Updated on 2024/04/25

写真a

 
KOYAMA, Yasumasa
 
Affiliation
Faculty of Science and Engineering
Job title
Professor Emeritus
Degree
工学博士 ( 東京工業大学 )
(BLANK)
工学修士 ( 東京工業大学 )
(BLANK)

Research Experience

  • 1994.04
    -
     

    Waseda University   School of Science and Engineering

  • 1990.04
    -
     

    早稲田大学材料技術研究所兼任研究員

  • 1989.04
    -
     

    Waseda University   School of Science and Engineering

  • 1985.04
    -
    1987.03

    ( 米国Purdue大学博士研究員)

  • 1981.04
    -
     

    Tokyo Institute of Technology   School of Engineering

Education Background

  •  
    -
    1981

    Tokyo Institute of Technology   Graduate School, Division of Science and Engineering  

  •  
    -
    1976

    Tokyo Institute of Technology   Faculty of Engineering  

Professional Memberships

  •  
     
     

    Japanese Society of Electron Microscopy

  •  
     
     

    The Crystallographic Society of Japan

  •  
     
     

    The Japan Society for Heat Treatment

  •  
     
     

    The Japan Institute of Metals

  •  
     
     

    The Physical Society of Japan

  •  
     
     

    The American Physical Society

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Research Areas

  • Metallic material properties / Magnetism, superconductivity and strongly correlated systems / Semiconductors, optical properties of condensed matter and atomic physics

Research Interests

  • Solid-State Physics I (Optical Properties, Properties, Semiconductor & Dielectrics)

Awards

  • 平成20年度 日本学術振興会 日本学術振興会 科学研究費補助金第1回審査員表彰

    2008  

  • 日本金属学会功績賞

    1996  

  • 村上記念研究奨励賞

    1990  

  • 本多記念研究奨励賞

    1982  

  • 日本金属学会論文賞

    1982  

  • 手島記念研究奨励賞

    1981  

  • 日本金属学会ジェフリース賞

    1979  

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Papers

  • Formation of the $alpha$-Mn and R structures from the $beta$-Mn structure in Mn-Si alloys

    T. Doi, T. Kobayashi, M. Tanimura, Y. Koyama

    Physical Reveiw B / The American Physical Society   82   184205  2010.11  [Refereed]

  • Crystallographic Features of Ferroelectric States around the Tetragonal/Rhombohedral Phase Boundary in Pb1-xLax(Zr1-yTiy)O3

    M. Arao, Y. Inoue, R. Ando, Y. Koyama

    Materials Science Forum   638-642   1737 - 1742  2010.01  [Refereed]

  • Stability of the orbital-modulated state in the layered perovskite manganite Sr2-xRxMnO4 (R = Pr, Nd)x</SUB>RxMnO4 (R = Pr, Nd)

    Y. Inoue, M. Arao, G. Shindo, Y. Koyama

    Materials Science Forum   638-642   1760 - 1765  2010.01  [Refereed]

  • Metastable dodecagonal-atomic-column state related to the formation of the σ structure in the Cr-Co alloy system

    T. Doi, M. Tanimura, Y. Koyama

    Physical Reveiw B / The American Physical Society   79 ( 13 ) 134204  2009.04  [Refereed]

  • Phase transition with non-deterministic nature in the Ni(3)Al(0.45)V(0.50) alloy

    M. Tanimura, T. Doi, Y. Koyama

    EPL   82 ( 4 ) 40007  2008.05  [Refereed]

     View Summary

    We demonstrate a thermodynamic state change dominated by a non-thermodynamic variable found in the Ni(3)Al(0.45)V(0.50) alloy. Specifically, the L1(2) -&gt; L1(2) + D0(22) evolution, which is a sub-phenomenon of the fcc -&gt; L1(2) + D0(22) phase transition, takes place only when lattice defects are introduced to more than a certain critical density in the L1(2) diffusion field. Because the defect density changes along with the variation in the thermodynamic path, the path variation can alter the evolved state even under an identical thermodynamic condition. This path dependence of the state change shows the non-deterministic (chaotic) nature of the phase transition. Copyright (C) EPLA, 2008.

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    1
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  • Crystal-structure units of the R (Fe(3)Mo(2)) phase in the Fe-Mo alloy system

    Toshihiro Doi, Makoto Tanimura, Yasumasa Koyama

    PHYSICAL REVIEW B   77 ( 13 ) 134205  2008.04

     View Summary

    Around 37 at. % Mo in the Fe-Mo alloy system, in the temperature range between 1473 and 1761 K, there exists the intermetallic-compound R phase, whose crystal structure involves 12-, 14-, 15-, and 16-coordination polyhedra. To determine the crystal-structure units of the R phase, the crystallographic features of the bcc-to-R structural change in the (bcc -&gt; bcc+R) reaction of the Fe-Mo system have been investigated by transmission electron microscopy. It was found that atomic columns, which were identified as the secondary structural unit, appeared first in the initial stage of the reaction. Based on this fact, the R phase is understood to have a crystal structure consisting of atomic columns, just as in the case of the mu structure [A. Hirata , Phys. Rev. B 74, 054204 (2006)]. Although only 12-coordination polyhedra form an atomic column in the mu structure, the column in the R structure consists of two kinds of polyhedra: that is, 16- and 12-coordination ones. Of these two polyhedra, in particular, the appearance of a 16-coordination polyhedron as the primary structural unit of the R structure is associated with the formation of covalent bonds in alloys.

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  • Crystallographic features of the orbital-ordered state in the layered perovskite manganite Sr2-xPrxMnO4

    Wataru Norimatsu, Go Shindo, Yasumasa Koyama

    PHYSICA B-CONDENSED MATTER   403 ( 5-9 ) 1585 - 1586  2008.04

     View Summary

    The crystallographic features of Sr2-xPrxMnO4 samples, prepared by a coprecipitation method, have been investigated by transmission electron microscopy to understand their electronic States for 0.10 &lt;= x &lt;= 0.30. The orbital-ordered (OO) state lacking charge ordering existed for 0.10 &lt;= x &lt;= 0.25, and accompanied C-type antiferromagnetic (CAF) ordering at lower temperatures. It was also found that the suppression of CAF ordering occurred in a variant with smaller orthorhombicity, which is one of two variants forming a banded domain structure in the OO state. (C) 2007 Elsevier B.V. All rights reserved.

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    5
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  • Role of the triclinic Al(2)Fe structure in the formation of the Al(5)Fe(2)-approximant

    Akihiko Hirata, Yuichiro Mori, Manabu Ishimaru, Yasumasa Koyama

    PHILOSOPHICAL MAGAZINE LETTERS   88 ( 7 ) 491 - 500  2008  [Refereed]

     View Summary

    The structural correspondence among the bcc, ordered-bcc B2, triclinic Al(2)Fe and approximant Al(5)Fe(2) structures have been examined on the basis of the experimentally obtained orientation relationship of (110)(bcc,B2) //(2 (2) over bar1)(Al2Fe) and [(1) over bar 11](bcc,B2) // N(241)(Al2Fe), where N(hkl) denotes the normal direction of the (hkl) plane. The formation of the triclinic Al(2)Fe structure from the bcc structure is directly associated with that of Al-Fe covalent bonds; that is, hybridization between Al and Fe atoms. The covalent bond formation results in the appearance of decagonal-like atomic arrangements in the Al(2)Fe structure, which are similar to those in the approximant Al(5)Fe(2) structure.

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    8
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  • Stability of electronic states in the layered perovskite Sr2-xNdxMnO4

    W. Norimatsu, Y. Koyama

    Physica C   463-465 ( 1 ) 115 - 119  2007.10  [Refereed]

  • Diffusion blocking, path variability, and bifurcation of the final state in the phase separation of Ni3(Al,V)1-δ alloys

    Makoto Tanimura, Yasumasa Koyama

    Physical Review B / The American Physical Society   76 ( 5 ) 54103  2007.08  [Refereed]

  • Ferroelectric domain structures around the morphotropic phase boundary of the piezoelectric material PbZr1-xTixO3

    T. Asada, Y. Koyama

    Physical Review B / The American Physical Society   75 ( 21 ) 214111  2007.06  [Refereed]

  • Crystallographic features of the orbital-ordered, and charge-and-orbital-ordered states in Sr2-xNdxMnO4

    W. Norimatsu, Y. Koyama

    Physical Review B / The American Physical Society   75 ( 23 ) 235121  2007.06  [Refereed]

  • Orientation relationship between the bcc and σ phases in Cr-Co alloys

    T. Doi, M. Tanimura, Y. Koyama

    Materials Research Society Symposium Proceedings   980  2007.04  [Refereed]

  • Doping Effect on Ferroelectric Microstructure in YMn1-xTixO3

    S. Mori, J. Tokunaga, Y. Horibe, T. Asada, Y. Koyama, T. Katsufuji

    Ferroelectrics   348   170  2007.03  [Refereed]

  • Domain-structure relaxation in the tetragonal-to-orthorhombic phase transition of the layered perovskite Sr1.8La0.2Mn1-yFeyO4

    W. Norimatsu, Y. Koyama

    Physical Review B / The American Physical Society   75 ( 10 ) 104416  2007.03  [Refereed]

  • Ferroelectric Domain Structures Near the MPB in (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3

    T. Asada, T. Shibata, Y. Koyama

    Ferroelectrics   347   223  2007.02  [Refereed]

  • The role of antiphase boundaries in the kinetic process of the L1(2) -&gt; D0(22) structural change of an Ni3Al0.45V0.50 alloy

    Makoto Tanimura, Yasumasa Koyama

    ACTA MATERIALIA   54 ( 16 ) 4385 - 4391  2006.09  [Refereed]

     View Summary

    The influence of antiphase boundaries (APBs) on the kinetic process of the L1(2) -&gt; D0(22) structural change in an Ni3Al0.45V0.50 alloy has been investigated by transmission electron microscopy. The structural change includes two kinetic factors, i.e., the periodic introduction of antiphase-shifted planes (APSPs) in the L1(2) structure and V diffusion in the L1(2) matrix. Our results revealed that APB migration was accompanied by V diffusion as if to sweep the V atoms out of the antiphase domains (APDs). It was also found that the APBs served as the nucleation sites of the D0(22) regions, because the combination of the migrated APBs produced the APSPs in the L1(2) structure. On this basis, it is proposed that the APBs bring together the two kinetic factors in the process of the L1(2) -&gt; D0(22) structural change, the physical origin of which is the variation in the charge density distribution of the APDs due to APB migration. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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  • Characteristic features of the Fe7Mo6-type structure in a transition-metal alloy examined using transmission electron microscopy

    A. Hirata, A. Iwai, Y. Koyama

    Physical Review B / The American Physical Society   74 ( 5 ) 54204  2006.08  [Refereed]

  • Evolution of orthorhombic domain structures during the tetragonal-to-orthorhombic phase transition in the layered perovskite Sr2-xLaxMnO4

    W. Norimatsu, Y. Koyama

    Physical Review B / The American Physical Society   74 ( 8 ) 85113  2006.08  [Refereed]

  • Influence of Si addition on microstructure evolution of Ni3Al0.50V0.50 alloy

    Makoto Tanimura, Kei-ichi Kondo, Toshihiro Doi, Yasumasa Koyama

    MATERIALS TRANSACTIONS   47 ( 7 ) 1755 - 1761  2006.07  [Refereed]

     View Summary

    The microstructure evolution related to the L1(2) (supersaturated Ni3Al1-xVx) -&gt; L1(2) (Ni3Al)+D0(22) (Ni3V) phase separation of the Ni3Al0.50V0.50 alloy, with the addition of a small amount of Si (less than 0.2 mol%), was examined by transmission electron microscopy. The data showed that the L1(2) single phase was formed via the appearance and subsequent disappearance of the D0(22) regions under an isothermally holding process, as in the case of the Ni3Al0.50V0.50 alloy without Si. A striking feature of the microstructure evolution of the present alloy was that the formation of the final L1(2) single phase was accomplished within an extremely brief period of less than 10 min. Such a brief period indicated that the microstructure evolution was not accompanied by atomic diffusion, which was quite different from the case of the Si-free alloy. On this basis, we propose that the Si addition induced a pinning effect on atomic migration (i.e., suppression of long-range atomic diffusion) in the supersaturated L1(2) matrix and thus accelerated the formation of the final L1(2) single phase.

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  • 圧電性酸化物Pb(Zr1-xTix)O3における強誘電単斜晶相の分域構造

    T. Asada, Y. Koyama

    熱処理   45 ( 6 ) 337 - 338  2005.12

  • Microstructure evolution in the phase separation of the Ni3Al0.40V0.60 alloy

    M Tanimura, T Doi, Y Koyama

    MATERIALS TRANSACTIONS   46 ( 12 ) 2988 - 2996  2005.12  [Refereed]

     View Summary

    It has been found that phase separation, i.e., D0(22) precipitation in the supersaturated Ll(2) matfix to form the Ll(2) + D0(22) equilibrium state, stagnates in the Ni3Al0.52V0.48 alloy under a certain thermodynamic condition. This stagnation originates from the suppression of the long-range vanadium diffusion in the Ll(2) matrix, so-called diffusion blocking. Because diffusion blocking is inherent to the Ll(2) structure, its occurrence depends largely on the morphological features of the Ll(2) matrix. In this study, the microstructure evolution during the phase separation of the Ni3Al0.40V0.60 alloy was examined from the viewpoint of the relation between the variation in the initial microstructure and the appearance of the effect of diffusion blocking. Our results showed that cuboidal domains of about 30 nm on a side formed in the initial Ll(2) matrix and that the D0(22), regions appeared at the domain boundaries. The microstructure evolution of the alloy was found to proceed via the rearrangement, combination, and growth of these D0(22), regions, accompanying vanadium migration of about 20 nm in the Ll(2) matrix. The requisite migration length was determined by the size and density of the initial D0(22) regions, which depend on the size of the Ll(2), cuboidal domains. A shorter migration length than that in the case of the Ni3Al0.52V0.48 alloy was presumed to be advantageous to concealing the effect of diffusion blocking. On that basis, it was concluded that the occurrence of the phase separation in the Ni3Al0.40V0.60 alloy was attributed to the reduction in the size of the cuboidal domains in the initial Ll(2) matrix.

  • Microstructures associated with dielectric and magnetic properties in (1-x)BiFeO3-xBaTiO(3)

    Y Horibe, M Nakayama, Y Hosokoshi, T Asaka, Y Matsui, T Asada, Y Koyama, S Mori

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   44 ( 9B ) 7148 - 7150  2005.09  [Refereed]

     View Summary

    We have investigated microstructures associated with the magnetic and dielectric properties of multiferroic mixed-perovskite compounds of the formula (1 - x)BiFeO3-xBaTiO(3) by transmission electron microscopy (TEM) combined with conventional dielectric and magnetic measurements. TEM observation revealed that there exist complicated ferroelectric (FE) domains 20-30 nm in size in the FE phase of BiFeO3. It was also found that, in an x = 0.25 compound, a weak spontaneous magnetization appears in the FE phase at 20K. From the results of the X-ray and electron diffraction experiments, it is suggested that this weak spontaneous magnetization is closely associated with the decrease in the extent of rhombohedral distortion by a partial substitution of BaTiO3 for BiFeO3.

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  • Diffusion kinetics originating a bifurcation of the final states in the phase separation of the Ni3Al1-xVx alloy

    Makoto Tanimura, Yasumasa Koyama

    JOURNAL OF PHYSICS: CONDENSED MATTER   17 ( 32 ) 4911 - 4924  2005.08  [Refereed]

  • Doping effect on the charge ordering in AlV2O4

    Y. Horibe, K. Kurushima, S. Mori, T. Asada, Y. Koyama, M. Shingu, T. Katsufuji

    Physical Review B / The American Physical Society   71 ( 5 ) 52411  2005.02  [Refereed]

  • Charge ordered microdomains in Cr doped AlV2O4

    K. Kurushima, Y. Horibe, S. Mori, T. Asada, Y. Koyama, M. Shingu, T. Katsufuji

    Transactions of the Materials Research Society of Japan   30 ( 1 ) 101 - 104  2005

  • Ni3Al-Ni3V系合金におけるL12+D022相分離に伴う組織形成

    熱処理   45   199  2005

  • Ni3Al-Ni3V系合金におけるL12+D022相分離に伴う組織形成

    M. Tanimura, T. Doi, Y. Koyama

    熱処理   45 ( 4 ) 199 - 200  2005

  • Crystallographic features in lightly-doped La1-xSrxMnO3

    M. Arao, T. Ueno, T. Asada, Y. Koyama, C. Minamisawa, M. Mogi, Y. Inoue

    Physica C: Superconductivity / Elsevier B.V.   412-414 ( 1 ) 171  2004.10  [Refereed]

  • Disordered atomic column state in Fe-Mo alloys

    A. Hirata, Y. Koyama

    Physical Review B / The American Physical Society   70 ( 13 ) 134203  2004.10  [Refereed]

  • Kinetic process of the phase separation in the alloy Ni3Al0.52V0.48

    Makoto Tanimura, Akihiko Hirata, Yasumasa Koyama

    Physical Review B / The American Physical Society   70 ( 9 ) 94111  2004.09  [Refereed]

  • Coexistence of ferroelectricity and antiferroelectricity in lead zirconate titanate

    T. Asada, Y. Koyama

    Physical Review B / The American Physical Society   70 ( 10 ) 104105  2004.09  [Refereed]

  • La-induced conversion between the ferroelectric and antiferroelectric incommensurate phases in Pb1-xLax(Zr1-yTiy)O3

    T. Asada, Y. Koyama

    Physical Review B / The American Physical Society   69 ( 10 ) 104108  2004.03  [Refereed]

  • Crystallographic features of ferroelectric domain structures in (Sr1-xBax)Bi2Ta2O9

    T Asada, T Sugawara, Y Koyama

    INTEGRATED FERROELECTRICS   62   229 - 232  2004  [Refereed]

     View Summary

    The crystallographic features of the ferroelectric domain structures in (Sr1-xBax)Bi2Ta2O9 single crystals have been examined by transmission electron microscopy. The unique 90degrees domain structures were found in these single crystals. In SrBi2Ta2O9 with x = 0, particularly the majority domains with the +/-[110](t) polarization vectors in the tetragonal notation are divided by the minority ones with the +/-[1(1) over bar 0](t) vectors, which were observed as a curved-line contrast in dark field images.

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  • Crystallographic correspondence between the bcc and C14-type structures in the Fe-Ti alloy system

    A. Hirata, Y. Koyama

    Mater. Sci. Eng. A   374   72  2004

  • Features of structural phase transition in La1-xSrFeO3

    M Mogi, Y Inoue, M Arao, Y Koyama

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   392 ( 1 ) 295 - 299  2003.10  [Refereed]

     View Summary

    The structural phase transition in the insulating phase of La1-xSrxFeO3 has been examined by X-ray powder diffraction. Based on the Rietveld analysis of the obtained profiles, the R $(3) over bar $c-Pbnm structural transition was confirmed to take place in this oxide system. The interesting feature of the transition is that it is characterized by the appearance of the antiferroelectric displacement of the La/Sr atom, in addition to the change in the rotational-displacement pattern, R-25(x) + R-25(y) + R-25(z) --&gt; R-25(x) + R-25(y) + M-3(z). It is thus understood that the low-temperature Pbnm phase can be identified as an antiferroelectric phase in oxides having a simple perovskite structure. (C) 2003 Elsevier B.V. All rights reserved.

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  • Crystallographic features of RTiO3 (R : rare-earth ion)

    M Arao, Y Koyama

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   392 ( 1 ) 300 - 305  2003.10  [Refereed]

     View Summary

    The crystallographic features of the simple perovskite oxides RTiO3 with R = La, Nd, Sm and Y, which are known to be correlated-electron insulators, have been examined mainly at room temperature by transmission electron microscopy. Electron diffraction patterns indicated that the oxides with R = La and Nd have the space group Pbnm. In electron diffraction patterns of SmTiO3 and YTiO3, on the other hand, the twin splitting of reflections due to the Pbnm structure is seen along one of the (100)(c) directions, where the subscript c denotes the cubic Brillouin zone. That is, it is understood that their crystal system is lowered from an orthorhombic one to a monoclinic one. The analysis of the patterns also suggested that the Jahn-Teller atomic displacement with q = (1/2 1/2 0). should be involved in the crystal structure for R = Sm and Y, in addition to the full symmetric Jahn-Teller lattice distortion in the structure for R = La and Nd. (C) 2003 Elsevier B.V. All rights reserved.

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  • Appearance of the icosahedral atomic cluster in the formation of the MgCu2-type structure from the bcc structure in Ti-Cr alloys

    A. Hirata, Y. Koyama, M. Tanimura

    Physical Review B / The American Physical Society   67 ( 14 ) 144107  2003.04  [Refereed]

  • Features of the incommensurate phase in Pb(Zr1-xTix)O3

    S. Watanabe, Y. Koyama

    Physical Review B / The American Physical Society   66 ( 13 ) 134102  2002.10  [Refereed]

  • Crystallographic features of La1-xSrxMO3 with M = Cr and Fe in the vicinity of the R(3)over-bar-c/Pbnm phase boundary

    M Arao, H Shindo, T Asada, Y Koyama

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   378 ( 1 ) 137 - 141  2002.10  [Refereed]

     View Summary

    Crystallographic features of La1-xSrxMO3 with M = Cr and Fe in the vicinity of the R3c/Pbnm phase boundary have been examined mainly at room temperature by transmission electron microscopy. When the Sr content x increases from x = 0, the change in crystal structure from Pbnm to R (3) over barc was confirmed to occur at room temperature in La1-xSrxCrO3 around x = 0.2. In La1-xSrxFeO3, On the other hand, the incommensurate phase was found to exist between the Pbnm and the R (3) over barc phases. The analysis of the electron diffraction patterns obtained in the work indicated that the normal structure of the incommensurate structure and the direction of the modulation could, respectively, be identified as the Pbnm structure and one of the &lt;323&gt; directions. It was also suggested that the rotational displacement Of the oxygen octahedra involved in the Pbnm structure should be modulated in the incommensurate structure. (C) 2002 Elsevier Science B.V. All rights reserved.

  • Pattern bifurcation in the pseudobinary Ni-3(Al, V) alloy: evidence,of spontaneous L1(2) single crystallization

    M Tanimura, M Kikuchi, Y Koyama

    JOURNAL OF PHYSICS-CONDENSED MATTER   14 ( 29 ) 7053 - 7064  2002.07  [Refereed]

     View Summary

    The structural change between substitution-type ordered structures basically includes the competition of pairwise interactions, which can describe the stability of such structures. The purpose of this study is to. investigate the frustration effect resulting from competition related to the L1(2) --&gt; D0(22) structural change in the Ni3Al0.52V0.48 alloy and to discuss the pattern bifurcation in the pseudobinary Ni-3(Al, V) alloy. The present data obtained by transmission electron microscopy show that the change in microstructures during the structural change is characterized by the appearance and the subsequent annihilation of D0(22) regions in the L1(2) matrix. The final product of the structural change is the L1(2) single phase including topological defects, which is different from the equilibrium state known as the L1(2) + D0(22) checkerboard pattern. This indicates that the suppression of the L1(2) --&gt; D0(22) structural change occurs in this alloy. Such suppression can be explained as being due to an inability to accommodate the strain field in the vicinity of the L1(2)/D0(22) boundaries when D0(22) precipitation occurs. The trigger of this inability is thought to be strong frustration originating from a small concentration deviation in the initial L1(2) matrix from the stoichiometry of the L1(2) structure. It is concluded that the formation of the L1(2) single phase as the final state, i.e. spontaneous L1(2) single crystallization, results from an effort by the alloy to accommodate the strain field, the occurrence of which is related to the frustration effect appearing during the L1(2) --&gt; D0(22) structural change.

  • Formation of isomorphic Ir3+ and Ir4+ octamers and spin dimerization in the spinel CuIr2S4

    PG Radaelli, Y Horibe, MJ Gutmann, H Ishibashi, CH Chen, RM Ibberson, Y Koyama, YS Hor, Kiryukhin, V, SW Cheong

    NATURE   416 ( 6877 ) 155 - 158  2002.03  [Refereed]

     View Summary

    Inorganic compounds with the AB(2)X(4) spinel structure have been studied for many years, because of their unusual physical properties. The spinel crystallographic structure, first solved by Bragg in 1915(1), has cations occupying both tetrahedral (A) and octahedral (B) sites. Interesting physics arises when the B-site cations become mixed in valence. Magnetite (Fe3O4) is a classic and still unresolved example, where the tendency to form ordered arrays of Fe2+ and Fe3+ ions competes with the topological frustration of the B-site network(2). The CuIr2S4 thiospinel is another example, well known for the presence of a metal-insulator transition at 230 K with an abrupt decrease of the electrical conductivity on cooling accompanied by the loss of localized magnetic moments(3-7). Here, we report the determination of the crystallographic structure of CuIr2S4 below the metal-insulator transition. Our results indicate that CuIr2S4 undergoes a simultaneous charge-ordering and spin-dimerization transition-a rare phenomenon in three-dimensional compounds. Remarkably, the charge-ordering pattern consists of isomorphic octamers of Ir83+S24 and Ir84+S24 (as isovalent bi-capped hexagonal rings). This extraordinary arrangement leads to an elegant description of the spinel structure, but represents an increase in complexity with respect to all the known charge-ordered structures, which are typically based on stripes, slabs or chequerboard patterns.

  • Commensuration of the antiferroelectric incommensurate phase in Pb(Co1/2W1/2)O3

    S. Watanabe, Y. Koyama

    Physical Review B / The American Physical Society   65 ( 6 ) 64108  2002.02  [Refereed]

  • New Chemical Layered Structure in Ti-Cr alloys

    Mater, Trans.   43   1689  2002  [Refereed]

  • Features of the Jahn-Teller displacements in LaTiO3

    M. Arao, Y. Inoue, Y. Koyama

    Journal of Physics and Chemistry of Solids   63 ( 6-8 ) 995 - 997  2002

     View Summary

    Atomic displacements involved in a crystal structure of LaTiO3 have been investigated by means of transmission electron microscopy mainly at room temperature. Electron diffraction patterns taken from LaTiO3 were found to be characterized by the presence of the forbidden reflections for the Pnma structure. The extinction rule of these reflections including fundamental ones indicated that a space group of the crystal structure was determined to be P21/a with a monoclinic symmetry. A characteristic feature of the P21/a structure is that there exist the Jahn-Teller atomic displacement with q = 〈1/21/2 0〉c as well as the Jahn-Teller distortion. An additional displacement with q = 〈1/2 0 0〉c should also be needed. © 2002 Elsevier Science Ltd. All rights reserved.

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    13
    Citation
    (Scopus)
  • Roles of ferroelectricity, antiferroelectricity, and rotational displacement in the ferroelectric incommensurate phase of Pb(Zr1-xTix)O3

    S. Watanabe, Y. Koyama

    Physical Review B / The American Physical Society   63 ( 13 ) 134103  2001.04  [Refereed]

  • Change in Al concentration profile related to the D0(19)-&gt; L1(0) structural change in Ti-Al alloys

    M Tanimura, Y Inoue, Y Koyama

    SCRIPTA MATERIALIA   44 ( 2 ) 365 - 373  2001.02  [Refereed]

  • Pb(Zr1-xTix)O3における不整合相の分域構造

    熱処理   41   289  2001

  • Nanometer-size polycrystallization in bcc -> hcp+C15 structural change of a Ti-30 mol % Cr alloy

    Mat. Trans.   42   2553  2001  [Refereed]

  • Features of the rhombohedral-to-orthorhombic structural phase transition in La1-xSrxMnO3

    M Arao, Y Koyama, Y Inoue, Y Moritomo

    PHYSICAL REVIEW B   62 ( 9 ) 5399 - 5405  2000.09  [Refereed]

     View Summary

    The R (3) over bar c-to-Pbnm structural phase transition in La1-xSrxMnO3, which is characterized by changes in the rotational displacement of the MnO6 octahedra and in the crystal system, has been investigated by transmission electron microscopy. The experimental data indicate that the former change results in the appearance of the antiphase boundary and the latter produces the {110} and {112} twin structures. In addition, the structural phase transition can be reproduced on the basis of the Landau theory, in which a normal structure and an order parameter no, respectively, assumed to be the Pm3m structure and normal coordinates of the rotational displacements.

  • Direct observation of dynamic local structure in La2-xSrxCuO4 around x=0.12

    Y Horibe, Y Inoue, Y Koyama

    PHYSICAL REVIEW B   61 ( 18 ) 11922 - 11927  2000.05  [Refereed]

     View Summary

    In situ observation of the dynamic local structure in La2-xSrxCuO4 has been conducted at lower temperatures by transmission electron microscopy. The observation clearly showed that around x=0.12 dynamic fluctuation of the local Pccn/LTT-tilt region in the interior of the LTO domain occurs, even at 12 K, while Pccn/LTT regions nucleated along the twin boundary are rather static. The dynamic local Pccn/LTT region is thought to play a crucial role in the slight suppression of T-C and in the appearance of the incommensurate magnetic structure, both of which have been reported to occur around x=0.12.

  • Pb(Zr1-xTix)O3における強誘電不整合相の結晶学的特徴

    固体物理   Vol.35, 881  2000

  • 強相関電子系酸化物における構造相転移と分域構造

    熱処理技術協会誌/日本熱処理技術協会   39 ( 6 ) 273  1999.12

  • Imcomplete sructural phase transition and lattice destruction in Sr1-xlaxTio3+δ

    Journal of Applied Physics/American Institute of Physics   86 ( 5 ) 2759  1999.09  [Refereed]

  • 高温超伝導体における構造ゆらぎ—誘電体、合金と対比して—

    電子顕微鏡/日本電子顕微鏡学会   34 ( 2 ) 76  1999.07

  • 低温構造相転移と超伝導

    化学工業/化学工業社   50 ( 4 ) 266  1999.04

  • LOCAL STRUCTURAL FLUCTUATION IN THE SUPERCONDUCTING STATE OF La1-xSrxCuO4

    Transactions of the Materials Research Society of Japan/日本MRS   24   133  1999

  • Chemical composition of the fine alpha(2)+gamma lamellar structure in a Ti-44at.%Al alloy

    M Tanimura, Y Inoue, Y Koyama

    SCRIPTA MATERIALIA   39 ( 7 ) 907 - 913  1998.09  [Refereed]

  • Modulated structures in La2-xSrxMO4(M=Ni,Mn)

    Advances in Superconductivity   10   227 - 230  1998

  • La系酸化物超伝導体における低温構造相転移のその場観察

    熱処理   38;3, p127-128  1998

  • La系酸化物超伝導体における低温構造相転移での微細組織

    日本金属学会会報   37 ( 12 ) 1012  1998

  • Features of crystal structures in Sr1-xLaxVO3

    Advances in Superconductivity   10, p223-226  1998

  • Features of crystal structures in Sr1-xLaxVO3

    Advances in Superconductivity   10, p219-222  1998

  • Mechanism of α→α2+γ phase separation in hypo-eutectoid Ti-37 at% Al alloy

    Materials Trans. JIM/The Japan Instutute of Metals   39 ( 1 ) 57  1998.01  [Refereed]

  • Evolution of low-temperature phases in a low-temperature structural transition of a La cuprate

    Y Inoue, Y Horibe, Y Koyama

    PHYSICAL REVIEW B   56 ( 21 ) 14176 - 14179  1997.12  [Refereed]

     View Summary

    The microstructure produced by a low-temperature structural phase transition in La1.5Nd0.4Sr0.1CuO4 has been examined by transmission electron microscopy with the help of imaging plates. The low-temperature transition was found to be proceeded not only by the growth of the Pccn/low-temperature-tetragonal phases nucleated along the twin boundary but also by the nucleation and growth of the phases in the interior of the low-temperature-orthorhombic domain. In addition, because the map of the octahedron tilt as an order parameter is not identical to that of the spontaneous strain accompanied by the transition, the microstructure below the transition is-understood to be a very complex mixture of the low-temperature phases.

  • La系酸化物における低温構造相転移と超伝導

    日本金属学会誌/日本金属学会   61;9(1006)  1997.09

  • Microstructure and Tc-suppression in La-cuprates

    Y. Horibe, Y. Inoue, Y. Koyama

    Physica C: Superconductivity / Elsevier B.V.   282-287 ( 2 ) 1071 - 1072  1997.08  [Refereed]

  • Features of crystal structures in the correlated electron system Sr1-xLaxTiO3

    M. Arao, F. Munakata, Y. Koyama

    Physica C: Superconductivity / Elsevier B.V.   282-287 ( 2 ) 1111 - 1112  1997.08  [Refereed]

  • New aspect of low-temperature phase transitions of La-cuprates

    J.Superconductivity/Plenum Press   10;4(461)  1997.08  [Refereed]

  • Characteristic features of low-temperature structural phase transitions in La2-xSrxCuO4

    J.Superconductivity/Plenum Press   10;4(361)  1997.08  [Refereed]

  • Electronic state and valence control of LaCoO3: Difference between La-deficient and Sr-substituting effects

    F Munakata, H Takahashi, Y Akimune, Y Shichi, M Tanimura, Y Inoue, R Itti, Y Koyama

    PHYSICAL REVIEW B   56 ( 3 ) 979 - 982  1997.07  [Refereed]

     View Summary

    The difference between La-deficient and Sr-substituting effects for La in LaCoO3 has been investigated by using x-ray photoelectron spectroscopy (XPS). On the basis of the relationship between the intensity of the T-2(2) line and the amount of low-spin Co3+ ions in these compounds, it is pointed out that La0.9CoO3-delta is in a highly covalent low-spin state and La0.8Sr0.2CoO3-delta is in a mixed spin state. From the Co 2p and valence-band XPS results, it is pointed out that the electronic state of La0.9CoO3-delta is quite different from that of Sr-substituting La0.8Sr0.2CoO3-delta. In order to explain the difference between La-deficiency and Sr-substitution effects in valence controlling of LaCoO3, it is considered that Sr substitution produces the Co 3d holes and La deficiency yields the O 2p holes.

  • Two-dimensional character of an amorphous state in a eutectiod reaction of a Ti-40 at% Al alloy.

    Physical Review B / The American Physical Society   55 ( 21 ) 14063  1997.06  [Refereed]

  • Low-temperature transition in barium sodium niobate Ba2NaNb5O15

    S Mori, N Yamamoto, Y Koyama, Y Uesu

    PHYSICAL REVIEW B   55 ( 17 ) 11212 - 11217  1997.05  [Refereed]

     View Summary

    In order to examine a low-temperature transition in barium sodium niobate Ba2NaNb5O15 (BSN), an in situ observation was made in the temperature range between room temperature and 25 K by a transmission electron microscope. It was revealed that satellite reflection spots characterizing the incommensurate structure, which appear around 573 K, do not disappear even around 25 K. On the other hand, it was found that microdomains elongating along the [010] direction with a width of about 10 nm appear around 200 K, together with the appearance of both new satellite spots at the (h,k,l+1/2) points with h,k,l integers and new faint diffuse streaks through the (h,k+ 1/2, l+1/2) points along the [100] direction. It is concluded that the low-temperature transition in BSN is a transition from the Iq quasicommensurate state to the 2q one in which two modulated waves arise along the [100] and [010] directions.

  • Nature of incommensurate phases in real systems

    Ferroelectrics/Gordon and Breach Science Publishers   vol.203(389)  1997  [Refereed]

  • Microstructure related to a low-temperature transition in Ba2NaNb5O15

    Ferroelectrics/Gordon and Breach Science Publishers   vol.190(13)  1997  [Refereed]

  • Microstructure in low-temperture structural phase transitions of La1.60-xSrxNd0.40CuO4

    Advances in Superconductivity Ⅸ/Springer-Verlag   -281  1997

  • Crystallographic features of Sr3-xCaxRu2Oy

    Advances in Superconductivity Ⅸ/Springer-Verlag   -265  1997

  • Antiphase boundaries, inversion, and ferroelastic domains in the striped-type superstructure of gamma-brass Cu-Al alloys

    Y Koyama, M Hatano, M Tanimura

    PHYSICAL REVIEW B   53 ( 17 ) 11462 - 11468  1996.05  [Refereed]

     View Summary

    Features of the striped-type superstructure in gamma-brass Cu-Al alloys have been investigated mainly by transmission electron microscopy in order to understand the role of a rhombohedral distortion in its stability. From dark field images taken from alloys exhibiting the superstructure, it was found that there exist two types of ferroelastic rhombohedral domains, which are in a twin relation. In other words, the superstructure is characterized by a periodic array of the ferroelastic domains as well as inversion antiphase boundaries. Because charge density waves should be responsible for the formation of the superstructure, as suggested in our previous paper [Phys. Rev. B 40, 5378 (1989)], the rhombohedral distortion must play a crucial role in their appearance. Presumably the distortion enlarges parallel portions of the Fermi surface. On the basis of the present experimental data, the interplay between the striped-type superstructure and the rhombohedral gamma-brass structure is also discussed.

  • Modulated Structure in Al65Cu20Fe15-xMnx Alloys,Studied by Transmission Electron Microscopy

    Materials Transaction JIM/日本金属学会   37;5  1996.05  [Refereed]

  • Change in Microstructure during Aging at 1273K in Ti-40at.% Al alloy

    Materials Transaction JIM/日本金属学会   37;5  1996.05  [Refereed]

  • 日本金属学会功績賞

    日本金属学会    1996

  • Low-Temperature Structural Transitions and Tc-suppression in La2-x-yBaxNdyCuO4

    Advances in Superconductivity VIII/Spring-Verlag    1996

  • Features of Microstructure in Sr2RuO4

    Advances in Superconductivity VIII/Spring-Verlag    1996

  • Features of a eutectoid reaction in a Ti-40 at% Al alloy: Evidence for an amorphous-state formation from a crystal

    M Tanimura, Y Inoue, Y Koyama

    PHYSICAL REVIEW B   52 ( 21 ) 15239 - 15243  1995.12  [Refereed]

     View Summary

    In an initial stage of a eutectoid reaction of a Ti-40 at. % Al alloy, hcp--&gt;D0(19)+L1(0), an amorphous state was found to form around an intermediate precipitate with the B-19 structure, together with a change in crystal structure of the matrix from the hcp structure to the D0(19) one. Characteristic features of the amorphous state are that the destruction of the crystal lattice mainly takes place in the (001) plane of the D0(19) structure but is very slightly extended along the [001] direction. The shape of the amorphous region is just like a thin film. On the basis of the present data, the origin of the two-dimensional-like character of the lattice destruction is simply discussed.

  • ABSENCE OF INTERACTION BETWEEN FERROELECTRIC AND INCOMMENSURATE TRANSITIONS IN BA2NANB5(1-X)TA5XO15

    S MORI, N YAMAMOTO, Y KOYAMA, S HAMAZAKI, M TAKASHIGE

    PHYSICAL REVIEW B   52 ( 13 ) 9117 - 9120  1995.10  [Refereed]

     View Summary

    Features of both the ferroelectric and incommensurate transitions in Ba2NaNb5(1-x)Ta5xO15 were investigated by means of a transmission electron microscope in order to find the relationship between these transitions. An in situ observation of the transitions revealed that the ferroelectric transition takes place around 373 K in x=0.57 while the incommensurate transition occurs around 543 K. Because the ferroelectric transition does not basically accompany a change in microstructures related to the incommensurate lattice modulation, there is no interaction between these two transitions.

  • EVOLUTION OF A FERROELASTIC DOMAIN-STRUCTURE IN AN INCOMMENSURATE PHASE OF BARIUM SODIUM NIOBATE (BA2NANB5O15)

    S MORI, N YAMAMOTO, Y KOYAMA, Y UESU

    PHYSICAL REVIEW B   52 ( 9 ) 6158 - 6161  1995.09  [Refereed]

     View Summary

    Features of a ferroelastic domain structure in an incommensurate phase of barium sodium niobate [Ba2NaNb2O15 (BSN)] that appear in the cooling process were investigated by means of a transmission electron microscope. The in situ observation revealed that there exists an abrupt change in the domain structure around 503 K. The ferroelastic domain structure above 503 K basically consists of two types of Iq ferroelastic microdomains with a size of about 20 nm while below it large ferroelastic domains with flat domain boundaries are formed.

  • FEATURES OF INCOMMENSURATE PHASES IN K2SEO4, K2ZNCL4, AND RB2ZNCL4

    Y KOYAMA, T NAGATA, K KOIKE

    PHYSICAL REVIEW B   51 ( 18 ) 12157 - 12164  1995.05  [Refereed]

  • LOW-TEMPERATURE STRUCTURAL TRANSITIONS AND T-C SUPPRESSION IN LA(2-X)M(X)CUO(4) (M=BA, SR)

    Y KOYAMA, Y WAKABAYASHI, K ITO, Y INOUE

    PHYSICAL REVIEW B   51 ( 14 ) 9045 - 9051  1995.04  [Refereed]

  • Memory effect in an incommensurate phase of barium sodium niobate

    Ferroelectrics/Gordon and Breach Science Publishers   169  1995

  • MEMORY EFFECT, DEFECT DENSITY WAVE, AND RELATED MICRODOMAIN STRUCTURE IN AN INCOMMENSURATE PHASE OF BARIUM SODIUM NIOBATE

    S MORI, N YAMAMOTO, Y KOYAMA, Y UESU

    PHYSICAL REVIEW B   51 ( 1 ) 73 - 76  1995.01  [Refereed]

  • Structural phase transitions in La1.885Sr0.115CuO4

    Y. Koyama, Y. Wakabayashi, Y. Inoue

    Physica C: Superconductivity / Elsevier B.V.   235-240 ( 2 ) 833 - 834  1994.12  [Refereed]

  • Crystallographic features of a low-temperature- tetragonal structure in La2-xBaxCuO4

    Y. Inoue, Y. Wakabayashi, K. Ito, Y. Koyama

    Physica C: Superconductivity / Elsevier B.V.   235-240 ( 2 ) 835 - 836  1994.12  [Refereed]

  • Features of structural phase transitions in Ca2-xSmxMnO4

    H. Saito, Y. Inoue, F. Munakata, M. Yamanaka, J. Takahashi, N. Kamegashira, Y. Koyama

    Physica C: Superconductivity / Elsevier B.V.   235-240 ( 2 ) 753 - 754  1994.12  [Refereed]

  • バリウム-ソジュウム-ナイオベイトにおける不整合構造の特徴

      155/,293  1994

  • GINZBURG-LANDAU THEORY OF AN INCOMMENSURATE-TO-COMMENSURATE TRANSITION IN BARIUM SODIUM NIOBATE

    S MORI, Y KOYAMA, Y UESU

    PHYSICAL REVIEW B   49 ( 1 ) 621 - 624  1994.01  [Refereed]

     View Summary

    Features of an incommensurate-to-commensurate transition in barium sodium niobate, which is obtained in a cooling process from a tetragonal phase, have been theoretically examined on the basis of the Ginzburg-Landau theory. The present theory which takes into account the contribution of higher-order distortion waves produced from higher-order harmonics via the umklapp process, well reproduces an experimentally obtained change in an incommensurability delta on cooling. The theory also predicts both a discommensuration with a phase slip of 2pi/4 and the incommensurate-to-commensurate transition.

  • STRUCTURAL TRANSITIONS AND RELATED FLUCTUATIONS IN LA1.875-XSMXSR0.125CUO4

    Y KOYAMA, Y WAKABAYASHI, SI NAKAMURA, Y INOUE, K SHINOHARA

    PHYSICAL REVIEW B   48 ( 13 ) 9710 - 9716  1993.10  [Refereed]

     View Summary

    Features of both structural transitions and related fluctuations in La1.875-xSmxSr0.125CuO4 have been examined by means of electron diffraction in a temperature range between about 600 K and about 20 K. As the temperature is lowered, a structural phase transition from a high-temperature tetragonal (HTT) structure to a low-temperature orthorhombic (LTO) one takes place in 0 less-than-or-equal-to x less-than-or-equal-to 0.2 and HTT --&gt; LTO --&gt; Pccn --&gt; LTT successive transitions in 0.3 less-than-or-equal-to x less-than-or-equal-to 0.4 where the Pccn and low-temperature tetragonal (LTT) structures are characterized as a rotation of an oxygen octahedron about two [110] directions. An enhancement of a structural fluctuation in the LTO phase, which indicates a soft-phonon mode in the LTO-to-Pccn transition, occurs on cooling even in 0 less-than-or-equal-to x less-than-or-equal-to 0.2 as well as in 0.3 less-than-or-equal-to x less-than-or-equal-to 0.4. Because an increase in the Sm content results in two effects, a lowering of T(c) and an enhancement of the structural fluctuation, the fluctuation is understood to suppress the superconductivity. Eventually, a dip in a composition dependence of T(c) in La-Sr-Cu-O, which was experimentally found by Oda et al. [J. Phys. Soc. Jpn. 58, 1137 (1989)], is concluded to be due to the structural fluctuation related to the LTO-to-Pccn transition.

  • DIFFUSE-SCATTERING IN ND-CE-SR-CU-O

    Y INOUE, M HASEGAWA, F MUNAKATA, M YAMANAKA, Y KOYAMA

    PHASE TRANSITIONS   41 ( 1-4 ) 231 - 236  1993  [Refereed]

     View Summary

    Detailed features of the structural disorder in Nd-Ce-Sr-Cu-O with the T*-structure have been investigated by diffuse scattering in electron diffraction. There exist three types of characteristic diffuse scatterings: a cylindrical-shape, a spike-shape, and spots in addition to the fundamental spots due to the T*-structure. The cylindrical-shape and spike-shape scatterings can be concluded to result from both atomic displacements of O(2) atoms along the [110] directions, on planes through the apical positions of the CuO5 pyramids, and the ordering of Nd and Sr atoms. On the other hand, the extra spots are understood to be related to displacements of O(1) or O(3) atoms along the c axis.

  • DIFFUSE-SCATTERING IN LA-SM-SR-CU-O

    T IKEDA, Y KOYAMA, Y INOUE

    PHASE TRANSITIONS   41 ( 1-4 ) 237 - 241  1993  [Refereed]

     View Summary

    Diffuse scattering, reflecting structural disorder in La1.8-xSmxSr0.2CuOz, have been investigated by means of electron diffraction. In the T*-structure, in addition to the fundamental spots due to the T*-structure there exists cylindrical shape diffuse scattering due to static displacements of the apical oxygen atoms in the CuO5 pyramids. On the other hand, both thermal diffuse scattering and superlattice spots have been found in diffraction patterns obtained from the T-structure. The thermal diffuse scattering is distributed on the {100}* planes of reciprocal space. In addition, the superlattice spots appear in a range of 0.3 &lt; x &lt; 0.6 and are located at a zone boundary of the first Brillouin zone along the [110]* directions. On the basis of the extinction rule for the superlattice spots, the crystal system of the T-structure for 0.3 &lt; x &lt; 0.6 is concluded to be orthorhombic while the system for 0 &lt; x &lt; 0.3 is tetragonal.

  • Structural transitions in bkb and brb

    Shu-Ichi Nakamura, Yasumasa Koyama, Manabu Ishimaru

    Phase Transitions   41 ( 1-4 ) 243 - 247  1993.01

     View Summary

    Features of the crystal structures at room temperature and structural transitions in both Ba1-xKxBiO3 and Ba1-xRbxBiO3 with 0.1 &lt
    ϗ ≤ 0.5 have been examined in detail by means of electron diffraction. From the analysis of diffraction patterns, it has been confirmed that at room temperature the crystal system changes with respect to ϗ cubic system for ϗ ≥ 0.35 and orthorhombic for 0.1 &lt
    ϗ &lt
    0.35. Furthermore, the cubic to orthorhombic structural transition with changing temperature was found to be due to the condensation of the Rx 25 and the RY 25 modes. © 1993, Taylor &amp
    Francis Group, LLC. All rights reserved.

    DOI

    Scopus

  • OBSERVATION OF A STRUCTURAL-FLUCTUATION ENHANCEMENT IN THE VICINITY OF SUPERCONDUCTING TRANSITIONS IN BA1-XMXBIO3 (M = K,RB)

    Y KOYAMA, SI NAKAMURA, Y INOUE

    PHYSICAL REVIEW B   46 ( 14 ) 9186 - 9189  1992.10  [Refereed]

     View Summary

    Features of structural fluctuations related to the cubic-to-orthorhombic transition in Ba1-xMxBiO3 (M = K, Rb) have been investigated in detail by means of electron diffraction. A fluctuation due to the softening of the R25 mode, which is responsible for the transition, is detected as diffuse scattering at the R point in the reciprocal space of the cubic phase. When x is reduced from x = 0.5 to a transition composition in the cubic phase, the fluctuation is enhanced remarkably at lower temperatures. Because the fluctuation of the R25 mode results in those of spontaneous strains, the fluctuations of the A1g and E(g) strains are understood to be enhanced near the superconducting transition.

  • DIRECT OBSERVATION OF A STRUCTURAL-FLUCTUATION ENHANCEMENT IN A SUPERCONDUCTING STATE OF TL2BA2CA2CU3O10 BY ELECTRON-DIFFRACTION

    Y KOYAMA, SI NAKAMURA, Y INOUE, T OHNO

    PHYSICAL REVIEW B   46 ( 9 ) 5757 - 5759  1992.09  [Refereed]

     View Summary

    Features of temperature-dependent diffuse scattering showing a structural fluctuation in Tl2Ba2Ca2Cu3O10 have been investigated in detail in a temperature range between about 20 K and room temperature by means of electron diffraction. As the temperature is lowered, the intensity of diffuse scattering increases remarkably below about 130 K, which is taken to be the onset temperature of superconductivity. Because the scattering is obviously due to a structural fluctuation, Tl2Ba2Ca2Cu3O10 is understood to exhibit an enbancement of the fluctuation in a superconducting state. In addition, a careful analysis of the features in the scattering suggests that a dynamic displacement of an apical O atom in the CuO5 pyramid along the c axis is responsible for the structural fluctuation in the superconducting state.

  • STRUCTURAL TRANSITIONS IN THE SUPERCONDUCTING OXIDES BA-PB-BI-O

    Y KOYAMA, M ISHIMARU

    PHYSICAL REVIEW B   45 ( 17 ) 9966 - 9975  1992.05  [Refereed]

     View Summary

    The crystal structures at room temperature and the structural transitions in BaPb1-xBixO3 (Ba-Pb-Bi-O) have been investigated in detail by means of electron diffraction. From an analysis of the crystal structures on the basis of electron diffraction patterns obtained experimentally, it is shown that, as the temperature is lowered, Ba-Pb-Bi-O undergoes a cubic-to-tetragonal transition resulting from the condensation of the triply degenerate R25 mode, R25x, in the lower-Bi-content range 0 less-than-or-equal-to x &lt; 0.35, and successive transitions from the cubic to the tetragonal and to a monoclinic structure in the intermediate-Bi-content range 0.35 &lt; x &lt; 0.90. In the successive transitions, the cubic-to-tetragonal transition is understood to be due to the first condensation of the R25 mode, R25x, and the tetragonal-to-monoclinic transition is due to the second condensation of the R25 mode, R25y. The soft-phonon-mode behavior of the R25 mode in the cubic and tetragonal phases was actually detected as a change in an intensity of diffuse scattering appearing at the R point in the first Brillouin zone of the cubic perovskite structure. In addition, the crystal structure at room temperature in the higher-Bi-content range 0.90 &lt; x less-than-or-equal-to 1.00 is confirmed to involve the breathing displacements resulting from the appearance of charge-density waves as well as the R25x + R25y displacements. Moreover, a simple explanation of the physical origin of the superconductivity in Ba-Pb-Bi-O is presented on the basis of the present experimental results.

  • ONE-DIMENSIONAL LONG-PERIOD SUPERLATTICES IN ALLOYS

    Y KOYAMA, S MORI

    PHYSICAL REVIEW B   44 ( 15 ) 7852 - 7862  1991.10  [Refereed]

     View Summary

    One-dimensional (1D) long-period superlattices (LPSL's) in alloys, which consist of a periodic array of antiphase boundaries in the L1(0) or L1(2) ordered structure, have been examined theoretically from the viewpoint of the appearance of charge-density waves (CDW's) in a three-dimensional system. The Fermi surface of the LPSL has flat portions along the &lt;110&gt; directions, and the Fermi-surface nesting results in two nesting vectors in the L1(0) structure and four vectors in the L1(2) Structure. This means that the 1D LPSL is not a single first-order CDW state but is characterized by the superposition of multiple CDW's. The response of the lattice system to the CDW is further understood to take place in two ways: One is the introduction of a periodic array of antiphase boundaries as the atomic arrangement, and the other is the periodic lattice distortion as the atomic displacement. Unlike low-dimensional materials such as 2H-TaSe2, the former way plays a particularly important role in the response for the LPSL's. As a result of the appearance of the CDW's, the change in the period with respect to the composition of an alloy, the electron-atom ratio, is determined by the size of the Fermi surface along the &lt;110&gt; directions and can be basically explained on the basis of the same equation as that in the Sato-Toth theory [Phys. Rev. 124, 1833 (1961)]. In addition, a Ginzburg-Landau free-energy functional is proposed in order to understand the features of an incommensurate structure, such as the temperature dependence of the period and the discommensurate structures in the LPSL's. It is assumed in the theory that the normal structure is the normal ordered structure and the order parameter is the CDW. Note that the temperature dependence of the period is closely related to the phase modulation of the first-order CDW by means of the higher-order CDW's produced from the higher-order harmonics via the umklapp process. Hence the LPSL is classified into two groups in the L1(0) structure and three groups in the L1(2) structure on the basis of the positional relation between the first- and third-order spots in diffraction patterns. The temperature dependence of the period and the phase modulation of the first-order wave are then calculated by the derived free-energy expression for each group, and are found to be in good agreement with those obtained experimentally. That is, the present theory can well reproduce overall features of the LPSL's. This means that the 1D LPSL's are concluded to be the CDW state in the three-dimensional system.

  • THEORY OF THE INCOMMENSURATE-TO-COMMENSURATE TRANSITION IN LONG-PERIOD SUPERLATTICES OF A3B-TYPE ALLOYS

    Y KOYAMA, M ISHIMARU

    PHYSICAL REVIEW B   41 ( 12 ) 8522 - 8525  1990.04  [Refereed]

  • MODULATED STRUCTURE IN BI2SR2CA1-XYXCU2OY

    Y INOUE, Y SHICHI, F MUNAKATA, M YAMANAKA, Y KOYAMA

    PHYSICAL REVIEW B   40 ( 10 ) 7307 - 7309  1989.10  [Refereed]

  • STRIPED-TYPE SUPERSTRUCTURE IN GAMMA-BRASS ALLOYS

    Y KOYAMA, J YOSHIDA, H HOSHIYA, Y NAKAMURA

    PHYSICAL REVIEW B   40 ( 8 ) 5378 - 5386  1989.09  [Refereed]

  • Soft Phonon Mpde in the high-Tc superconductor Tl-Ba-Ca-Cu-O

    Y. Koyama, H. Hoshiya

    Physical Review B / The American Physical Society   39  1989.04  [Refereed]

  • DIFFUSE-SCATTERING IN YBA2CU3-XCOXO7-Y

    Y KOYAMA, H HOSHIYA

    PHYSICAL REVIEW B   39 ( 7 ) 4764 - 4767  1989.03  [Refereed]

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Research Projects

  • Crystallographic relation between coordination-polyhedron structures involving dodecagonal atomic columns and approximant-phase structures related to the dodecagonal quasicrystal

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2015.04
    -
    2018.03
     

    Koyama Yasumasa, INOUE Yasuhide

     View Summary

    Dodecagonal atomic columns and dodecagonal structural units have been so far proposed to be involved in an atomic arrangement of the dodecagonal quasicrystal. The purpose of our study thus focuses on the crystallographic relation between the dodecagonal quasicrystal and the intermetallic-compound H, ν, and σ phases in the Mn-Si-V alloy system. Remark that the H and ν phases can be identified as approximant phases for the dodecagonal quasicrystal. Concretely, we have investigated the crystallographic features of the (σ → H) structural change and the crystallographic relation between the ν and H phases mainly by transmission electron microscopy. As a result, it was found that the crystal structures of the H and ν phases involved structural hierarchies, which were characterized by dodecagonal structural units as well as dodecagonal atomic columns. It was also proposed that the structural hierarchies should be directly associated with self-similarity in the dodecagonal quasicrystal

  • 12角形原子コラムを含む配位多面体構造および近似結晶構造の特徴

    科学研究費助成事業(早稲田大学)  科学研究費助成事業(基盤研究(C))

    Project Year :

    2015
    -
    2017
     

  • Crystallographic relation between coordination-polyhedron structures involving dodecagonal atomic columns and approximant-phase structures related to the dodecagonal quasicrystal

    Project Year :

    2015
    -
    2017
     

     View Summary

    Dodecagonal atomic columns and dodecagonal structural units have been so far proposed to be involved in an atomic arrangement of the dodecagonal quasicrystal. The purpose of our study thus focuses on the crystallographic relation between the dodecagonal quasicrystal and the intermetallic-compound H, ν, and σ phases in the Mn-Si-V alloy system. Remark that the H and ν phases can be identified as approximant phases for the dodecagonal quasicrystal. Concretely, we have investigated the crystallographic features of the (σ → H) structural change and the crystallographic relation between the ν and H phases mainly by transmission electron microscopy. As a result, it was found that the crystal structures of the H and ν phases involved structural hierarchies, which were characterized by dodecagonal structural units as well as dodecagonal atomic columns. It was also proposed that the structural hierarchies should be directly associated with self-similarity in the dodecagonal quasicrystal

  • Are higher order structures present in coordination-polyhedron structures of alloys, just like proteins?

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2006
    -
    2007
     

    KOYAMA Yasumasa, HIROTSU Yoshihiko, HIRATA Akihiko

     View Summary

    Coordination-polyhedron structures found in intermetallic compounds involving 3d-transition metals are characterized by the presence of complex coordination(CN) polyhedra such as a CN12 polyhedron. Because these polyhedra can be regarded as structural units, our motivation of this study is based on a simple question whether higher order structures are present in coordination-polyhedron structures or not, just as in the case of proteins. To get an answer of this question, in this study, we have tried to identify higher order structures in the g. and R structures of the Fe-Mo alloy system, where CN12, 14, 15, and 16 polyhedra are involved Concretely higher order structures in these two structures were determined on the basis of atomic arrangements of metastable states appearing in solid-state reactions from the bcc structures to them. The present experimental data obtained by transmission electron microscopy clearly revealed that there existed metastable states for both cases : the disordered atomic-column state and the short-range ordered state of atomic-column blocks in the μ case, and another disordered atomic-column state and the domain-structure state consisting of nanometer-sized atomic-column regions in the R case. Based on the appearance of these metastable states, we tried to determine higher order structures in the μ and R structures. As a result, four higher order structures related to a CN12 polyhedron are present in the structure, while there are two structures in the R structure, one of which is a CN16 polyhedron as a primary one. Thus the presence of these higher order structures in the μ and R structures is indicative of the fact that coordination-polyhedron structures in intermetallic compounds involving 3d-transition metals should have the hierarchy of crystal structures, which is directly associated with the formation and development of covalent bonds in a metallic system

  • Are higher order structures present in coordination-polyhedron structures of alloys, just like proteins?

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2006
    -
    2007
     

    KOYAMA Yasumasa, HIROTSU Yoshihiko, HIRATA Akihiko

     View Summary

    Coordination-polyhedron structures found in intermetallic compounds involving 3d-transition metals are characterized by the presence of complex coordination(CN) polyhedra such as a CN12 polyhedron. Because these polyhedra can be regarded as structural units, our motivation of this study is based on a simple question whether higher order structures are present in coordination-polyhedron structures or not, just as in the case of proteins. To get an answer of this question, in this study, we have tried to identify higher order structures in the g. and R structures of the Fe-Mo alloy system, where CN12, 14, 15, and 16 polyhedra are involved Concretely higher order structures in these two structures were determined on the basis of atomic arrangements of metastable states appearing in solid-state reactions from the bcc structures to them. The present experimental data obtained by transmission electron microscopy clearly revealed that there existed metastable states for both cases : the disordered atomic-column state and the short-range ordered state of atomic-column blocks in the μ case, and another disordered atomic-column state and the domain-structure state consisting of nanometer-sized atomic-column regions in the R case. Based on the appearance of these metastable states, we tried to determine higher order structures in the μ and R structures. As a result, four higher order structures related to a CN12 polyhedron are present in the structure, while there are two structures in the R structure, one of which is a CN16 polyhedron as a primary one. Thus the presence of these higher order structures in the μ and R structures is indicative of the fact that coordination-polyhedron structures in intermetallic compounds involving 3d-transition metals should have the hierarchy of crystal structures, which is directly associated with the formation and development of covalent bonds in a metallic system

  • ナノ粒子添加による超高度金属材料の創成

    文部科学省 

    Project Year :

    2000
    -
    2004
     

  • リラクサーの機能発現機構の解明と巨大誘電応答特性をもつ新リラクサーの開発

    文部科学省 

    Project Year :

    2000
    -
    2004
     

  • Piezoelectricity and hetero-structure fluctuations in ferroelectric materials having composition near a morphotropic phase boundary

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    2001
    -
    2003
     

    KOYAMA Yasumasa

     View Summary

    In Pb(Zr_<1-x>Ti_x)O_3 (PZT), there exist the ferroelectric tetragonal/monoclinic and monoclinic/rhombohedral phase boundaries around x=0.50, where the very high piezoelectric response has been reported. Among these two boundaries, the former is characterized by the nearly vertical boundary called the morphotropic phase boundary (MPB). In order to understand the physical origin of the excellent piezoelectric property found near the MPB, we have investigated the crystallographic features of the PZT samples around x=0.50 mainly by transmission electron microscopy.The preliminary examination by means of X-ray powder diffraction revealed that the x=0.50 and 0.48 sample exhibited the tetragonal to monoclinic transition, while in the x=0.46 sample near the monoclinic/rhombohedral boundary the rhombohedral phase was basically found in the temperature range below room temperature. Based on the preliminary data, the details of the crystallographic features of these samples were examined by in-situ observation using a transmission electron microscope. The observation indicated that in the tetragonal to monoclinic transition the monoclinic regions with a lenticular shape were nucleated at the {110} twin boundary in the tetragonal phase. The characteristic features are that the nucleated regions do not basically exhibit the growth process, and that the transition proceeds by the repeated nucleation of the monoclinic regions. The monoclinic phase after the transition is then characterized by the aggregation of the lenticular domains. In the x=0.48 sample, on the other hand, there is no crystallographic domain structure. Only the tweed-type structure was observed in the whole area of the sample. This suggests that the x=0.46 sample consists of nm-sized regions with the monoclinic symmetry. In other words the rhombohedral symmetry indicating by X-ray diffraction is just on average area. The excellent piezoelectric property is thus understood be directly associated with the tweed-type microstructure consisted of the nm-size monoclinic regions

  • Structural phase transition in the heavy fermion system Cu_6Ce

    Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research

    Project Year :

    1990
    -
    1992
     

    KOYAMA Yasumasa

     View Summary

    In the present work, crystallographic features of phase transition in The heavy fermion system CeCu_6 have been in detail investigated by means of transmission electron microscopy equipped with both liquid nitrogen and liquid helium reservoirs. Specimens for electron microscopy were prepared by an ion-thinning method. When the temperature is lowered, a one-dimensional modulated structure appears around 170 K instead of the orthorhombic-to-monoclinic transition reported so far. Features of the modulated structure are that a direction of the modulation is nearly parallel to the [410] direction and its period is about 15 nm at 104 K. Further, a physical origin of the appearance of the modulated structure is discussed in relation to both a lattice softening of the elastic constant C_<66> and lattice defects introduced during the preparation of the specimen by the ion-thinning method

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Internal Special Research Projects

  • 混晶系強誘電体Ba(Ti1‐xZrx)O3における巨大誘電応答とヘテロ構造ゆらぎ

    2011   塚崎 裕文

     View Summary

     変位型強誘電体BaTiO3 でのTiイオンの一部をZrイオンで置換した混晶系強誘電体Ba(Ti1-xZrx)O3 (BTZ)では、室温付近でのZr組成の増加に伴い、強誘電正方晶(FT)相から強誘電斜方晶(FO)相、強誘電菱面体晶(FR)相、さらに常誘電立方晶(PC)相へと相変化することが報告されている。ここでBTZでの興味深い特徴は、x=0.08組成付近において、3万を越える巨大な誘電異常を生じることである。しかし、この巨大応答を生む物理的起源については、現在でもその詳細は不明のままである。そこで本研究では、巨大応答の起源を理解するため、回折法におけるフリーデル則の破れを利用して、BTZにおける強誘電状態の結晶学的特徴を明らかにした。具体的には、透過型電子顕微鏡を用いて、BTZにおけるZr置換、すなわちランダム場の導入に伴う強誘電状態から常誘電状態への変化、ならびにヘテロ構造であるナノ・メソスケール強誘電分域構造の特徴について、その詳細を調べた。 観察の結果、BTZにおける強誘電状態は、室温でのZr組成xの増加に伴い、BaTiO3のFT状態からx=0.02付近でFO状態、x=0.06付近ではMA型単斜晶状態へと変化することが明らかとなった。また、x≧0.11にはPC状態からMA状態への直接転移も存在した。ここでMA状態が存在する組成域の中で、0.06<x<0.11での注目すべき特徴は、加熱による(MA→PC)逆変態過程においてナノスケール分域(NSD)状態が出現すること、冷却によるMA状態の出現が特異な緩和現象を伴うことである。特にNSD状態では、分域全体での平均としての立方対称性が、ナノ分域での単斜晶系とは異なっており、階層による対称性の相違が生じていた。冷却速度に依存する緩和現象については、冷却速度の遅い(PC→MA)順変態の場合、中間温度域に(PC+FT)共存状態が出現すること、一方、速い場合において順変態は完全に抑制されることが分かった。結局、BTZで観察される巨大誘電応答は、冷却過程で生じる特異な緩和現象に直接関係することが理解された。 x=0.08付近での強誘電相転移に引き続き、x≧0.11で生じる(PC→MA) 直接転移の特徴を調べた。まず誘電率測定から、(PC→MA) 転移温度はZr組成の増加と伴に減少し、x=0.20付近において室温付近となることが分かった。そこで0.19≦x≦0.25組成を有するBTZ試料を室温で観察することにより、Zr組成の変化を通して(PC→MA) 相転移の特徴を明らかにした。その結果、PC/MA相境界に近いPC状態は、および分極成分を持つナノ領域から成ることが明らかとなった。さらに、と分極成分を合成することにより、各ナノ分域の分極ベクトルの方向を決定した。ここで合成によって得られる分極方向はFR状態の方向の一つに平行である必要はない。実際、本研究ではFR相を特徴付ける{110}双晶構造、すなわち109°分域構造の存在を見出すことはできなかった。よって、PC/MA相境界に近いPC状態は、単斜晶ナノ分域から成るナノ強誘電分域状態であり、PC状態での立方晶系の対称性は、分域全体を平均化して得られる巨視的な対称性であることが分かった。結局、(PC→MA)強誘電相転移は、単斜晶系の対称性を持つナノ分域の合体、再配列、および成長を通して進行することが結論された。

  • 階層性を有するMn系合金の結晶構造と物性の特徴

    2009  

     View Summary

     金属Mnのα-Mnとβ-Mn相は、12配位多面体等の複雑な配位多面体から成る配位多面体構造を有している。ここで興味深い特徴は、金属MnにSiを添加したMn-Si合金にもR相と呼ばれる配位多面体構造相が出現し、その結果、Mn-Si合金の7at.%Si組成付近には (β-Mn→α-Mn+R)共析反応が存在することである。これら相の物理的特性については、α-Mn相でのノンコリニアーな反強磁性磁気秩序、R相ではその構造中に共有結合性ボンドの存在が指摘されている。そこで本研究課題では、(β-Mn→α-Mn+R)共析反応での配位多面体構造間の構造変化に注目し、その結晶学的特徴を透過型電子顕微鏡で調べ、さらに得られた結果を基に、これら構造変化の特徴および物理的特性との相関について検討した。 本研究では、まず構造変化での方位関係を決定した。その結果、(β-Mn→α-Mn)構造変化では一種類、(β-Mn→R)変化では三種類の方位関係の存在が明らかとなった。ここで後者での三種類の方位関係の中で、二種類の関係は、より高いMn組成域で出現すること、その出現確率は全体の30%程度であることが分かった。このため、これらの関係はSi置換によって誘起された、付加的なものと判断した。そこで、得られた方位関係を用いて構造変化での原子変位を決定したところ、両構造変化とも単純な原子変位によって説明できることが明らかとなった。特に興味深い特徴は、得られた方位関係が、β-Mn構造の14配位多面体対からα-MnおよびR構造での16配位多面体対への、構造単位の変化によって決定されることである。さらにα-MnとR相での物理的特性の違いについては、16配位多面体の内部に存在するFriauf多面体の相違によることも理解された。具体的には、α-Mn相でのFriauf多面体の原子サイトは基本的にMn原子によって、一方、R相ではすべて混合サイトによって特徴付けられている。

  • 強誘電ランダムアクセスメモリー材料での強誘電疲労特性とその分域構造

    2005  

     View Summary

    強誘電体Pb(Zr1-xTix)O3(PZT)やSrBi2Ta2O9(SBT)等を用いた強誘電ランダムアクセスメモリ材料は、次世代を担うメモリ材料として広く注目を集めている。ここでメモリ材料として要求される特性は、電場による分極反転に際して、誘電疲労と生じないということである。実際、パルスレーザー堆積法で作製したBi4-yLayTi3O12(BLT)の薄膜試料は、誘電疲労を示さないことが示されている。そこで本研究では、誘電疲労のない強誘電分域構造の特徴を明らかにするため、代表例であるPZTとBLTについてその結晶学的特徴を透過型電子顕微鏡を用いて調べた。具体的には、動力学的効果によるフリーデル則の破れを利用し、これら酸化物での強誘電分域構造の決定を行った。[PZTでの強誘電分域構造] 本研究では、まずPZTにおける強誘電菱面体晶相の室温での分域構造を、x=0.42試料を用いて決定した。その結果、分極ベクトルの方向は方向、分域構造は大きさ100nm程度の109°および180°分域構造から成ることを確認した。そこで、この分域構造の形成過程を調べるため、室温の分域構造を加熱したところ、分極成分領域の大きさの減少に伴い638K付近で10nm程度の斑点状分域へと連続的に変化し、Tc=653Kを越えた700Kにおいて斑点状分域は消失することが明らかとなった。また、この変化は可逆的に生じることも示された。[BLTでの強誘電分域構造] PZTに関する研究に引続き、誘電疲労を生じることのないBLTでの強誘電分域構造の特徴について、La量y=0および0.6を有する試料を用いてその詳細を調べた。ここでは以下に、y=0試料の結果について述べる。まずBLTの強誘電単斜晶相から得られた電子回折図形中には、I4/mmm構造による基本格子反射に加え、q=[1/2 1/2 0]と[1/2 1/2 l]に超格子反射が存在し、その構造中に酸素八面体の回転変位および傾斜変位を含むことが分かった。さらに基本格子反射を用いた暗視野像から、分極ベクトルの方向はI4/mmm構造の[100]方向であること、その分域の形状は方向に伸びたバンド状の形状を有していることが明らかとなった。

  • 強誘電ランダムアクセスメモリー材料での強誘電疲労特性とその分域構造

    2004  

     View Summary

    次世代を担う記録媒体として、近年強誘電体を用いた強誘電ランダムアクセスメモリー(FeRAM)材料が、広く注目を集めている。ここで、この材料に要求される最も重要な因子は、電場による分極反転に際して誘電疲労を生じないという特性で、このため候補材料としてのPb(Zr1-xTix)O3(PZT)、SrBi2Ta2O9(SBT)、およびBi4Ti3O12(BT)に関して、その誘電特性の詳細が検討されている。その結果、Laを添加したBi4-yLayTi3O12(BLT)において、実際、誘電疲労を示さない薄膜材料の開発が行われている。そこで本研究では、より良質なFeRAM材料の開発を目指し、誘電疲労を生むことのない強誘電分域構造の特徴を明らかにするため、上述のPZTとBLTについてその結晶学的特徴を透過型電子顕微鏡を用いて調べた。具体的には、動力学的効果によるフリーデル則の破れを利用し、二波励起の条件のもとで暗視野像を撮影することにより、これら酸化物での強誘電分域構造の決定を行った。[PZTでの強誘電分域構造] 本研究では、まずPZTのx=0.50付近に存在する強誘電正方晶、単斜晶相、および菱面体晶相について、その強誘電分域構造の決定を行った。決定したこれら相での分極ベクトルの方向は、正方晶相が[001]方向、単斜晶相が[001]+[110]、および菱面体晶相が[111]方向である。分域構造については、正方晶相が{110}90°と{100}180°、また菱面体晶相は{110}109°および{112}180°分域壁から成る構造を有していることが確認された。一方、正方晶/単斜晶相境界付近に現われる単斜晶相は、ナノスケールを持つ正方晶領域と2つの単斜晶領域の交互配列によって特徴付けられていることが明らかとなった。[BLTでの強誘電分域構造] PZTに関する研究に引続き、誘電疲労を生じることのないBLTでの強誘電分域構造について、La量y=0.75および1.0を有するBLT試料を用いてその詳細を調べた。まず強誘電斜方晶相から得られた電子回折図形中には、I4/mmm構造による基本格子反射に加え、q=[1/2 1/2 0]位置に超格子反射が存在し、その構造は酸素八面体の傾斜変位を含むことが分かった。さらに200および-200基本格子反射による暗視野像はBLTにおける強誘電分域構造の存在を明らかにした。その特徴は、分極ベクトルの方向がI4/mmm構造の[100]方向であること、その分域の形状が方向に伸びた楕円形状あるいはバンド形状を有していることである。

  • ラーベス相合金における階層間構造変化の結晶学的特徴

    2003  

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    本研究では、主にTi-30at.%Cr合金における(bcc→hcp+C15)反応での構造変化に注目し、その結晶学的特徴を透過型電子顕微鏡を用いて調べた。具体的には、この反応において出現する準安定平衡相および平衡相の結晶学的特徴、ならびにその出現の順序について明らかにした。まず、構造変化は、bcc→bcc+Zone→bcc+LBS→bcc+CSS→bcc+CSS+hcp→bcc+hcp+NSR→bcc+hcp+C15→hcp+C15の7段階で進行することが示された。ここで、ZoneはCr一原子層から成るゾーン構造、LBRは局所的なbct領域(local bct region)、CSSはCr 1原子層とTi 3原子層が交互に配列した化学的縞状構造(chemical stripe structure)、そしてNSRはC15型構造に類似した構造を持つ、大きさ2nm程度のナノメーターサイズ領域(nm-size region)である。この構造変化での特徴については、hcp領域がCSSを核生成サイトとして、またNSRはhcp領域を取りまくように出現することである。特に、NSRはC15型構造形成の前駆段階に対応し、ラーベス相形成に重要な役割を果たすことが示された。また、最終的に得られた(hcp+C15)状態は非常に微細な組織を有することも明らかとなった。上述の実験結果を踏まえて、Ti-40at%Cr合金を用いbccおよびC15型構造の結晶方位関係を調べた。その結果、形成の初期段階における二十面体原子クラスターの出現を示す、7つの方位関係の存在が明らかとなった。そこで、Ti-30at%Cr合金におけるNSR領域を詳細に調べたところ、この形成は二十面体対称性を有することが分かった。よって、ラーベス相の形成は二十面体クラスターを介して生じることが結論された。

  • 秩序変数と自発歪が競合する相転移での動的構造ゆらぎ

    2000  

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     La系酸化物超伝導体には、低温構造相転移と呼ばれる低温斜方晶(LTO)相からPccn相、さらに低温正方晶(LTT)相への逐次相転移が存在する。以前我々が行ったLa1.5Sr0.1Nd0.4CuO4に関する研究から、低温構造相転移の進行は、低温相であるPccn/LTT傾斜領域の動的ゆらぎを伴うことが明かにされている。この動的ゆらぎは、相転移の主秩序変数であるCuO6八面体の傾斜と副秩序変数である自発歪e4の競合によるものであり、このため低温構造相転移が報告されていないLa2-xSrxCuO4においても、動的平衡状態として、その存在が期待される。そこで本研究では、La2-xSrxCuO4におけるこの動的ゆらぎの存在を透過型電子顕微鏡を用いて明かにした。 本研究で用いたLa2-xSrxCuO4試料は、クエン酸共沈法により作製したx=0.12セラミック試料である。透過型電子顕微鏡用試料には、Arイオン・シンニング法を用いて薄片化したものを用いた。低温構造相転移のその場観察は、液体ヘリウム二軸冷却ステージを備えたH-800型透過型電子顕微鏡を用いて、電子回折図形および明・暗視野像を撮影することにより、12Kから室温の温度範囲で行なった。 得られた実験結果から、x=0.12を有するLa1.88Sr0.12CuO4において低温構造相転移の存在が明らかとなった。具体的には、低温相であるPccn/LTT傾斜領域は、転移点TL≒135KでLTO分域界面に沿って核成長し、その後幅広がり成長を起こすとともに、LTO分域内にも出現することが示された。特に、LTO分域内のPccn/LTT領域は、静的なものではなく、動的ゆらぎとして存在することが分かった。さらに、動的挙動はPccn/LTT領域の大きさにより異なることも示された。結局、これらの実験結果から、LTO分域内のPccn/LTT傾斜領域の動的挙動は、主秩序変数の平衡ゆらぎであると結論した。

  • Pb<SUB>1-x</SUB>La<SUB>x</SUB>(Zr<SUB>1-y</SUB>Ti<SUB>y</SUB>)O<SUB>3</SUB>の反強誘電相における結晶構造と微細組織

    1998  

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     ペロブスカイト型構造を有するPb(Zr1-xTix)O3は,室温においてx0.05では強誘電性を示すことが報告されている。このため,x=0.05において反強誘電性と強誘電性との競合が生じ,新たな状態の出現が予想される。そこで,本研究では,この競合によって生じる状態の詳細を明らかにするため,Pb(Zr0.95Ti0.05)O3を取り上げ,室温での結晶構造および微細組織の特徴を透過型電子顕微鏡を用いて調べた。 室温での電子回折図形中には,q=1/2位置周りに不整合構造の存在を示す超格子反射の分裂が認められた。分裂した超格子反射の消滅則を調べたところ,超格子反射はすべてのq=1/2位置周りに存在することがわかった。この事実は,Vielandらが指摘している,酸素八面体の回転変位であるM3変位だけでは,その消滅則を説明することができない。そこで,q=1/2での既約表現を検討した結果,Pb原子によるM5'変位を考慮することの必要性が明らかとなった。ここで,このM5'変位は方向に沿ったPb原子の反強誘電変位である。 M3およびM5'変位によって特徴付けられる微細組織の特徴を調べるために,分裂した超格子反射を用いて暗視野像を撮影した。得られた像中には,約8nmの間隔で並ぶ縞状コントラストが観察された。また,縞状コントラスとの先端部はヘアピン状をしており,このコントラストが2π/2の位相すべりを持つディスコメンシュレーションであることを示している。よって,不整合相の微細組織はディスコメンシュレーションの周期的配列からなると結論される。 ディスコメンシュレーションの解析は,M3およびM5'変位による超格子反射で結像した暗視野像を用いて行った。まず,M3変位の超格子反射による暗視野像から不整合相を特徴づける縞状コントラストは,M3回転変位の回転軸が90°異なる,2つのバリアントからなることが分かった。さらに,M5'変位による反射で結像した像からは,一方のM3ドメインがM5'変位の反位相境界に対応することも示された。結局,反強誘電性と強誘電性との競合によって生じた状態,すなわち,Pb(Zr1-xTix)O3における不整合相でのディスコメンシュレーションは,M3分域とM5'反位相境界から形成されていることがわかった。

  • 強相関電子材料La<SUB>x</SUB>Sr<SUB>1-x</SUB>VO<SUB>3</SUB>の構造相転移と金属-絶縁体転移

    1997  

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    3d遷移金属酸化物LaMO3は、多体効果によって特徴付けられる強相関電子材料の一つであり、単純ペロブスカイト型構造を有している。本研究では、これら遷移金属酸化物の中でLaxSr1-xVO3を取り上げる。ここで、本酸化物ではx=0.8付近で金属-絶縁体転移を起こすことが知られているものの、この金属-絶縁体転移での格子系の役割、特に本酸化物での構造相転移と電子状態との相関については明らかではない。そこで本研究では、LaxSr1-xVO3における構造相転移の有無、その特徴、さらに強相関電子状態への構造相転移の影響を明らかにするため、本酸化物での結晶構造ならびに局所構造を反映する微細組織の特徴を透過型電子顕微鏡を用いて調べた。 室温で得られた電子回折図形の解析から、LaxSr1-xVO3の室温での結晶構造は、0.0≦x<0.1で立方晶Pm3m構造、0.1≦x≦0.6で菱面体晶Rc構造、さらに0.6<x<0.8の二相共存域を経て0.8≦x≦1.0では斜方晶Pbnm構造へと変化することが分かった。ここで、この結果から、x=0.7付近においてRc―Pbnm構造相転移の存在が予想される。そこで、室温でRc構造を有するx=0.6試料を用いて、低温領域での相転移の有無ならびにその特徴を調べた。その結果、冷却により約150KでPbnm構造へ転移すること、相転移点以下でのRcとPbnm領域の共存が明らかとなった。このことは、Rc―Pbnm構造相転移が強い一次相転移であることを示している。また、Pbnm分域中にはVO6八面体の傾斜に関係する反位相境界も観察された。

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