

    Supersaturated ZnO-Al(2)O(3) (&gt;20 at.% Al) thin films are grown by pulsed laser deposition technique on silica glass substrates at 600 degrees C. They are characterized by combining x-ray diffraction, Al-K edge x-ray absorption near edge structures (XANESs), high resolution transmission electron microscope (TEM) imaging, TEM analysis, and a series of first principles calculations. The films are composed of textured wurtzite grains with c planes parallel to the substrate. The distance between c planes expands significantly when the Al concentration is greater than 10 at. %. The expansion disappears after annealing the films at above 800 &apos;C. High density of dislocationlike defects is found in the as deposited film. Any segregation of Al cannot be detected either at the grain boundaries or inside the grains. The lattice expansion toward c axis and the experimental XANES can be satisfactorily explained by taking a hypothetical homologous model with the composition of (ZnO)(3)(Al(2)O(3)) as the local environment of Al in the supersaturated solid solution. Simplified substitutional models with Al at the Zn site in wurtzite ZnO cannot explain these experimental results. First principles calculations show that the homologous phase is energetically more favorable than the simplified substitutional models, although decomposition into ZnO and ZnAl(2)O(4) is more favorable than the homologous phase. The local atomic structures of the supersaturated solid solution are therefore concluded to be analogous to the metastable homologous phase. (c) 2008 American Institute of Physics.

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  • First-principles XANES simulations of spinel zinc ferrite with a disordered cation distribution

    Seisuke Nakashima, Koji Fujita, Katsuhisa Tanaka, Kazuyuki Hirao, Tomoyuki Yamamoto, Isao Tanaka

    PHYSICAL REVIEW B   75 ( 17 ) 174443  2007.05  [Refereed]

     View Summary

    Theoretical calculations of Zn K and Fe K x-ray absorption near-edge structures (XANES) using a first-principles method have been performed to evaluate the degree of cation disordering in spinel zinc ferrite (ZnFe2O4) thin film prepared by a sputtering method, ZnFe2O4 thin films annealed at elevated temperatures, and ZnFe2O4 bulk specimen prepared by a solid-state reaction. Using the full-potential linearized augmented plane-wave + local orbitals method, a theoretical spectrum is generated for the tetrahedral and octahedral environments for each of the two cations. The experimental XANES spectrum of the thin film annealed at 800 degrees C as well as that of bulk specimen is successfully reproduced by using either the theoretical spectrum for Zn2+ on the tetrahedral site (A site) or that for Fe3+ on the octahedral site (B site), which is indicative of the normal spinel structure. For the as-deposited film, on the other hand, excellent agreement between theoretical and experimental spectra is obtained by considering the presence of either ion in both the A and B sites. The degree of cation disordering, x, defined as [Zn1-x2+Fex3+](A)[Znx2+Fe2-x3+](B)O-4, is estimated to be approximately 0.6 in the as-deposited film, which is consistent with the analysis of the extended x-ray absorption fine structure on the Zn K edge. Curious magnetic properties as we previously observed for the as-deposited thin film-i.e., ferrimagnetic behaviors accompanied by large magnetization at room temperature and cluster spin-glass-like behavior-are discussed in connection with disordering of Zn2+ and Fe3+ ions in the spinel-type structure.

    DOI

  • Thermal annealing effect on magnetism and cation distribution in disordered ZnFe2O4 thin films deposited on glass substrates

    Seisuke Nakashima, Koji Fujita, Katsuhisa Tanaka, Kazuyuki Hirao, Tomoyuki Yamamoto, Isao Tanaka

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   310 ( 2 ) 2543 - 2545  2007.03  [Refereed]

     View Summary

    The effect of thermal annealing on magnetic properties has been investigated for disordered zinc ferrite (ZnFe2O4) thin films deposited on silica glass substrates using a sputtering method. The magnetization at room temperature is enhanced by annealing at relatively low temperature such as 300 degrees C and decreased by annealing at higher temperatures. X-ray absorption near edge structures indicate that ZnFe2O4 thin film annealed at 300 degrees C possesses disordered cation distribution similar to that for as-deposited thin film. It is presumed that the enhancement of magnetization by annealing at 300 degrees C is due to precipitation of disordered ZnFe2O4 crystal from an amorphous matrix formed in the as-deposited thin film. (C) 2006 Elsevier B.V. All rights reserved.

    DOI

  • Awaking of ferromagnetism in GaMnN through control of Mn valence

    S. Sonoda, I. Tanaka, F. Oba, H. Ikeno, H. Hayashi, T. Yamamoto, Y. Yuba, Y. Akasaka, K. Yoshida, M. Aoki, M. Asari, T. Araki, Y. Nanishi, K. Kindo, H. Hori

    APPLIED PHYSICS LETTERS   90 ( 1 ) 012504  2007.01  [Refereed]

     View Summary

    Room temperature ferromagnetism of GaMnN thin film is awaked by a mild hydrogenation treatment of a sample synthesized by molecular beam epitaxy. Local environment of Mn atoms is monitored by Mn-L-2,L-3 near edge x-ray absorption fine structure technique. Doped Mn ions are present at substitutional sites of Ga both before and after the hydrogenation. No secondary phase can be detected. Major valency of Mn changes from 3+ to 2+ by the hydrogenation. The present result supports the model that the ferromagnetism occurs when Mn2+ and Mn3+ are coexistent and holes in the midgap Mn band mediate the magnetic coupling. (c) 2007 American Institute of Physics.

    DOI

  • On a dendrite generated by a zero-dimensional weak self-similar set

    CHAOS SOLITONS &amp; FRACTALS   34 ( 5 ) 1732 - 1735  2007

  • Dilute Ga dopant in TiO2 by X-ray absorption near-edge structure

    Toshihiro Okajima, Tomoyuki Yamamoto, Masahiro Kunisu, Satoru Yoshioka, Isao Tanaka, Norimasa Umesaki

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   45 ( 9A ) 7028 - 7031  2006.09  [Refereed]

     View Summary

    As a model of aliovalent impurity in functional ceramic, the local environment of dilute Ga in rutile-structured TiO2 is investigated by Ga K-edge X-ray absorption near-edge structure (XANES) spectroscopy. In conjunction with the experiments, first-principles calculations by two methods are systematically made. The projector augmented wave method is used to optimize the local structure and obtain the solution energy. The augmented plane wave plus local orbitals method is adopted to obtain theoretical XANES spectra. A comparison between experimental and theoretical XANES spectra shows that Ga dopants are located at the Ti4+ sites forming Ga3+. Oxygen vacancies are present to maintain the charge balance of the solid solution. Ga atoms and oxygen vacancies are not present at the nearest neighbor sites but stay apart.

    DOI

  • Coexistence of Mn2+ and Mn3+ in ferromagnetic GaMnN

    S. Sonoda, I. Tanaka, H. Ikeno, T. Yamamoto, F. Oba, T. Araki, Y. Yamamoto, K. Suga, Y. Nanishi, Y. Akasaka, K. Kindo, H. Hori

    JOURNAL OF PHYSICS-CONDENSED MATTER   18 ( 19 ) 4615 - 4621  2006.05  [Refereed]

     View Summary

    Considerable efforts have been devoted recently to synthesizing diluted magnetic semiconductors having ferromagnetic properties at room temperature because of their technological impacts for spintronic devices. In 2001 successful growth of GaMnN films showing room temperature ferromagnetism and p-type conductivity was reported. The estimated Curie temperature was 940 K at 5.7% of Mn, which is the highest among diluted magnetic semiconductors ever reported. However, the electronic mechanism behind the ferromagnetic behaviour has still been controversial. Here we show experimental evidence using ferromagnetic samples that Mn atoms are substitutionally dissolved into the GaN lattice and they exhibit mixed valences of +2 (majority) and +3. The p-type carrier density decreases significantly at very low temperatures. At the same time, magnetization dramatically decreases. The results imply that the ferromagnetic coupling between Mn atoms is mediated by holes in the mid-gap Mn band.

    DOI

  • First-principles calculations of migration energy of lithium ions in halides and chalcogenides

    Kishida, I, Y Koyama, A Kuwabara, T Yamamoto, F Oba, Tanaka, I

    JOURNAL OF PHYSICAL CHEMISTRY B   110 ( 16 ) 8258 - 8262  2006.04  [Refereed]

     View Summary

    Migration of Li+ ions via the vacancy mechanism in LiX (X = F, Cl, Br, and 1) with the rocksalt and hypothetical zinc blende structures and Li2X (X = O, S, Se, and Te) with the antifluorite structure has been investigated using first-principles projector augmented wave calculations with the generalized gradient approximation. The migration paths and energies, determined by the nudged-elastic-band method, are discussed on the basis of two idealized models: the rigid-sphere and charged-sphere models. The trajectories and energy profiles of the migration in these lithium compounds vary between these two models, depending on the anion species and crystal structure. The migration energies in LiX with both the rocksalt and hypothetical zinc blende structures show a tendency to decrease with increasing periodic number of the anion species in the periodic table. This is consistent with the widely accepted view that anion species with large ionic radii and high polarizabilities are favorable for good ionic conduction. In contrast, Li2O exhibits the lowest migration energy among Li2X compounds, although O is the smallest among the chalcogens, indicating that electrostatic attractive interactions play the dominant role in the inter-ion interactions in Li2O and, therefore, in the ion migration.

    DOI

  • First-principles study of cation disordering in MgAl2O4 spinel with cluster expansion and Monte Carlo simulation

    A Seko, K Yuge, F Oba, A Kuwabara, Tanaka, I, T Yamamoto

    PHYSICAL REVIEW B   73 ( 9 ) 094116  2006.03  [Refereed]

     View Summary

    The temperature dependence of cationic disorder in MgAl2O4 spinel is investigated using a combination of first-principles total-energy calculations, a cluster expansion, and canonical Monte Carlo simulations. The formation energies of the possible cation-disordered structures within the spinel unit cell are predicted to be all positive, suggesting that the ground state is the normal spinel in consistency with a widely accepted view. The temperature dependence of cationic disorder is well reproduced by considering many effective cluster interactions up to quadruplets. The order-disorder transition temperature is estimated at about 860 K based on the anomaly of specific heat. The cluster expansion of the volume of MgAl2O4 indicates that it decreases as more cations exchange.

    DOI

  • XANES and ELNES in ceramic science

    Tanaka, I, T Mizoguchi, T Yamamoto

    JOURNAL OF THE AMERICAN CERAMIC SOCIETY   88 ( 8 ) 2013 - 2029  2005.08  [Refereed]

     View Summary

    Both X-ray absorption near edge structures (XANES) and electron energy loss near edge structure (ELNES) are important toots in ceramic science offering information on local environment of selected elements not only in crystals but also in amorphous materials. Recent technological progress enables measurements of XANES of ppm-level dopants using modern synchrotron facilities. Combined with transmission electron microscopy, ELNES can be used to analyze the local structures with sulmanometer spatial resolution. First principles methods to reproduce and interpret the spectra have been established just recently. When a core-hole is adequately taken into account, most of K-edge spectra can be well reproduced using a modern band-structure method within one-electron approximation. The same is true for L-2,L-3-edge spectra of non-transition metal compounds. However, multi-electron calculations are mandatory to reproduce L-2,L-3-edge spectra of 3d transition-metal elements because of strong electronic correlations. In this paper, some recent results obtained in our group by the combination of XANES/ELNES experiments and theoretical calculations are reviewed.

    DOI

  • Soft X-ray emission spectra of argon atoms doped in solid matrices

    Y Muramatsu, T Yamamoto, JD Denlinger, RCC Perera

    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA   144   799 - 802  2005.06  [Refereed]

     View Summary

    Soft X-ray emission spectra in the Ar L region of various solid substrates doped with Ar atoms were measured to investigate the chemical states of the Ar atoms in the solid matrices. Ar ions were implanted into the solid matrices of Si(111), SiO2, highly oriented pyrolytic graphite (HOPG), Ti, Cr, Ni, and Cu with an acceleration voltage of 5 kV at room temperature. Soft X-ray emission spectra in the Ar L region were measured using synchrotron radiation. Low-energy tailing was observed at the L-3-M-1 and L-2-M-1 X-ray emission peaks in Ar-doped transition metals. The density of states (DOS) of the Ar 3s orbitals indicated that the low-energy tailed DOS can be formed by hybridization with 3d orbitals in the overpressurized Ar clusters. (c) 2005 Elsevier B.V. All rights reserved.

    DOI

  • Near-edge X-ray absorption fine structure of PdO at OK-edge

    M Mogi, Y Inoue, T Yamamoto, Tanaka, I, P Nachimuthu, RCC Perera

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   44 ( 6A ) 4057 - 4059  2005.06  [Refereed]

     View Summary

    The first-principles band structure calculations are carried out to interpret the spectral fine structure of the near-edge X-ray absorption fine structure (NEXAFS) spectrum of PdO at the 0 K-edge by the full-potential augmented plane wave plus local orbitals (APW+lo) method. The observed NEXAFS fine structure of PdO is quantitatively reproduced by our calculations. Three types of PdO polymorph are examined, which show a significant difference in theoretical NEXAFS profile. They can be used as theoretical finger prints for future studies.

    DOI

  • Characterization of nanotextured AIN thin films by x-ray absorption near-edge structures

    T Suga, S Kameyama, S Yoshioka, T Yamamoto, Tanaka, I, T Mizoguchi

    APPLIED PHYSICS LETTERS   86 ( 16 ) 163113  2005.04  [Refereed]

     View Summary

    AlN thin films have been grown on c-cut sapphire substrates by pulsed-laser deposition. The film epitaxially grown at 1073 K under vacuum of 5 x 10(-4) Pa was used to examine the crystallographic orientation dependence of Al K-edge x-ray absorption near-edge structures (XANES), which satisfactorily agrees with theoretical spectra obtained by first-principles calculations. The film grown at 1073 K with N-2 backfill of 7 x 10(-2) Pa shows nanotextured structure with its c plane parallel to the substrate. Although the nanotexture is not evident by x-ray diffraction, XANES can unambiguously indicate the texturing. Cross-sectional high-resolution electron microscopy provides the evidence of the nanostructure. &COPY; 2005 American Institute of Physics.

    DOI

  • Local environment of Mn dopant in ZnO by near-edge x-ray absorption fine structure analysis

    M Kunisu, F Oba, H Ikeno, Tanaka, I, T Yamamoto

    APPLIED PHYSICS LETTERS   86 ( 12 ) 121902  2005.03  [Refereed]

     View Summary

    High-resolution near-edge x-ray absorption fine structure (NEXAFS) at Mn K edge is employed to probe the local environment of Mn dopant in ZnO. First-principles supercell calculations are systematically made to obtain theoretical NEXAFS. Mn is found to substitute for Zn up to 5 at.%Mn in polycrystalline samples sintered at 1623 K in air. Presence of Mn3O4 is apparent for samples with higher Mn content. The NEXAFS does not change in the range of Mn concentration from 0.01 to 5 at. %, indicating the absence of Mn precipitates. The results are confirmed by examining the polarization dependence of the NEXAFS for a 5 at. %-doped ZnO thin film. (C) 2005 American Institute of Physics.

    DOI

  • Distribution of solute atoms in β- and spinel Si <inf>6-z</inf>Al <inf>z</inf>O <inf>z</inf>N <inf>8-z</inf> by Al K-edge x-ray absorption near-edge structure

    Kazuyoshi Tatsumi, Teruyasu Mizoguchi, Satoru Yoshioka, Tomoyuki Yamamoto, Takeo Suga, Toshimori Sekine, Isao Tanaka

    Physical Review B - Condensed Matter and Materials Physics   71 ( 3 )  2005.01

     View Summary

    Local environments of solutes in β- and spinel Si 6-zAl zO zN 8-z are investigated by means of Al K x-ray absorption near-edge structure. The experimental spectra are found to be the same throughout the wide solubility range. This suggests that the local environments of Al are independent of the solute concentration. First-principles band-structure calculations are systematically made to interpret the experimental spectra. Effect of a core hole was included into the calculation. Theoretical spectra were obtained using variety of different model structures constructed by a set of plane-wave pseudopotentials calculations in our previous study [K. Tatsumi, I. Tanaka, H. Adachi, and M. Yoshiya, Phys. Rev. B 66, 165210 (2002)]. The numbers of models were 51 and 45 for both β and spinel, respectively. They are classified and averaged according to the local atomic structure of Al solutes. The combination of experimental spectra and theoretical results can unambiguously lead to the conclusion that Al atoms are preferentially coordinated by O atoms in both β and spinel phases. This is consistent with the conclusion obtained by the first-principles total-energy calculations. In the spinel phase, Al atoms are found to be located preferentially at the octahedral cationic site. This agrees with the conclusion in a recent report on the nuclear magnetic resonance experiment. ©2005 The American Physical Society.

    DOI

  • General rule for displacive phase transitions in perovskite compounds revisited by first principles calculations

    Phys. Rev. Lett.   94   035502  2005

  • Ab initio lattice dynamics and phase transformations of ZrO2

    Phys. Rev. B   71   064301  2005

  • Distribution of solute atoms in beta- and spinel Si(6-)zAl(z)O(z)N(8-z) by AlK-edge x-ray absorption near-edge structure

    K Tatsumi, T Mizoguchi, S Yoshioka, T Yamamoto, T Suga, T Sekine, Tanaka, I

    PHYSICAL REVIEW B   71 ( 3 ) 033202  2005.01  [Refereed]

     View Summary

    Local environments of solutes in beta- and spinel Si6-zAlzOzN8-z are investigated by means of Al K x-ray absorption near-edge structure. The experimental spectra are found to be the same throughout the wide solubility range. This suggests that the local environments of Al are independent of the solute concentration. First-principles band-structure calculations are systematically made to interpret the experimental spectra. Effect of a core hole was included into the calculation. Theoretical spectra were obtained using variety of different model structures constructed by a set of plane-wave pseudopotentials calculations in our previous study [K. Tatsumi, I. Tanaka, H. Adachi, and M. Yoshiya, Phys. Rev. B 66, 165210 (2002)]. The numbers of models were 51 and 45 for both beta and spinel, respectively. They are classified and averaged according to the local atomic structure of Al solutes. The combination of experimental spectra and theoretical results can unambiguously lead to the conclusion that Al atoms are preferentially coordinated by O atoms in both beta and spinel phases. This is consistent with the conclusion obtained by the first-principles total-energy calculations. In the spinel phase, Al atoms are found to be located preferentially at the octahedral cationic site. This agrees with the conclusion in a recent report on the nuclear magnetic resonance experiment.

    DOI

  • Core-hole effect on dipolar and quadrupolar transitions for SrTiO3 and BaTiO3 at the Ti K-edge

    Phys. Rev. B   71   245113  2005

  • Theoretical investigation of AlK-edge X-ray absorption spectra of Al, AlN and Al2O3

    M Mogi, T Yamamoto, T Mizoguchi, K Tatsumi, S Yoshioka, S Kameyama, Tanaka, I, H Adachi

    MATERIALS TRANSACTIONS   45 ( 7 ) 2031 - 2034  2004.07  [Refereed]

     View Summary

    High-resolution X-ray absorption spectra of Al, AIN and Al2O3 are measured at the Al K-edge, which have revealed a chemical shift in the threshold energy and significant differences in the spectral fine structures among the three compounds. In order to interpret the chemical shift and the fine structures of these spectra, first-principles calculations using the full-potential linearized augmented plane wave method within the density functional theory are carried out, taking into account the core-hole effect. The resultant theoretical spectra quantitatively reproduce both the chemical shift and the spectral fine structures of the experimental ones. The dependence of the theoretical spectra on the supercell size is also examined.

  • First-principles calculation of L-3 X-ray absorption near edge structures (XANES) and electron energy loss near edge structures (ELNES) of GaN and InN polymorphs

    T Mizoguchi, T Yamamoto, T Suga, M Kunisu, Tanaka, I, H Adachi

    MATERIALS TRANSACTIONS   45 ( 7 ) 2023 - 2025  2004.07  [Refereed]

     View Summary

    First principles calculations of L-3 XANES/ELNES of GaN and InN with both wurtzite and zinc-blende structures have been made using OLCAO (orthogonalized linear combinations of atomic orbitals) method. Supercells with more than 100 atoms were employed. A core-hole was rigorously included in the calculation, and the photo absorption cross section (PACs) between the initial and final states was computed. Quantitative reproduction of experimental spectrum that is available in literature can be found when the PACs was computed. Although spectral shapes of two phases took similar, characteristic differences are predicted to appear at the first peak of the L3 XANES/ELNES. The first peak is notably broader in the zinc-blende phases. The origin of the broadness is analyzed using partial density of unoccupied states (PDOS) and Mulliken charge. We then conclude that the broadness can be related to greater covalency of the zinc-blende phase as compared to the wurtzite phase.

  • Atomic and electronic structures of hydrated polymolybdates by first principles calculations

    A Togo, Tanaka, I, K Murase, T Yamamoto, T Suga, E Matsubara

    MATERIALS TRANSACTIONS   45 ( 7 ) 1982 - 1986  2004.07  [Refereed]

     View Summary

    First principles calculations of hydrated polymolybdates complexes have been made with an atomic orbital basis molecular orbital method. Hydration effects are taken into account by the conductor-like screening model (COSMO) using dielectric constant of water. Hydrated heptapolymolybdate, Mo7O246-, shows a low symmetry structure, which agrees well to experimental results, i.e., X-ray diffraction of crystalline salts and X-ray absorption fine structure of the hydrated complex. Contrary to that, the hydrated heteropolymolybdate, NiMo6O2410- prefer to exhibit the high symmetry structure. Inspection of the electronic states found that the Ni ion exhibits trivalent state or d(7) configuration in a formal sense. Jahn-Teller distortion around Ni is therefore evident. Such distortion cannot be found in CrMo6O249- or CoMo6O249-.

  • X-ray absorption near edge structures of silicon nitride thin film by pulsed laser deposition

    T Suga, T Mizoguchi, M Kunisu, K Tatsumi, T Yamamoto, Tanaka, I, T Sekine

    MATERIALS TRANSACTIONS   45 ( 7 ) 2039 - 2041  2004.07  [Refereed]

     View Summary

    Silicon nitride thin film was fabricated by pulsed laser deposition using KrF excimer laser and a silicon nitride compact as a target. The deposition was carried out on Al2O3 (0001) at 1173 K in N-2 gas pressure of 0.27 Pa. The X-ray diffraction did not provide any structural information of the deposited thin films except that it is composed of amorphous and/or micro-crystalline structure. X-ray absorption near edge structures at Si-K edge revealed that local arrangement of Si is not random. It should be composed of SiN4 unit similar to the case of alpha-Si3N4 crystal. Metallic Si component cannot be found in XANES.

  • First principles study of core-hole effect on fluorine K-edge X-ray absorption spectra of MgF2 and ZnF2

    T Yamamoto, T Mizoguchi, K Tatsumi, Tanaka, I, H Adachi, Y Muramatsu, EM Gullikson, RCC Perera

    MATERIALS TRANSACTIONS   45 ( 7 ) 1991 - 1993  2004.07  [Refereed]

     View Summary

    First principles calculations have been carried out to investigate the core-hole effects on the theoretical fine structures of the X-ray absorption spectra of MgF2 and ZnF2 at F K-edge. Significant differences are found between the calculated spectral fine structures with and without core-holes. Experimental profiles of the near-edge X-ray absorption fine structures are well reproduced by the theoretical ones when the core-hole effect is introduced. The dependence of supercell size on the theoretical fine structures is also examined.

  • Electronic states of sulfur doped TiO2 by first principles calculations

    T Yamamoto, F Yamashita, Tanaka, I, E Matsubara, A Muramatsu

    MATERIALS TRANSACTIONS   45 ( 7 ) 1987 - 1990  2004.07  [Refereed]

     View Summary

    First principles calculations of rutile-type TiO2:S have been performed to investigate the effect of sulfur solutes on the electronic structure. Plane-wave pseudopotentials method has been employed and atomic relaxations were fully taken into account. All possible geometric configurations for sulfur solutes within a 12-atoms supercell have been examined changing sulfur concentration of x = 0, 0.25, 0.5. 0.75 and 1. Theoretical direct band gap is found to decrease as sulfur concentration is increased. The dependence on the sulfur concentration is weaker than that was predicted in literature. Both the optimization of solute configuration and atomic relaxation are found to be essential for quantitative evaluation of the electronic structures in the alloy.

  • First-principles calculations of ELNES and XANES of selected wide-gap materials: Dependence on crystal structure and orientation

    T Mizoguchi, Tanaka, I, S Yoshioka, M Kunisu, T Yamamoto, WY Ching

    PHYSICAL REVIEW B   70 ( 4 ) 045103  2004.07  [Refereed]

     View Summary

    Theoretical calculations of electron energy-loss near-edge structure (ELNES) and x-ray absorption near-edge structure (XANES) of selected wide-gap materials including TiO2, AlN, GaN, InN, ZnO, and their polymorphs are performed using a first-principles method. Calculations of 39 K and L-3(L-2,L-3) edges are made using large supercells containing 72 to 128 atoms. A core hole is included in the final state, and the matrix elements of the electric dipole transition between the ground state and the final state are computed. Structures of some metastable crystals are optimized by a plane-wave basis pseudopotential method. Spectral differences in ELNES and XANES among polymorphs are quantitatively reproduced in this way. The origin of the spectral differences is pursued from the viewpoint of chemical bondings. Crystallographic orientation dependence of ELNES and XANES is also examined both by experiment and theory. The dependence is found to be much larger in K edges than that in L-3(L-2,L-3) edges.

    DOI

  • The formation of a rock-salt type ZnO thin film by low-level alloying with MgO

    M Kunisu, Tanaka, I, T Yamamoto, T Suga, T Mizoguchi

    JOURNAL OF PHYSICS-CONDENSED MATTER   16 ( 21 ) 3801 - 3806  2004.06  [Refereed]

     View Summary

    A high pressure form of ZnO with a rock-salt structure can be synthesized as a thin film by a pulsed laser deposition technique when greater than or equal to 15 mol% of MgO is alloyed. The phase is identified both by x-ray diffraction and near edge x-ray absorption fine structure (NEXAFS) measurements. NEXAFS is interpreted with the aid of first principles calculations employing a supercell composed of 108 atoms with a Zn-1s core-hole. The rock-salt phase can be formed only when an MgO(100) substrate is used because of a favourable lattice matching. As a result of the heteroepitaxy, the crystal is distorted tetragonally by c/a = 1.02. The energy increase due to the tetragonal deformation is estimated to be 2 meV/ZnO by a first principles calculation.

    DOI

  • Fine structure analysis of SiKL2,3V Auger spectra of Si, SiC and SiO2

    T Yamamoto, C Sato, M Mogi, Tanaka, I, H Adachi

    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA   135 ( 1 ) 21 - 25  2004.03  [Refereed]

     View Summary

    High-resolution Si KL2.3V Auger spectra of Si, SiC and SiO2 are measured to investigate changes in the electronic structure of the valence band in these materials. Significant differences between the lineshapes of these spectra are observed. First-principles electronic structure calculations are also carried out, which reproduce the lineshapes of the KL2.3V Auger spectra of Si, SiC and SiO2 observed experimentally. The core-hole effects on the theoretical spectral lineshapes are also taken into account in the present calculations. (C) 2003 Elsevier B.V. All rights reserved.

    DOI

  • Identification of ultradilute dopants in ceramics

    Tanaka, I, T Mizoguchi, M Matsui, S Yoshioka, H Adachi, T Yamamoto, T Okajima, M Umesaki, WY Ching, Y Inoue, M Mizuno, H Araki, Y Shirai

    NATURE MATERIALS   2 ( 8 ) 541 - 545  2003.08  [Refereed]

     View Summary

    The properties of ceramic materials are strongly influenced by the presence of ultradilute impurities (dopants). Near-edge X-ray absorption fine structure (NEXAFS) measurements using third-generation synchotron sources can be used to identify ultradilute dopants, provided that a good theoretical tool is available to interpret the spectra. Here, we use NEXAFS analysis and first-principles calculations to study the local environments of Ga dopants at levels of 10 p.p.m in otherwise high-purity MgO. This analysis suggests that the extra charge associated with substitutional Ga on a Mg site is compensated by the formation of a Mg vacancy. This defect model is then confirmed by positron lifetime measurements and plane-wave pseudopotential calculations. This powerful combination of techniques should provide a general method of identifying the defect states of ultradilute dopants in ceramics.

    DOI

  • Substitution mechanism of Al ions in MgSiO3 perovskite under high pressure conditions from first-principles calculations

    T Yamamoto, DA Yuen, T Ebisuzaki

    EARTH AND PLANETARY SCIENCE LETTERS   206 ( 3-4 ) 617 - 625  2003.02  [Refereed]

     View Summary

    Large-scale first-principles density functional theory (DFT) calculations have been carried out to investigate how Al3+ can be incorporated into MgSiO3 perovskite under high pressure and to study the resultant change in the compressional mechanism Of MgSiO3 perovskite. We examined two types Of MgSiO3 models with 6.25 mol% Al2O3: charge-coupled substitution and oxygen-vacancy mechanisms. Five pressure points from 0 to 100 GPa have been considered. At each pressure point, we have calculated five models of the oxygen vacancy and five models of the charge-coupled mechanisms. We also change the internal positions of the substituted Al in the calculated cells, which have 80 atoms. Our free energy calculations show Al3+ replaces the nearest-neighbor cation pairs (Mg2+ and Si4+) at all pressures investigated. The calculated bulk modulus of the most energetically favorable model is 3.4% lower than that of the Al-free MgSiO3 perovskite. These results may have important implications for discriminating between thermal and compositional effects of I-D Earth models and the possible influence of aluminum perovskite. (C) 2002 Elsevier Science B.V. All rights reserved.

    DOI

  • First principles calculations of pressure-induced structural phase transition of Co

    T Yamamoto

    ADVANCES IN QUANTUM CHEMISTRY, VOL 42   42   199 - 207  2003  [Refereed]

  • Soft X-ray emission and absorption spectroscopy in the Si L region of polysilanes

    Y Muramatsu, M Fujino, T Yamamoto, EM Gullikson, RCC Perera

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   199   260 - 264  2003.01  [Refereed]

     View Summary

    Soft X-ray emission and absorption spectra in the Si L region of a number of substituted polysilanes, (SiR2)n, have been obtained using synchrotron radiation to investigate their electronic structures. Studied polysilanes were substituted with methyl (R = CH3), ethyl (C2H5), propyl (n-C3H7), butyl (n-C4H9), pentyl (n-C5H11) and phenyl (C6H5) groups. Although similar spectral features in both X-ray emission and absorption are observed among alkyl-substituted polysilanes, slight differences are distinguished between alkyl- and phenyl-substituted ones. These spectral features are qualitatively reproduced by summing calculated density-of-state spectra for Si3s- and Si3d-orbital.s. Thus, spectral features are explained through the hybridization of electronic orbitals in both backbone Si atoms and substituent C atoms. (C) 2002 Elsevier Science B.V. All rights reserved.

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Books and Other Publications

  • Fundamentals of Materials Science

    ITO, Kimihisa, HIRATA, Akihiko, YAMAMOTO, Tomoyuki( Part: Joint author)

    2020.10 ISBN: 9784339066524

  • 量子物質科学入門

    山本知之

    コロナ社  2010.03 ISBN: 9784339066173

Misc

  • Electronic structure analysis of La_<1-x>AE_xMnO_3 by XANES spectra

    Inaba Yuta, Yoshioka Tsuyoshi, Hirose Kazuyuki, Yamamoto Tomoyuki

    Meeting abstracts of the Physical Society of Japan   68 ( 2 ) 538 - 538  2013.08

    CiNii

  • 29aXL-1 Electronic structure analysis of In_2O_3:(Mn,TM)(TM=Cr,Fe,Co)

    Yoshioka Tsuyoshi, Nishida So, Yamashita Daisuke, Yamamoto Tomoyuki

    Meeting abstracts of the Physical Society of Japan   68 ( 1 ) 782 - 782  2013.03

    CiNii

Research Projects

  • Development of high-efficiency up-conversion oxide phosphor material and the analysis of its light-emitting mechanism

    Project Year :

    2020.04
    -
    2023.03
     

  • Study on the mechanism of ferromagnetism in dilute magnetic oxides

    Project Year :

    2014.04
    -
    2017.03
     

     View Summary

    Some of the nonmagnetic materials show ferromagnetism by doping dilute magnetic elements at the level of a few at%, which are called as dilute magnetic materials. Application of dilute magnetic elements for the spintronics, which uses the transportation of spins, are highly required after the current great applications of electrons and photons for electronics and photonics, which can support our developed life. In the current project, local environment of magnetic elements in dilute magnetic materials has been investigated using the X-ray absorption near-edge structure measurements and the first principles calculations in order to understand the mechanism of appearance of ferromagnetism, which cannot be explained only by the established magnetism theory

  • 第一原理計算による地球マントル物質に及ぼすプロトン存在の影響に関する研究

    Project Year :

    2011.04
    -
    2013.03
     

     View Summary

    平成24年度に行った,地球上部マントルの主成分であるMg2SiO4の高圧力下での弾性的性質に対するプロトン含有の影響に関する検討に加えて,実在の地球マントルの組成により近い状態を考えるために,Mgシリケートに地球マントル中に存在すると考えられている,他の元素を添加したモデルを構築し,それらの物質における欠陥形成機構を第一原理計算により検討した.添加する元素としては,Fe,Al,Caなど地球マントル中における存在比が高いと考えられている元素を添加したモデルの計算を進め,特に,Fe3+やAl3+等を添加する場合については,Mg2+,Si4+とは価数が異なるため,例えば,Si4+とAl3+の置換を考える場合には,電荷補償のために酸素空孔を伴うモデルが,またMg2+とAl3+の置換の場合には,Mg2+空孔を伴うモデルの構築が必要となるので,それぞれの元素添加に応じて空孔を導入したモデルを構築し計算を行った.特に,下部マントルの主成分であるMgSiO3ペロブスカイトに対するAl3+を含有する場合のプロトン含有量について詳細な検討を行った.以上,本年度の研究について総括すると,第一原理計算法を用いて,<Mgシリケート>+<マイナー元素>+<プロトン>の高温・高圧下での弾性的性質を評価し,そして,それらのマイナー元素の存在による地球マントル物質の結晶構造相転移の変化を評価し,最新の高温・高圧実験との比較ならびに地震波データから得られる地球内部の三次元構造との対応について検討を進めた.本研究における研究成果は,国内の日本地球惑星科学連合大会において発表すると共に,Dubunaで開催されたMSSMBS2012における招待講演ならびにSan Franciscoで開催されたAmerican Geophysical UnionのFall Meetingにおいて発表した.24年度が最終年度であるため、記入しない。24年度が最終年度であるため、記入しない

Presentations

  • Phase Stability of Organic-Inorganic Lead-Halide Perovskite

    Tomoyuki Yamamoto  [Invited]

    Joint 5th Int’l Symposium on Frontiers in Materials Science & 3rd International Symposium on Nano-materials, Technology and Applications 

    Event date:
    2019.11
     
     
  • XANES analysis for functional materials

    T. Yamamoto  [Invited]

    8th International Symposium on Molecular studies in Material and Biological Sciences  (St. Petersburg, Russia) 

    Presentation date: 2017.09

  • Local environment of dilute 3d transition elements in optical materials -XANES measurements and DFT calculations-

    T. Yamamoto  [Invited]

    International Conference "Functional materials and Nanotechnology-2017"  (Tartu, Estonia) 

    Presentation date: 2017.04

  • Local environment analysis of dilute rare-earth dopants in phosphor materials

    T.Yamamoto  [Invited]

    Pacific Rim Meeting on Electrochemical and Solid State Science  (Honolulu, USA) 

    Presentation date: 2016.10

  • X-ray Absorption Near-Edge Structure Analysis of Magnetic Elements in Dilute Magnetic Materials

    Tomoyuki Yamamoto  [Invited]

    3rd International Symposium on Frontiers in Materials Science  (Hanoi, Vietnam) 

    Presentation date: 2016.09

  • Origin of Room Temperature Ferromagnetism in Dilute Magnetic Oxides

    Tomoyuki Yamamoto  [Invited]

    Khujand Symposium on Computational Materials and Biological Sciences  (Khujand, Tajikistan) 

    Presentation date: 2016.09

  • X-ray Absorption Near-Edge Structure Analysis for Functional Materials with the Aid of First-Principles Calculations

    Tomoyuki Yamamoto  [Invited]

    The 4th International Symposium on Science of Molecular Assembly and Biomolecular Systems  (Nagoya, Japan) 

    Presentation date: 2015.09

  • Core-Level spectroscopy - experiments and first-principles calculations -

    Tomoyuki Yamamoto  [Invited]

    22nd International Workshop on WIEN2k 

    Presentation date: 2015.07

  • X-ray absorption near-edge structure analysis for functional materials

    Tomoyuki Yamamoto  [Invited]

    International Symposium on Nano-Materials, Technology and Applications  (Hanoi, Vietnam) 

    Presentation date: 2014.10

  • Electronic structure analysis of functional materials by synchrotron radiations and first-principles calculations

    Tomoyuki Yamamoto  [Invited]

    1st international Dushanbe symposium on computational materials and biological sciences  (Dushanbe, Tajikistan) 

    Presentation date: 2014.09

  • Electronic structure of magnetic elements in dilute maganetic materials

    Tomoyuki Yamamoto  [Invited]

    7th International workshop on molecular simulation studies in materials and biological sciences 

    Presentation date: 2014.09

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Specific Research

  • 希土類フリー高効率蛍光体の評価と創製

    2021  

     View Summary

    本研究では,白色LEDへの応用を目的とした希土類フリーの赤色蛍光体材料の開発を目指し,その候補として酸化物,フッ化物のホスト材料にMnを添加した物質群を中心として,合成および評価を行った.試料合成には主に固相反応法を用い,実験的評価にはX線回折測定,拡散反射スペクトル測定,蛍光スペクトル測定,電子スピン共鳴測定等を行い,更に第一原理計算による電子状態解析を行った.対象となる材料には,Mnを添加したCaAl12O19とK2SiF6などを選び,Mn以外の元素の共添加による発光強度の増大や,Mnの局所環境解析等を実験,計算の双方からアプローチし,その成果の一部を国際共同論文として発表した.

  • マテリアルズインフォマティクスを用いた結晶構造推定

    2020   Mikail G. Brik, Ngyuen Hai Chau, 久保雅俊

     View Summary

    機能性材料の開発において必要とされる物性の発現機構を考えるためには,その原子配列,結晶構造を知ることは必要不可欠である.しかし,新物質の開発過程においては,未知の結晶構造を予測する必要が出てくる.そのような場合,第一原理計算法などの計算科学的手法を用いた推定が可能であるが,複雑な結晶構造を高い確度で推定するのは困難を伴う場合も多い.そこで,近年の発展が著しいマテリアルズインフォマティクスの手法を用いて,複雑な結晶構造の推定を目指し,本年は機械学習を用いたガーネット構造の構造パラメーターの推定を行った.

  • アップコンバージョン蛍光体によるエネルギーハーべスティング太陽電池システムの開発

    2019  

     View Summary

    近赤外線等の長波長(低エネルギー)の光を可視光などのより短波長(高エネルギー)の光に変換するアップコンバージョン型の蛍光体を太陽電池システムに組み込むことを目指し,本研究では,アップコンバージョン型酸化物セラミックス蛍光体の発光効率の向上を中心に研究を進めた.具体的には,近赤外光に対してアップコンバージョン特性を示すEr添加CaZrO3などに更にNaなどのアルカリ元素を加えることにより,アップコンバージョン発光強度を増大させ,そのメカニズムを検討するために,粉末X線回折,走査型電子顕微鏡観察,紫外可視拡散反射スペクトルなどの分析を進めた.

  • 高耐久マルチカチオンハイブリッドペロブスカイトの創製

    2019  

     View Summary

    有機無機ハイブリッドペロブスカイトは,次世代太陽電池の吸光材料として注目を集めているが,実用環境下における安定性が課題となっている.ペロブスカイト構造の安定性は,カチオンとアニオンのイオン半径を基に評価できるので,カチオンサイトに複数のイオンを用いたマルチカチオン型のハイブリッドペロブスカイトの開発が進められている.本研究では,スピンコート法を用いて,複数のカチオンを導入したマルチカチオン型のハイブリッドペロブスカイトの合成,ならびに大気中,窒素中および真空中における安定性の評価を行った.

  • 3d遷移金属元素を微量添加した機能性材料の創製とその機能発現機構の解明

    2018  

     View Summary

    3d遷移金属元素を微量添加することにより新規物性を発現させた機能性材料は,種々の応用が期待されているが,元素添加量が微量であるため,その添加によってどのように新規物性が発現したのかが明らかにされていないものが多い.本研究では,シンクロトロン放射光を用いた原子レベルでの微量添加した3d遷移金属元素の局所環境解析を出発点として,その物性発現機構の検討を行った.特に,本年度は赤色蛍光体への応用を前提とした酸化物にMnを微量添加した物質群の合成と評価を行った.

  • アップコンバージョン蛍光体における微量添加元素の局所環境解析

    2017  

     View Summary

    近赤外光照射によって,可視光を発光するアップコンバージョン型の蛍光体が近年注目を集めているが,その発光メカニズムについては未だに充分な理解が得られていないものが多い.そこで,本研究では,近赤外光照射により緑色発光を示すErを添加したCaZrO3及びそれにYbを共添加することによって発光強度及び発光色が変化するメカニズムの解明を目指し,X線吸収端近傍微細構造測定と第一原理計算を用いて,添加したErとYbの局所環境を明らかにした.

  • 有機無機ハイブリッドペロブスカイトの電子状態解析

    2016  

     View Summary

    次世代太陽電池として注目を集めている有機無機ハイブリッドペロブスカイトの中でも,現時点で特に発電効率が高いと報告されているCH3NH3PbI3への軟X線(Al-Ka線)照射の影響について,時間依存X線光電子分光測定と第一原理計算を用いた検討を行った.真空中での軟X線長時間照射に対して,徐々にCH3NH3の量が減り,最終的には,PbI2へと変化することが確認できた.

  • 無機フォトクロミック物質における添加元素の局所環境解析

    2015  

     View Summary

    Recently efficient inorganic photochromic oxide materials were found, in which it was mentioned that 3d transition metal and/or rare earth dopings enhance photochromic property. However, these mechanisms have not yet been clearly understood. In the current study, local environment analysis of doped ions in inorganic photochromic oxide materials has been investigated. Samples were synthesized by a conventional solid-state reaction method in air and reducing atmosphere (Ar:97%+H2:3%). Crystal structures of the synthesized samples were examined by the powder X-ray diffraction technique and the color-change due to the irradiation of UV-ray and visible light was examined by diffuse reflectance spectrum using a UV-Vis spectrometer. Local environment analysis was carried out by measuring the X-ray absorption near-edge structure (XANES) at K-edge of 3d transition metal and L-edge of rare earth element. To analyze the fine structure of the observed XANES spectra, we also performed the all-electron first-principles calculations within a density functional theory. This method has been applied to several types of the materials. Among them, we could find out the necessary condition to show strong photochromism in Fe doped BaMgSiO4, that is Fe must substitute at Mg2+ site as divalent ions.

  • マルチフェロイック酸化物における磁性元素の電子状態解析

    2013  

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    近年,再生可能エネルギーに関する研究が活発に進められてきており,その中の一つとして圧電素子に関する研究が活発に進められてきている.圧電素子の材料 としては,高い圧電性能を示すことからこれまで鉛を含む材料が中心に用いられてきたが,環境に対する意識の高まりから,近年鉛を含まない非鉛系の圧電材料 の開発が進められてきている.非鉛系の圧電材料としては,KNbO3-NaNbO3-LiNbO3系,(Bi1/2Na1/2)TiO3や(Bi1 /2K1/2)TiO3系,ビスマス層状構造強誘電体(BLSF),(K,Na,Li)(Nb,Ta)O3などがそれぞれに特徴を持った特性に対して研究 が進められている.上記の非鉛系圧電材料の中で,ビスマス系層状強誘電体Bi4Ti3O12にMnやFeなどの3d遷移金属及び希土類元素を共添加することで,圧電(誘電)特性が向上するという報告がある.それと同時に,強誘電性と共に強磁性を発現するマルチフェロイック特性を示すという報告もある.従来の研究では,添加元素の組合せや添加量を変えた試料を作製し,物性を評価し,求める物性値を最適化することを目標とする研究が主として行われてきているが,添加元素の局所環境や添加元素のイオン価数状態,更には元素添加 による電子状態の変化等に関する研究は,圧電(誘電)特性の変化に関するメカニズムを理解する上で必要不可欠であるにもかかわらず,これらの研究はほとんど行われていないのが現状である.本研究では,ビスマス系層状構造強誘電体の一つであるBi4Ti3O12にFeを添加することによりマルチフェロイック特性を示す物質を固相反応法により作製し,粉末X線回折実験による結晶構造評価を行い,Feの電子状態並びに局所環境について,シンクロトロン放射光を用いて評価した.また,比較のためにMnを添加したものも同様の方法にて作製し,評価した.具体的には,Mn及びFeの電子状態(価数)を評価するために分子科学研究所UVSORのBL-4BにてMnおよびFeのL3端X線吸収端近傍微細構造(XANES)スペクトルを測定した.また,大型放射光施設SPring-8のBL01B1にてFe-K端,第一原理計算法を用いたスペクトル形状の解析を行い,Feの局所環境について評価した.更に,第一原理計算による固溶エネルギーの評価を行い,上記の実験結果と合わせて,Fe及びMnの局所環境解析が行えた.これらの成果をまとめて,下記の国際会議及び論文発表を行うことができた.

  • 遷移金属元素を含む新規機能性ハイブリッドソフトマテリアルの評価と設計

    2012   吉岡 剛志

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    本研究の目的は,異なる遷移金属元素を含む2種類以上の分子からなる分子性共結晶を対象として,その共結晶が遷移金属間の相互作用によって,新たな磁気的,電気的,光学的物性を示すような新規物質の探索を行うことである.また,それと同時に,計算科学的手法を用いた電子状態と物性との関連を検討し,新規機能性ハイブリッドソフトマテリアルとしての分子性共結晶の設計指針を構築することも目指している.分子性結晶の結晶構造については,これまで分子間の相互作用を量子化学計算ならびに第一原理計算によって見積もることが困難であったが,近年開発が進められている新しい計算手法を分子性結晶に適用し,本研究の研究対象である分子性共結晶の計算科学的構造解析手法の確立も目指し,新規機能性物質の設計を試みることも目指している.本年度は,上記の目標を達成するためにフェロセンとポルフィリン錯体の共結晶,ならびに異なる磁性元素を含むメタロセン共結晶の合成を溶媒蒸発法を用いて試みた.条件や組み合わせを変えて,数十種類の合成を試みた中で,一部の試料に置いて結晶化した試料が確認できたので,それらに対して単結晶X線回折実験による構造解析を行った.また,一方で磁性元素を含む機能性酸化物(PrCoO3:Ca,Sr)の合成も行い,それらの物質中での磁性元素の電子状態解析をシンクロトロン放射光を用いたX線吸収端微細構造測定ならびに第一原理計算を用いて行った.前者の実験は,SPring-8のBL01B1における磁性元素のK吸収端を,またUVSORのBL4Bにおける磁性元素のL吸収端スペクトルの測定を行い,後者の計算にはVASPおよびWIEN2kコードを用いた.これらの実験と計算とを組み合わせた解析により,ハイブリッドソフトマテリアルにおける,新規機能の発現機構について検討する手がかりを得るところまで到達できており,研究成果の一部を国際会議ならびに国際学術誌へ発表することができた.現時点までで,今後の物質設計手法の確立に向けた一定の成果をあげることができたものと考えている.

  • 室温強磁性希薄磁性体における強磁性発現機構に関する研究

    2011  

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    非磁性の半導体や絶縁体に磁性元素を数at%程度添加することにより強磁性を発現する物質があり,希薄磁性体と呼ばれており,近年活発に研究が進められている.希薄磁性体における強磁性発現のメカニズムは未だに明らかにされていない部分が多い.特に,添加した元素の局所的な環境,すなわちマトリックス中の元素と置換しているか,格子間に存在しているか,また析出物として存在しているかを知ることが極めて重要であるが,それらは十分に検討が進められていない.また,磁性元素のスピン状態,価数などの電子状態についても添加した元素量が最大でも数at%と希薄なため,未知なものがほとんどである.これらのような背景を踏まえて,本年度は,置換固溶型の希薄磁性体における強磁性発現機構を検討するために,室温で強磁性を発現する酸化インジウムにFeとMnを数at%添加した多結晶焼結体を作製し,その中における磁性元素の電子状態解析をシンクロトロン放射光を用いた検討を進めた.試料は固相反応法を用いて作製し,MnとFeの価数を調整する元素として更にSn4+を数%添加した試料も同様に作製した.作製した試料は,室温において強磁性を示すことが確認されたが,Sn4+の添加量が増えるにしたがって,飽和磁化が減少することが確認され,Mnの添加量とほぼ同じ量のSn4+を添加した時点で,ほぼ常磁性となることも確認した.作製した試料の結晶構造について,粉末X線回折実験を行いビックスバイト構造の回折パターンのみが観測され,また,添加したMn,FeとInとのイオン半径の関係から,添加したMn,FeはInサイトに置換しているものと確認した.ただし,X線回折測定のみでは,結晶子の小さな析出物として存在している場合は検出することができないが,これまでのシンクロトロン放射光を用いた実験にてInサイトに置換することを既に確認している.今回は,MnとFeのL3端X線吸収スペクトルを測定することによりMnとFeの価数の評価を行った.これらの測定は分子科学研究所の放射光施設であるUVSORのBL-4Bにて全電子収量法により測定した.すべての試料においてFeは3+であることが示唆されたが,強磁性を示す資料においてはMnはMn2+とMn3+が混在していることが見いだされた.また,Sn4+の添加によりMn3+の量が減少し,MnとSnの量がほぼ同量となった時点で,ほぼすべてのMnが2+の状態になっていることがわかり,Mnの2+と3+の価数混在が強磁性発現に必要不可欠であることを見出すことができ,その結果を学術誌(Solid State Communications)に出版した.

  • 室温強磁性希薄磁性多結晶焼結体セラミックスの創製

    2008  

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    3d遷移金属元素などの磁性元素を希薄(数at%)に含み強磁性を示すものが希薄磁性体と呼ばれ,近年その強磁性発現機構に関する基礎研究のみにとどまらず,スピトロニクスへの応用を視野に入れた応用研究も活発に進められている.特に,応用の面から考えても希薄磁性体のキュリー温度が高い方が良いことから,室温で強磁性を示す希薄磁性体の探索が進められている.これらの多くは,非平衡プロセスによって作製された薄膜であり,多結晶焼結体で強磁性を示す希薄磁性体は数少ないが,MnとFeを共添加したIn2O3の多結晶焼結体が室温で強磁性を示すことが報告されていることから,本年度はその物質における強磁性発現機構の検討,ならびにIn2O3系の希薄磁性体の創製を試みた.磁性元素の局所環境については,様々な環境における試料作製を行い,第一原理計算とXANES(X-ray Absorption Near-Edge Structure:X線吸収端微細構造)測定を用いて,それらの 環境を考慮した第一原理計算による磁性元素の局所環境の予測及びXANESスペクトルの解析を行った.具体的には,焼成温度ならびに添加する磁性元素の濃度,種類を変えて,固相反応法により大気中及びAr雰囲気中で焼成し試料作製を行い,作製した試料を,粉末X線回折法を用いて磁性元素の置換固溶に伴う結晶構造の変化に関する構造解析,ならびに析出物の有無及び同定等を行った.本実験により,焼成温度と析出物の種類ならびに量を系統的なデータを得ることが出来た.更に,第一原理擬ポテンシャル計算に よって得られた磁性元素周辺の構造を用いて,磁性元素の局所環境を非常に敏感に検知することが可能であるXANESスペクトルの予測を行った.その結果, 添加した磁性元素の局所環境を特定することができ,Mnの価数の混在が強磁性発現に大きく寄与していることを見出すことができた.

  • 希薄磁性半導体中の磁性元素の局所環境解析

    2005  

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    磁性元素を希薄に含む半導体(希薄磁性半導体)中における磁性元素の局所環境を第一原理計算とXANES(X-ray Absorption Near-Edge Structure:X線吸収端微細構造)測定を用いた解析を目指し,本年度においては,様々な環境(焼成温度および雰囲気)における試料作製,それらの環境を考慮した第一原理計算による磁性元素の局所環境の予測及びXANESスペクトルの予測を行った.500~1000℃の範囲内で焼成温度を変えてFe, Co添加SnO2を固相反応法により大気中及びAr雰囲気中で焼成し試料作製を行った.作製した試料を,粉末X線回折法を用いて磁性元素の置換固溶に伴う格子定数の変化などの構造解析,ならびに析出物の有無及び同定等の検討を行った.本実験により,焼成温度と析出物の種類ならびに量を系統的なデータを得ることが出来た.粉末X線回折実験により高温焼成による磁性元素(Fe, Co)の置換固溶に伴う格子定数の変化が見られ,その変化は第一原理擬ポテンシャル計算の結果と調和的であった.そこで,第一原理擬ポテンシャル計算によって得られた磁性元素周辺の構造を用いて,磁性元素の局所環境を非常に敏感に検知することが可能であるXANESスペクトルの予測を行った.その結果,粉末X線回折実験では検知することが困難であるnmオーダーの析出物やSnO2中に置換固溶しているFe,CoのXANESスペクトルは,標準物質となるFe, Coの酸化物のそれらとは形状が大きく異なることが予測され,XANESスペクトルを系統的に比較することで,磁性元素の局所環境を十分に解析可能であることがわかった.

  • シンクロトロン放射光を使った軟X線分光

    1998  

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    特性X線の微細構造には物質の化学結合状態が反映され、物質のキャラクタリゼーションに広く応用されている。しかし、我々の身近に存在する炭素、窒素、酸素などの軽元素から発生する特性X線のエネルギーは約200~600電子ボルト(eV)程度であり、それらの測定は従来型の分析装置では非常に困難であった。近年のシンクロトロン放射光装置の発達により、これらの軽元素に対する分析感度は格段に進歩し、従来のモデルでは解析できない新しい実験結果が現れている。アメリカ合衆国University of California, Berkeley校に併設されているLawrence Berkeley National Laboratoryにあるシンクロトロン放射光施設Advanced Light Source(ALS)は、世界で最も明るい低エネルギー領域(軟X線)の放射光源であり、今回その施設で実験を行う機会を得た。主として、酸素、フッ素を含む化合物のX線発光および吸収スペクトルの測定を行い、今までに報告されていない実験結果を多数得ることができた。特に、フッ化カルシウム(CaF2)とフッ化ストロンチウム(SrF2)のF K線中に現れるサテライト線強度の入射エネルギー依存性およびサテライト線の影響を受けないスペクトルの測定に成功した。これらのX線スペクトル解析に有力な計算法の一つであるDV-Xa分子軌道法を用いて、後者のスペクトルを解析し、実験結果を非常に良く説明することができた。これらの結果の詳細については本年度のALS Compendiumに掲載される予定である。また、これらの実験結果以外にも多数(特に酸化物)の実験結果があり、それらを説明するために、分子軌道計算法を用いた解析を進めている。

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