Ken Niwa

Nagoya University, Nagoya, Aichi, Japan

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Publications (31)69.23 Total impact

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    ABSTRACT: Nb hydrides formation in 10-GPa supercritical water has been investigated by angle-dependent micro-beam hard x-ray photoemission spectroscopy. In the Nb 3d core-level spectra, Nb hydride components are found in the slightly high binding energy side of the metallic components, and the oxide ones are observed even though little oxides are recognized in x-ray diffraction patterns. Obtained emission-angle dependence of the Nb 3d core-level spectra of Nb hydride specimens shows that the Nb hydride components increase with the emission angle decreased i.e. the sampling depth increased, while the oxide ones decrease. The bulk valence-band spectrum is obtained by decomposing the measured valence-band spectra into a bulk and surface components with use of the emission-angle dependence of the core-level and valence-band spectra; it consists of two bands. This implies the Nb-H chemical bond formation and Nb in an oxidation state, consistent with reported band structure calculations and the observed core-level chemical shifts. Thus it is confirmed by valence-band and core-level photoelectron spectroscopy that the Nb hydrides are formed inside the specimen, irrespective to the well-known high oxidation ability of supercritical water.
    No preview · Article · Apr 2015 · Journal of Alloys and Compounds
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    ABSTRACT: BaTiO 3 –SnO solid solutions have been investigated from the viewpoints of synthesis and Sn ion valence. First-principles calculations show that the solution energy of Sn 21 into the Ba sites in BaTiO 3 is less than that into the Ti sites under high pressure. The BaTiO 3 –SnO solid solutions have been synthesized under high pressure (;20 GPa) and temperatures using a laser-heated diamond anvil cell. The synthesized materials have been characterized using x-ray diffractometry, scanning transmission electron microscopy, and energy-dispersive x-ray spectroscopy. It is found from these various methods that we have successfully synthesized uniform solid solutions of BaTiO 3 –SnO. Furthermore, it is also clarified by the Sn L 3 -edge electron energy loss spectra measurements that the valences of the Sn ions in the BaTiO 3 –SnO solid solution are 21. These results indicate that the Sn 21 ions are substituted into the Ba sites, according to the ion size. Consequently, the Sn ions can be substituted into the Ba sites of the shrinking BaTiO 3 lattice under high pressure, which is similar to the Ca and Sn co-substitution into Ba sites under ambient pressure as reported previously.
    No preview · Article · Oct 2014 · Journal of Materials Research
  • Ken NIWA · Masashi HASEGAWA
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    ABSTRACT: Diamond-anvil cell (DAC) provides opportunity to explore the new insight of material science under extreme high pressure. In particular, DAC combined with laser heating system is a powerful tool for synthesizing novel materials. In this article, we review the recent advanced researches on the synthesis of novel metal nitride and the crystal growth of rectangular hollow tube oxides by means of the high-pressure experiments using laser-heated diamond anvil cell. © 2014 Japan Society of High Pressure Science and Technology. All Rights Reserved.
    No preview · Article · Sep 2014 · The Review of High Pressure Science and Technology
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    ABSTRACT: The last remaining marcasite-type RuN2 was successfully synthesized by direct chemical reaction between ruthenium and molecular nitrogen above the pressure of 32 GPa. For the first time, we found that Ru 4d is weakly hybridized with N 2p in the structure by using transmission electron microscopy equipped with electron-energy-loss spectroscopy. Our finding give important knowledge about the platinum-group pernitride with respect to the chemical bonding between platinum-group element and nitrogen.
    No preview · Article · Sep 2014 · Chemistry - A European Journal
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    ABSTRACT: Super critical fluids are well known as suitable solvents for dissolution and extraction processes, because it exhibits extreme high solubility and reactivity. However further experimental development using supercritical fluid would offer new insight in material science, especially the synthesis and crystal growth of novel materials. We report the successful growth of single crystals with the rutile-type structure (TiO2, Co-doped TiO2, SiO2, GeO2 and SnO2) in supercritical fluids (water or oxygen) using a laser heated diamond-anvil cell up to a pressure of 7 GPa. The resultant product showed the rectangular hollow tube morphology, a several tens of microns in length and a wall thickness of less than 500 nm. TEM analyses demonstrated that this rectangular hollow tube single crystals were surrounded by the {110} faces and grown along the [001] direction. The preferential growth of {110} faces is consistent with the lowest surface energy of {110} faces of the rutile-type structure. In addition, the rapid cooling rate in LHDAC and high solubility of supercritical fluids also play an important role for the formation of the rectangular hollow tube. The details of the synthesis procedure, characterization and growth mechanism are discussed in this paper.
    No preview · Article · May 2014 · Journal of Physics Conference Series
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    ABSTRACT: Marcasite-type rhodium nitride was successfully synthesized in a direct chemical reaction between a rhodium metal and molecular nitrogen at 43.2 GPa using a laser-heated diamond-anvil cell. This material shows a low zero-pressure bulk modulus of K0 = 235(13) GPa, which is much lower than those of other platinum group nitrides. This finding is due to the weaker bonding interaction between metal atoms and quasi-molecular dinitrogen units in the marcasite-type structure, as proposed by theoretical studies.
    No preview · Article · Jan 2014 · Inorganic Chemistry
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    ABSTRACT: High-pressure behaviors of Linde-type A (LTA) zeolite were investigated using a quasi-hydrostatic pressure medium of noble gases (helium or argon). High-pressure in situ X-ray diffraction measurements combined with the use of a diamond anvil cell did not observe any structural phase transition for LTA zeolites up to a pressure of approximately 12 GPa at room temperature. However, it was found that the compressibility strongly depended not only on the pressure medium but also on the pressurization process. LTA zeolites showed significant incompressible behaviors at pressures between 2 and 7 GPa when it was compressed in helium. Based on careful analyses of the data together with the results of previous high-pressure studies, the incompressibility of LTA zeolites was induced by the penetration of a large amount of noble gas atoms into the cages of LTA zeolites under high pressures. The present results offer interesting and important information on the atomic diffusion process for open-structured materials under high pressures and thus have great implications for the development of new noble-gas-atom filled materials, as well as geochemical implications.
    No preview · Article · Dec 2013 · Microporous and Mesoporous Materials
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    ABSTRACT: We discuss the doping effect of Sn2þ on ATiO3 perovskites by comparing with those of Pb and Ba. The solution energy of Sn into A sites of ATiO3 perovskites shows minimum when the lattice size is almost the same as that of SrTiO3 (STO), as determined by first-principles calculations. In dielectric measurements of these STO-based ceramics, the Sn2þ-doped STO ceramics showed a higher peak temperature than the Pb2þ- or Ba2þdoped STO ceramics at the same doping amount. In addition, the maximum polarization obtained in the Sn2þ-doped STO ceramics was larger than that obtained in the Pb2þ- or Ba2þ-doped ceramics in terms of the relationship between the polarization and electric field curve measurement at room temperature. The piezoresponse was also observed in the Sn2þ-doped STO ceramics at room temperature by piezoresponse force microscopy. We propose that these effects originate from the bond length between Sn2þ and O2� which is smaller than that between Pb2þ and O2�. The Sn2þ ion prefers the off-center position at the Sr2þ site because of the retention of the stable bond length between Sn2þ and O2�. These leads to the formation of polar regions and which can assist in the generation of a larger polarization.
    No preview · Article · Sep 2013 · Japanese Journal of Applied Physics
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    ABSTRACT: We have measured the solubility of argon in SiO2 melt under high pressure by melting a SiO2 crystal at the argon pressure as high as 19 GPa using laser-heated diamond anvil cell (LHDAC). SiO2 was successfully melted at the pressure of about 12 GPa and recovered with a completely glassy phase. Above 15 GPa, the sample contained a large amount of crystalline SiO2, although it was completely melted and spherical due to the surface tension. This may arise from the change in the coordination number of Si in SiO2 melt at around 12–15 GPa, which is close to the triple point of coesite, stishovite, and liquid SiO2. Argon content in the recovered SiO2 was measured by energy-dispersive spectroscopy and mass spectrometry. It increased at pressures up to approximately 5–7 GPa and exhibited a constant value up to at least 12 GPa; this implies that, at these conditions, the noble gas behaves as an incompatible element. The result is completely different from those of previous LHDAC studies, which claimed that the increased solubility then dramatically drops above the pressure of about 5 GPa. Here, we report the new details of experimentally determined pressure dependence of noble gas solubility by LHDAC, and discuss the experimental difficulties and the structure change of silicate melt under high pressures.
    No preview · Article · Feb 2013 · Earth and Planetary Science Letters
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    ABSTRACT: We have characterized Nb hydrides synthesized in high-pressure supercritical water by hard X-ray photoelectron spectroscopy. Comparison is made, in particular, of the Nb 2p core level spectra and valence band ones for the Nb hydride specimens in different stages of hydrogenation with those for Nb oxide and metallic references. The Nb 2p core level spectra of the Nb hydride specimens synthesized at relatively low temperature show an intense Nb2O5 component and a shoulder structure, which is attributed to Nb hydrides, at the high binding energy side of a metallic component of the Nb metal. The valence band spectra of the Nb hydride specimens also show a broad band at the binding energy EB between 5 and 9 eV, which is ascribed mainly to Nb oxides. The present results indicate that the surface of the synthesized Nb hydrides is covered with several 10 nm thick Nb oxides and suggest that the Nb hydrides are formed deep inside the specimens. The Nb 2p chemical shift implies the Nb valence of +1.4 for the synthesized hydride NbHx.
    No preview · Article · Feb 2013 · Journal of Electron Spectroscopy and Related Phenomena
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    ABSTRACT: The perovskite (Pv) to post-perovskite (PPv) phase transition and the deformation texture of the PPv phase were investigated on the basis of a high-pressure X-ray diffraction (XRD) study of CaIrO3 using a diamond-anvil cell in a pressure range up to 31 GPa. The development of a crystallographic preferred orientation (CPO) in the PPv phase was observed after the plastic deformation from 8 or 9 GPa to 31 GPa at both room and high (∼1500 K) temperatures. The observed CPOs in the present study indicate that the (0 1 0) plane worked as an active slip plane in the PPv phase over the entire pressure and temperature range of the present experiment. We also confirmed that the Pv to PPv phase transition proceeds at room temperature under high-stress conditions. The phase transition under high-stress condition is in stark contrast to the results of a previous hydrostatic experiment in which the Pv-CaIrO3 remained stable in a helium media at 31 GPa and room temperature. This indicates that shear stress plays an important role in the Pv to PPv phase transition, and this effect should be taken into account when the thermal structure at the D″ layer is discussed on the basis of the high-pressure experiments.
    No preview · Article · Mar 2012 · Physics of The Earth and Planetary Interiors
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    ABSTRACT: Hydrostatic pressure induces structural changes in proteins, including denaturation, the mechanism of which has been attributed to water penetration into the protein interior. In this study, structures of 3-isopropylmalate dehydrogenase (IPMDH) from Shewanella oneidensis MR-1 were determined at about 2 Å resolution under pressures ranging from 0.1 to 650 MPa using a diamond anvil cell (DAC). Although most of the protein cavities are monotonically compressed as the pressure increases, the volume of one particular cavity at the dimer interface increases at pressures over 340 MPa. In parallel with this volume increase, water penetration into the cavity could be observed at pressures over 410 MPa. In addition, the generation of a new cleft on the molecular surface accompanied by water penetration could also be observed at pressures over 580 MPa. These water-penetration phenomena are considered to be initial steps in the pressure-denaturation process of IPMDH.
    Full-text · Article · Mar 2012 · Acta Crystallographica Section D Biological Crystallography
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    ABSTRACT: The rock salt (B1) structure of binary oxides or chalcogenides transforms to the CsCl (B2) structure under high pressure, with critical pressures P(s) depending on the cation to anion size ratio (R(c)/R(a)). We investigated structural changes of A(2)MO(3) (A = Sr, Ca; M = Cu, Pd) comprising alternate 7-fold B1 AO blocks and corner-shared MO(2) square-planar chains under pressure. All of the examined compounds exhibit a structural transition at P(s) = 29-41 GPa involving a change in the A-site geometry to an 8-fold B2 coordination. This observation demonstrates, together with the high pressure study on the structurally related Sr(3)Fe(2)O(5), that the B1-to-B2 transition generally occurs in these intergrowth structures. An empirical relation of P(s) and the R(c)/R(a) ratio for the binary system holds well for the intergrowth structure also, which means that P(s) is predominantly determined by the rock salt blocks. However, a large deviation from the relation is found in LaSrNiO(3.4), where oxygen atoms partially occupy the apical site of the MO(4) square plane. We predict furthermore the occurrence of the same structural transition for Ruddlesden-Popper-type layered perovskite oxides (AO)(AMO(3))(n), under higher pressures. For investigating the effect on the physical properties, an electrical resistivity of Sr(2)CuO(3) is studied.
    No preview · Article · Nov 2011 · Inorganic Chemistry
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    ABSTRACT: Rectangular hollow tube crystals of the rutile-type TiO2 have been successfully grown in supercritical water at about 2 GPa for the first time using a diamond-anvil cell combined with the infrared laser heating system. The scanning electron microscope (SEM) and transmission electron microscope (TEM) observations indicate that they are grown along the [001] direction in a few tens of micrometer lengths. It is also found that they are surrounded by the {110} face with its wall thickness of 200 nm. Details of the morphology, optical property, and growth mechanism of the obtained crystals are discussed.
    No preview · Article · Sep 2011 · Crystal Growth & Design
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    Full-text · Article · Aug 2011 · Acta Crystallographica Section A Foundations of Crystallography
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    ABSTRACT: The binary skutterudite CoP(3) has a large void at the body-centered site of each cubic unit cell and is, therefore, called a nonfilled skutterudite. We investigated its room-temperature compression behavior up to 40.4 GPa in helium and argon using a diamond-anvil cell. High-pressure in situ X-ray diffraction and Raman scattering measurements found no phase transition and a stable cubic structure up to the maximum pressure in both media. A fitting of the present pressure-volume data to the third-order Birch-Murnaghan equation of state yields a zero-pressure bulk modulus K(0) of 147(3) GPa [pressure derivative K(0)' of 4.4(2)] and 171(5) GPa [where K(0)' = 4.2(4)] in helium and argon, respectively. The Grüneisen parameter was determined to be 1.4 from the Raman scattering measurements. Thus, CoP(3) is stiffer than other binary skutterudites and could therefore be used as a host cage to accommodate large atoms under high pressure without structural collapse.
    No preview · Article · Mar 2011 · Inorganic Chemistry
  • Ken Niwa · Takehiko Yagi
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    ABSTRACT: Compression behaviors of CaIrO3 with perovskite (Pv) and post-perovskite (pPv) structures have been investigated up to 31.0(1.0) and 35.3(1) GPa at room temperature, respectively, in a diamond-anvil cell with hydrostatic pressure media. CaIrO3 Pv and pPv phases were compressed with the axial compressibility of β a > β c > β b and β b > β a > β c, respectively and no phase transition was observed in both phases up to the highest pressure in the present study. The order of axial compressibility for pPv phase is consistent with the crystallographic consideration for layer structured materials and previous experimental results. On the other hand, Pv phase shows anomalous compression behavior in b axis, which exhibit constant or slightly expanded above 13 GPa, although the applied pressure remained hydrostatic. Volume difference between Pv and pPv phases was gradually decreased with increasing pressure and this is consistent with the results of theoretical study based on the ab initio calculation. Present results, combined with theoretical study, suggest that these complicate compression behaviors in CaIrO3 under high pressure might be caused by the partially filled electron of Ir4+. Special attention must be paid in case of using CaIrO3 as analog materials to MgSiO3, although CaIrO3 exhibits interesting physical properties under high pressure.
    No preview · Article · Jan 2011 · Physics and Chemistry of Minerals
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    ABSTRACT: The phase relation of FeTiO3 was investigated up to a pressure of about 67 GPa by synchrotron x-ray diffraction and analytical transmission electron microscopy. We found that FeTiO3 perovskite decomposes into Fe1−xTi0.5xO wüstite and orthorhombic FeTi3O7 phases (ideally FeTiO3→2/3FeO+1/3FeTi3O7) at conditions above 42 GPa and 2000 K. Our study also demonstrates that the phase assemblage of Fe1−xTi0.5xO and FeTi3O7 is the densest of the various systems reported so far for FeTiO3.
    No preview · Article · Sep 2010 · Physical review. B, Condensed matter
  • T. Okada · T. Yagi · K. Niwa · T. Kikegawa
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    ABSTRACT: Lattice-preferred orientations (LPOs) of post-perovskite (PPv)-type MgGeO3 formed by transformations from two different pre-phases, orthopyroxene (OPx) and perovskite (Pv), were studied using radial and axial X-ray diffraction techniques combined with a laser-heated diamond-anvil cell. When the PPv was made from Pv, strong LPO was formed in the PPv immediately after the transformation with an alignment of the (0 0 1) or (0 1 1) plane almost perpendicular to the compression axis. This present result contrasts markedly from that observed when the PPv was formed from OPx and those reported on MgGeO3-PPv and (Mg,Fe)SiO3-PPv transformed from OPx, in which the (1 0 0) or (1 1 0) plane was aligned to the compression axis. It was clarified that when the PPv-type MgGeO3 is formed in diamond-anvil cells, it has strong transformation-induced LPO from the beginning even before plastic deformation occurs, and that its characteristics depend on the pre-phase before the transformation. The formed transformation-induced LPO of PPv did not change in spite of further compression using a conventional gasket because the sample became too thin to deform. However, by using a diamond gasket technique, we were able to deform the PPv at room temperature, resulting in significant variations of the relative intensities of several diffraction peaks. The change in the axial diffraction patterns can be explained only by assuming that the (0 0 1) plane aligns almost perpendicularly to the compression axis. The present results suggest that the (0 0 1) is a dominant slip plane in MgGeO3-PPv at room temperature. If one makes the assumption that the dominant slip system(s) of silicate PPv deformed in the D″ layer is or are identical to that of germanate PPv at room temperature, then a large S-wave polarization anisotropy in the Earth's D″ layer would be explained by the small degree of deformation-induced LPO of the PPv phase that is expected to exist in this layer.
    No preview · Article · Jun 2010 · Physics of The Earth and Planetary Interiors
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    ABSTRACT: This paper reports a procedure to combine the focused ion beam micro-sampling method with conventional Ar-milling to prepare high-quality site-specific transmission electron microscopy cross-section samples. The advantage is to enable chemical and structural evaluations of oxygen dissolved in a molten iron sample to be made after quenching and recovery from high-pressure experiments in a laser-heated diamond anvil cell. The evaluations were performed by using electron energy-loss spectroscopy and high-resolution transmission electron microscopy. The high signal to noise ratios of electron energy-loss spectroscopy core-loss spectra from the transmission electron microscopy thin foil, re-thinned down to 40 nm in thickness by conventional Argon ion milling, provided us with oxygen quantitative analyses of the quenched molten iron phase. In addition, we could obtain lattice-fringe images using high-resolution transmission electron microscopy. The electron energy-loss spectroscopy analysis of oxygen in Fe0.94O has been carried out with a relative accuracy of 2%, using an analytical procedure proposed for foils thinner than 80 nm. Oxygen K-edge energy-loss near-edge structure also allows us to identify the specific phase that results from quenching and its electronic structure by the technique of fingerprinting of the spectrum with reference spectra in the Fe-O system.
    No preview · Article · May 2010 · Journal of Microscopy

Publication Stats

167 Citations
69.23 Total Impact Points


  • 2009-2015
    • Nagoya University
      • • Department of Crystalline Materials Science
      • • Department of Material Science
      • • Graduate School of Engineering
      • • Department of Materials, Physics and Energy Engineering
      Nagoya, Aichi, Japan
  • 2007-2010
    • The University of Tokyo
      • Institute for Solid State Physics
      Edo, Tōkyō, Japan