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Journal Articles

X-ray and neutron study on the structure of hydrous SiO$$_{2}$$ glass up to 10 GPa

Urakawa, Satoru*; Inoue, Toru*; Hattori, Takanori; Sano, Asami; Kohara, Shinji*; Wakabayashi, Daisuke*; Sato, Tomoko*; Funamori, Nobumasa*; Funakoshi, Kenichi*

Minerals (Internet), 10(1), p.84_1 - 84_13, 2020/01

 Times Cited Count:7 Percentile:74.72(Geochemistry & Geophysics)

The structure of hydrous amorphous SiO$$_{2}$$ is fundamental to investigate the effects of water on the physicochemical properties of oxide glasses and magma. The hydrous SiO$$_{2}$$ glass with 13 wt.% D$$_{2}$$O was synthesized under high-pressure and high-temperature conditions and its structure was investigated by small angle X-ray scattering, X-ray diffraction, and neutron diffraction experiments at pressures of up to 10 GPa and room temperature. This hydrous glass is separated into a SiO$$_{2}$$ rich major phase and a D$$_{2}$$O rich minor phase. Medium-range order of the hydrous glass shrinks compared to the anhydrous SiO$$_{2}$$ glass due to disruption of SiO$$_{4}$$ linkage by formation of Si-OD deuterioxyl, while the pressure response is similar. Most of D$$_{2}$$O molecules are in the small domains and hardly penetrate into SiO$$_{2}$$ major phase.

Journal Articles

Local structure analysis of KNbO$$_3$$ nanocubes by solvothermal synthesis

Yoneda, Yasuhiro; Kohara, Shinji*; Nakashima, Koichi*; Nagata, Hajime*; Wada, Satoshi*

Japanese Journal of Applied Physics, 54(10S), p.10NC01_1 - 10NC01_6, 2015/10

 Times Cited Count:4 Percentile:21.11(Physics, Applied)

The atomic-scale structure of KNbO$$_3$$ nanopowder synthesized by solvothermal method has been studied using high-energy X-ray diffraction, Rietveld refinement, and the atomic pair-distribution function (PDF) technique. It was found that the local structure of KNbO$$_3$$ nanoparticles deviates from the average structure. The local structure was found to be rhombohedral structure though the average structure was tetragonal structure. The rhombohedral distortion of NbO$$_6$$ octahedra was maintained in the annealed sample.

Journal Articles

Local structure analysis of BaTiO$$_3$$ modulated by Mn substitution

Yoneda, Yasuhiro; Kohara, Shinji*; Noguchi, Yuji*; Miyayama, Masaru*

Transactions of the Materials Research Society of Japan, 40(1), p.29 - 32, 2015/03

Barium titanate (BaTiO$$_3$$) with a high dielectric constant is widely used in the production of ceramic capacitors. However, during sintering large numbers of ionized oxygen vacancies and conduction electrons are created. Doping with manganese is one of several ways to confine the electrons to a small region. In Mn-doped BaTiO$$_3$$ there exist three valence states for the manganese ions, Mn$$^{2+}$$, Mn$$^{3+}$$, and Mn$$^{4+}$$. The Mn$$^{4+}$$ is nearly exactly incorporated into Ti$$^{4+}$$ sites and participates in the collective motion in the lattice. We performed local structure analysis by using atomic pair-distribution function (PDF) and X-ray absorption fine structure (XAFS). Figure 1 shows the obtained PDFs for pure BaTiO$$_3$$ and 0.5%-Mn-doped BaTiO$$_3$$. All data collected at room temperature. Two obtained PDF patterns are well in agreement. Mn doping has hardly affected local structure.

Journal Articles

Local structure analysis of $$A$$TiO$$_{3}$$ ($$A$$ = Sr, Ba, Pb)

Yoneda, Yasuhiro; Kohara, Shinji*

Ferroelectrics, 485(1), p.34 - 41, 2015/00

 Times Cited Count:2 Percentile:11.21(Materials Science, Multidisciplinary)

High-energy X-ray total scattering provides simultaneous information about long-range order through the Bragg peaks and short-range order through the diffuse scattering. In ferroelectric materials, short-range order structure inside the ferroelectric domains tends to deviate from the average structure. The local structure analysis is necessary to reveal the ferroelectric mechanism. We applied atomic pair-distribution function (PDF) analysis using synchrotron X-ray for the study of local structure of ferroelectrics having perovskite structure. In BaTiO$$_3$$, since local distortion is averaged out owing to the requirements of periodicity and harmonic lattice vibrations, the average structure showed a cubic structure. The actual structure of systems which displays so much instability and disorder can be revealed by the PDF analysis.

Journal Articles

Local structure analysis of Nb-related perovskite materials

Yoneda, Yasuhiro; Kohara, Shinji*; Nagata, Hajime*; Fu, D.*; Takenaka, Tadashi*

Transactions of the Materials Research Society of Japan, 39(4), p.455 - 458, 2014/12

NaNbO$$_3$$, KNbO$$_3$$, and AgNbO$$_3$$ have rhombohedral structure at low temperatures. The phase transition depends on ${it A}$-site ion with NbO$$_6$$ octahedron. NaNbO$$_3$$ and KNbO$$_3$$ have structural phase transition from rhombohedral to orthorhombic with increasing temperature. On the other hand, the phase transition of AgNbO$$_3$$ occurs from rhombohedral to monoclinic. Since the rhombohedral structure is a highly ordered average structure, it tends to be subject to the influence of local randomness. We performed local structure analysis by using atomic pair-distribution function (PDF). The Nb-O bond clearly appeared in the observed distribution for NaNbO$$_3$$, but degraded in that for AgNbO$$_3$$. The difference of local structures causes different successive phase transitions for the Nb-related perovskite materials.

Journal Articles

Local structure analysis of BaTiO$$_3$$ and KNbO$$_3$$ solid solution

Yoneda, Yasuhiro; Kohara, Shinji*; Kumada, Nobuhiro*; Wada, Satoshi*

Japanese Journal of Applied Physics, 53(9S), p.09PD01_1 - 09PD01_5, 2014/09

 Times Cited Count:3 Percentile:15.11(Physics, Applied)

The atomic-scale structure of solid solution of BaTiO$$_3$$ (BT) and KNbO$$_3$$ (KN) has been studied using high-energy X-ray diffraction, X-ray absorption fine structure, and atomic pair-distribution function analysis techniques. We prepared BT-KN solid solution through KNbTiO$$_5$$, in which Ti and Nb atoms are arranged randomly. The average structure of BT-KN solid solution was cubic structure and the local strcuture is also reproduced by the cubic structure. It is rare that a solid solution synthesized from ferroelectric materials has a local strcuture of a paraelectric material. Since the original correlation of BT or KN was lost, ferroelectricity disappeared in the BT-KN solid solution.

Journal Articles

Local structure analysis of NaNbO$$_3$$

Yoneda, Yasuhiro; Fu, D.*; Kohara, Shinji*

Journal of Physics; Conference Series, 502(1), p.012022_1 - 012022_4, 2014/04

 Times Cited Count:11 Percentile:97.99

NaNbO$$_3$$ has an antiferroelectric structure at room temperature and finds important technological applications. It exhibits an unusual complex sequence of temperature- and pressure-driven structural phase transitions. NaNbO$$_3$$ shows unambiguous evidence for the presence of the ferroelectric $$R3c$$ phase coexisting with an antiferroelectric phase ($$Pbcm$$) over a wide range of temperatures. We have carried out atomic pair-distribution function (PDF) analysis on NaNbO$$_3$$ to understand the phase transitions. High-energy X-ray PDF using powder samples were carried out at the SPring-8, which is provided to become rhombohedral structure if ${it A}$-site atoms did order and to become antiferroelectric phase if ${it A}$-site atoms did disorder.

Journal Articles

Local structure analysis of BaTiO$$_3$$ nanoparticles

Yoneda, Yasuhiro; Kohara, Shinji*; Kato, Kazumi*

Japanese Journal of Applied Physics, 52(9), p.09KF01_1 - 09KF01_5, 2013/09

 Times Cited Count:7 Percentile:33.17(Physics, Applied)

The atomic-scale structure of nanocrystalline BaTiO$$_3$$ powders has been studied using high-energy X-ray diffraction, X-ray absorption fine structure, and the atomic pair-distribution function technique. The studies show that the materials are well-ordered at nanometer length distance. The three-dimensional atomic ordering in BaTiO$$_3$$ is cubic-like on average, but locally shows slight distortions of a tetragonal-type, similar to that occurring in the coarsegrained bulk crystals. Ferroelectric coherence is facilitated in part by control of particle morphology. The spatial extent of cooperative ferroelectric distortions is found to be degraded in nanometer-sized building blocks.

Journal Articles

Local structure analysis of SmFe$$_2$$ and TbFe$$_2$$

Yoneda, Yasuhiro; Kohara, Shinji*; Ito, Masayoshi*; Abe, Hiroshi; Takeuchi, Mitsuaki*; Uchida, Hirohisa*; Matsumura, Yoshihito*

Transactions of the Materials Research Society of Japan, 38(1), p.109 - 112, 2013/03

We performed local structure analysis of SmFe$$_2$$ and TbFe$$_2$$ in the crystallization process from amorphous by using synchrotron X-ray pair-distribution function (PDF) method. The giant mangetostrictive materials of these compounds can be controlled its property by ion irradiation. In amorphous TbFe$$_2$$ and SmFe$$_2$$, the mosaic block size was small and the short-range order structure owing to Fe-Fe network maintained. In crystalline SmFe$$_2$$, a rigid long-range order structure of Fe-Fe and Sm-Sm network coexisted and it had a large mosaic block size. On the other hand, in crystalline TbFe$$_2$$, Fe-Fe network degraded by Tb-Tb network owing to its lattice tolerance.

Journal Articles

Local structure analysis of Bi$$_2$$WO$$_6$$

Yoneda, Yasuhiro; Kohara, Shinji*; Takeda, Hiroaki*; Tsurumi, Takaaki*

Japanese Journal of Applied Physics, 51(9), p.09LE06_1 - 09LE06_6, 2012/09

Local structure analysis of Bi$$_2$$WO$$_6$$ was performed by high-energy X-ray atomic pair-distribution function (PDF) analysis. We found a deviation between local and average structures owing to the different coherence length between Bi$$_2$$O$$_2$$ and WO$$_6$$ layers. Bi atoms were displaced toward the $$b$$-axis of the orthorhombic $$Pca2_1$$ structure. The local off-center shift of Bi atoms coupled with thermal factor and negrected in the average structure. The coherence length of Bi$$_2$$O$$_2$$ layer increased with increasing temperature and splead in the whole crystal when the average structure changed from $$Pca2_1$$ to $$Aba2$$ structure.

Journal Articles

Structural investigations of AgNbO$$_3$$ phases using high-energy X-ray diffraction

Yoneda, Yasuhiro; Yoshii, Kenji; Kohara, Shinji*

Transactions of the Materials Research Society of Japan, 37(1), p.73 - 76, 2012/03

The structures and phase transitions of AgNbO$$_3$$ were investigated using high-energy X-ray powder diffraction. Bragg reflections have revealed the high temperature M$$_3$$-O, O-T, and T-C phase transitions but have not given any significant evidence of low temperature M$$_1$$-M$$_2$$ and M$$_2$$-M$$_3$$ ones. The existence of structural disorder in the T and O phases was indicated by diffuse scattering. Here we again confirm the average structure of AgNbO$$_3$$. The refined structure was essentially the same as previous reports. With isotropic thermal parameters, refinements of the structures at most temperatures resulted in larger Ag thermal parameters. Disorder feature of Ag coupled with the thermal parameters and exhibit as diffuse scattering in T and O phases.

Journal Articles

Densified low-hygroscopic form of P$$_{2}$$O$$_{5}$$ glass

Brazhkin, V. V.*; Akola, J.*; Katayama, Yoshinori; Kohara, Shinji*; Kondrin, M. V.*; Lyapin, A. G.*; Lyapin, S. G.*; Tricot, G.*; Yagafarov, O.

Journal of Materials Chemistry, 21(28), p.10442 - 10447, 2011/07

 Times Cited Count:14 Percentile:44.78(Chemistry, Physical)

P$$_{2}$$O$$_{5}$$ compound is an archetypical glass-forming oxide with a high hygroscopicity. We found that the quenching from the P$$_{2}$$O$$_{5}$$melt under ultrahigh pressures enables obtaining densified P$$_{2}$$O$$_{5}$$ glasses with a residual densification up to 12% at normal conditions. These glasses have a low hygroscopicity and can exist under air conditions for several weeks. An examination of the structure of the new form of P$$_{2}$$O$$_{5}$$ glass reveals a cardinal decrease of the volume of nanovoids in the glassy matrix.

Journal Articles

Phase-change materials; Vibrational softening upon crystallization and its impact on thermal properties

Matsunaga, Toshiyuki*; Yamada, Noboru*; Kojima, Rie*; Shamoto, Shinichi; Sato, Masugu*; Tanida, Hajime*; Uruga, Tomoya*; Kohara, Shinji*; Takata, Masaki*; Zalden, P.*; et al.

Advanced Functional Materials, 21(12), p.2232 - 2239, 2011/06

 Times Cited Count:107 Percentile:95.64(Chemistry, Multidisciplinary)

Thermal properties of the amorphous and crystalline state of phase-change materials show remarkable differences such as higher thermal displacements and a more pronounced anharmonic behavior in the crystalline phase. These findings are related to the change of bonding upon crystallization.

Journal Articles

Local crystal structure of nano-manganese-oxide gold adsorbent

Iikubo, Satoshi*; Koyanaka, Hideki*; Shamoto, Shinichi; Takeuchi, Ken*; Kohara, Shinji*; Kodama, Katsuaki; Loong, C.-K.*

Journal of Physics and Chemistry of Solids, 71(11), p.1603 - 1608, 2010/11

 Times Cited Count:8 Percentile:39.13(Chemistry, Multidisciplinary)

The local crystal structure of dried and deuterated nano-manganese-oxide powder samples was studied via atomic pair distribution function analysis of X-ray and neutron powder diffraction data. The protonated sample shows ultrahigh efficiency as a gold adsorbent even from ppt-level aqueous solutions such as seawater. We show that the nano-manganese-oxide particles have an R-MnO$$_{2}$$-type local crystal structure. The possible role of the protons on the surface of the nano-particles is discussed.

Journal Articles

Reverse Monte Carlo simulation approach on the amorphous structure of C$$_{59}$$N

Ferdows, M.; Suzuya, Kentaro; Kawakita, Yukinobu; Kohara, Shinji*; Kodama, Katsuaki; Yamamoto, Kazunori; Shamoto, Shinichi; Hirata, Takamichi*; Omote, Kenji*; Kasama, Yasuhiko*; et al.

Abstracts of 7th International Symposium on Advanced Materials in Asia-Pacific (NT 2010)/JAIST International Symposium on Nano Technology 2010 (7th ISAMAP), p.25 - 28, 2010/09

We performed Reverse Monte Carlo analysis (RMCA) to study amorphous structure of a heterofullerene, C$$_{59}$$N by using pulsed neutron powder diffraction data. The scattering function S(Q) shows that the structure is amorphous, while the atomic pair distribution function (PDF) shows that the local structure is similar to that of C$$_{60}$$. Obtained structures strongly depend on the initial models. Problems for this RMCA method are discussed.

Journal Articles

Local structure modulation in the electronic ferroelectric oxide LuFe$$_{2}$$O$$_{4}$$

Hayakawa, Hironori*; Morimoto, Masanori*; Ikeda, Naoshi*; Yoneda, Yasuhiro; Kohara, Shinji*; Yoshii, Kenji; Matsuo, Yoji*; Michiuchi, Takamasa*; Mori, Shigeo*

Transactions of the Materials Research Society of Japan, 34(1), p.51 - 54, 2009/05

We have recently reported that LuFe$$_{2}$$O$$_{4}$$ becomes ferroelectric below 330 K, owing to a charge-ordering transition of Fe ions. As details of the properties of this material have not been reported so far, we have investigated its local structure in the ferromagnetic phase by means of synchrotron X-ray diffraction. The analysis of pair-distribution function showed the existence of displacement of Lu atoms; this structure is different from the average structure of this materials known thus far. This result encourages a detailed reinvestigation of crystal structure of LuFe$$_{2}$$O$$_{4}$$. We also show the results obtained from some other experimental methods, such as dielectric measurement and electron diffraction.

Journal Articles

Local structure of BiFeO$$_3$$-BaTiO$$_3$$ mixture

Yoneda, Yasuhiro; Yoshii, Kenji; Kohara, Shinji*; Kitagawa, Shuji*; Mori, Shigeo*

Japanese Journal of Applied Physics, 47(9), p.7590 - 7594, 2008/09

 Times Cited Count:22 Percentile:64.73(Physics, Applied)

Local structure analysis of (1-$$x$$)BiFeO$$_3$$-$$x$$BaTiO$$_3$$ mixture was carried out by synchrotron radiation X-ray pair-distribution function (PDF) method. The PDF peak is resolved as a doublet due to the presence of two distinct bond length, Bi-O and Ba-O. Cubic perovskite lattice of (1-$$x$$)BiFeO$$_3$$-$$x$$BaTiO$$_3$$ solid solution distorts locally to accomodate the distinct Bi-O and Ba-O bond lengths.

Journal Articles

AsS: Bulk inorganic molecular-based chalcogenide glass

Brazhkin, V. V.*; Gavrilyuk, A. G.*; Lyapin, A. G.*; Timofeev, Yu. A.*; Katayama, Yoshinori; Kohara, Shinji*

Applied Physics Letters, 91(3), p.031912_1 - 031912_3, 2007/07

 Times Cited Count:19 Percentile:59.84(Physics, Applied)

A high pressure method to produce bulk chalcogenide glasses of a unique AsS composition has been developed. The structure, optical properties, and stability of the obtained glasses have been studied. Glasses have an intrinsic deep-red color, opticalpseudogap Eg$$approx$$1.75 eV, a broad Urbach absorption tail W$$_{U}$$$$approx$$120 meV, and high temperature stability up to 130$$^{circ}$$C. AsS glasses show photoinduced transformations, including photocrystallization. The glass structure is largely based on the quasimolecular As$$_{4}$$S$$_{4}$$ units with partial polymerization providing an example of an inorganic molecular-based glass.

Journal Articles

Structural basis for the fast phase change of Ge$$_{2}$$Sb$$_{2}$$Te$$_{5}$$; Ring statistics analogy between the crystal and amorphous states

Kohara, Shinji*; Kato, Kenichi*; Kimura, Shigeru*; Tanaka, Hitoshi*; Usuki, Takeshi*; Suzuya, Kentaro; Tanaka, Hiroshi*; Moritomo, Yutaka*; Matsunaga, Toshiyuki*; Yamada, Noboru*; et al.

Applied Physics Letters, 89(20), p.201910_1 - 201910_3, 2006/11

 Times Cited Count:190 Percentile:97.84(Physics, Applied)

The three-dimensional atomic configuration of amorphous Ge$$_{2}$$Sb$$_{2}$$Te$$_{5}$$ were derived by reverse Monte Carlo simulation with synchrotron-radiation X-ray diffraction data. The authors found that amorphous Ge$$_{2}$$Sb$$_{2}$$Te$$_{5}$$ can be regarded as "even-numbered ring structure", because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge-Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge$$_{2}$$Sb$$_{2}$$Te$$_{5}$$ is the key for the fast crystallization speed of the material.

Journal Articles

Local structure of relaxor Pb(In$$_{0.5}$$Nb$$_{0.5}$$)O$$_3$$ ferroelectrics

Yoneda, Yasuhiro; Suzuya, Kentaro; Mizuki, Junichiro; Kohara, Shinji*

Journal of Applied Physics, 100(9), p.093521_1 - 093521_4, 2006/11

 Times Cited Count:12 Percentile:43.65(Physics, Applied)

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