Takaya Nagai

Hokkaido University, Sapporo, Hokkaidō, Japan

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Publications (61)115.56 Total impact

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    ABSTRACT: Crystal structures of high-pressure polymorphs of Ca(OD)2, (a) at room temperature (phase II′) and (b) at high temperature (phase II), were obtained from in situ neutron diffraction measurements.
    Journal of Solid State Chemistry 10/2014; 218:95–102. · 2.20 Impact Factor
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    ABSTRACT: Spin transition and substitution of Fe3+ in Fe3+AlO3-bearing MgSiO3 perovskite (Pv) and post-perovskite (PPv) were examined up to 200 GPa and 165 GPa, respectively, at room temperature by X-ray emission spectroscopy (XES) and XRD. The results of XES and XRD indicate that in Pv high spin (HS) Fe3+ at the dodecahedral (A) site replaces Al at the octahedral (B) site and becomes low spin (LS) between 50 and 70 GPa with pressure, while in PPv LS Fe3+ occupies the B-site and Al occupies the A-site above 80-100 GPa. The Fe3+-Al coupled substitution seems to be at work in both Pv and PPv. Combining these results on Fe3+ with the recent first-principles calculations on Fe2+ in Pv and PPv, the spin transition and substitution of iron in pyrolitic lower mantle minerals are proposed. Further, their effects on iron-partitioning among the lower mantle minerals are discussed.
    Physics of The Earth and Planetary Interiors 03/2014; · 2.40 Impact Factor
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    ABSTRACT: Experiments using laser-heated diamond anvil cells combined with synchrotron X-ray diffraction and SEM–EDS chemical analyses have confirmed the existence of a complete solid solution in the MgSiO3–MnSiO3 perovskite system at high pressure and high temperature. The (Mg, Mn)SiO3 perovskite produced is orthorhombic, and a linear relationship between the unit cell parameters of this perovskite and the proportion of MnSiO3 components incorporated seems to obey Vegard’s rule at about 50 GPa. The orthorhombic distortion, judged from the axial ratios of a/b and $ \sqrt{2}\,a/c, $ 2 a / c , monotonically decreases from MgSiO3 to MnSiO3 perovskite at about 50 GPa. The orthorhombic distortion in (Mg0.5, Mn0.5)SiO3 perovskite is almost unchanged with increasing pressure from 30 to 50 GPa. On the other hand, that distortion in (Mg0.9, Mn0.1)SiO3 perovskite increases with pressure. (Mg, Mn)SiO3 perovskite incorporating less than 10 mol% of MnSiO3 component is quenchable. A value of the bulk modulus of 256(2) GPa with a fixed first pressure derivative of four is obtained for (Mg0.9, Mn0.1)SiO3. MnSiO3 is the first chemical component confirmed to form a complete solid solution with MgSiO3 perovskite at the P–T conditions present in the lower mantle.
    Physics and Chemistry of Minerals 07/2013; · 1.40 Impact Factor
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    ABSTRACT: The skeletal texture and crystal morphology of the massive reef-building coral Porites lobata were observed from the nano- to micrometer scale using an analytical transmission electron microscope (ATEM). The skeletal texture consists of centers of calcification (COCs) and fiber area. Fiber areas contain bundles of needle-like aragonite crystals that are elongated along the crystallographic c-axis and are several hundred nanometers to one micrometer in width and several micrometers in length. The size distribution of aragonite crystals is relatively homogeneous in the fibers. Growth lines are observed sub-perpendicular to the direction of aragonite growth. These growth lines occur in 1-2μm intervals and reflect a periodic contrast in the thickness of an ion-spattered sample and pass through the interior of some aragonite crystals. These observations suggest that the medium filled in the calcification space maintains a CaCO(3)-supersaturated state during fiber growth and that a physical change occurs periodically during the aragonite crystals of the fiber area.
    Journal of Structural Biology 10/2012; · 3.37 Impact Factor
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    ABSTRACT: There are still large discrepancies among the previous reports on the spin transition of iron in Mg–perovskite (Pv). To alleviate this problem, we examined the spin state of Fe3 + in Mg0.85Fe3 +0.15Al0.15Si0.85O3 Pv up to 200 GPa by X-ray emission spectroscopy (XES) and X-ray diffraction (XRD). The gradual decrease of the high spin (HS) ratio of Fe3 + by low temperature annealing of the samples above ~ 60 GPa in XES and the change of the trend of unit cell volumes with pressure by annealing at 50–60 GPa in XRD indicate that Fe3 + occupies the A-site and is HS below ~ 50 GPa but above 50–60 GPa it gradually replaces Al at the B-site through cation exchange reaction by annealing and becomes low spin (LS), while Fe3 + remaining at the A-site is HS up to 200 GPa. This means that the spin state of Fe3 + depends on Fe3 + occupancies between the A- and B-sites and these Fe3 + occupancies are strongly controlled by the synthesis condition and annealing temperature of the samples through the cation exchange reaction. The present results combined with the previous reports indicate that in Al-bearing Mg–Pv in the lower mantle Fe2 + occupies the A-site and remains HS for the whole lower mantle, while Fe3 + occupies the A-site and is HS below ~ 50 GPa but above 50–60 GPa it replaces Al at the B-site and becomes LS, on the assumption that spin transition pressure of Fe2 + at the A-site is higher than that of Fe3 + at the same site.
    Earth and Planetary Science Letters 02/2012; s 317–318:407–412. · 4.72 Impact Factor
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    ABSTRACT: The pressure responses of portlandite and the isotope effect on the phase transition were investigated at room temperature from single-crystal Raman and IR spectra and from powder X-ray diffraction using diamond anvil cells under quasi-hydrostatic conditions in a helium pressure-transmitting medium. Phase transformation and subsequent peak broadening (partial amorphization) observed from the Raman and IR spectra of Ca(OH)2 occurred at lower pressures than those of Ca(OD)2. In contrast, no isotope effect was found on the volume and axial compressions observed from powder X-ray diffraction patterns. X-ray diffraction lines attributable to the high-pressure phase remained up to 28.5GPa, suggesting no total amorphization in a helium pressure medium within the examined pressure region. These results suggest that the H–D isotope effect is engendered in the local environment surrounding H(D) atoms. Moreover, the ratio of sample-to-methanol–ethanol pressure medium (i.e., packing density) in the sample chamber had a significant effect on the increase in the half widths of the diffraction lines, even at pressures below the hydrostatic limit of the pressure medium. KeywordsPortlandite–Isotope effect–Phase transition–Hydrostaticity–Hydrogen bonding
    Physics and Chemistry of Minerals 12/2011; 38(10):777-785. · 1.40 Impact Factor
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    ABSTRACT: A ferroelectric KNbO3 has been investigated to 11.4 GPa at room temperature by angle-dispersive powder diffraction with diamond anvil cell using synchrotron radiation, which was performed by a high-resolution evaluation setting. KNbO3 transforms from orthorhombic to tetragonal lattice at around 6 GPa, and further transforms to a cubic lattice at about 9.2 GPa. Single-crystal X-ray diffraction studies under pressures clarified the space groups of the three phases and their lattice constants. The extinction rule of the observed reflections and diffraction intensity distribution proved Cm2m, P4mm and Pm3m for orthorhombic, tetragonal and cubic polymorphs, respectively.
    Ferroelectrics 03/2011; 337(2006):189-195. · 0.38 Impact Factor
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    ABSTRACT: The El Niño/Southern Oscillation (ENSO) system during the Pliocene warm period (PWP; 3-5 million years ago) may have existed in a permanent El Niño state with a sharply reduced zonal sea surface temperature (SST) gradient in the equatorial Pacific Ocean. This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for near-term climate change, ENSO variability--and related global climate teleconnections-could have been radically different from that today. Yet, owing to a lack of observational evidence on seasonal and interannual SST variability from crucial low-latitude sites, this fundamental climate characteristic of the PWP remains controversial. Here we show that permanent El Niño conditions did not exist during the PWP. Our spectral analysis of the δ(18)O SST and salinity proxy, extracted from two 35-year, monthly resolved PWP Porites corals in the Philippines, reveals variability that is similar to present ENSO variation. Although our fossil corals cannot be directly compared with modern ENSO records, two lines of evidence suggest that Philippine corals are appropriate ENSO proxies. First, δ(18)O anomalies from a nearby live Porites coral are correlated with modern records of ENSO variability. Second, negative-δ(18)O events in the fossil corals closely resemble the decreases in δ(18)O seen in the live coral during El Niño events. Prior research advocating a permanent El Niño state may have been limited by the coarse resolution of many SST proxies, whereas our coral-based analysis identifies climate variability at the temporal scale required to resolve ENSO structure firmly.
    Nature 03/2011; 471(7337):209-11. · 42.35 Impact Factor
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    ABSTRACT: MgSiO3 dominant perovskite is believed to be the most abundant constituent mineral in the Earth’s lower mantle. Generally minerals form solid solutions and their nature should affect on physical properties of minerals. In this paper, we will introduce our recent studies about incorporation mechanism of FeAlO3 component into MgSiO3 perovskite and its crystal chemistry.
    Nihon Kessho Gakkaishi. 01/2011; 53(1):8-12.
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    ABSTRACT: High-pressure and high-temperature experiments of albitic plagioclase up to 41 GPa and 270 °C were carried out using an externally heated diamond anvil cell. Raman spectroscopy and transmission electron microscopy of the recovered samples revealed that the amorphization of albite was complete at ∼37 GPa and room temperature. The amorphization pressure at 170 °C was nearly the same as that at room temperature. In contrast, the pressure largely decreased to ∼31 GPa at 270 °C. In comparison with the amorphization pressure of albite in laboratory shock experiments, that in the present static compression experiments is significantly lower (>10 GPa) even at room temperature. This suggests that shorter pressure duration results in a lower degree of amorphization of plagioclase. The formation of maskelynite in shocked meteorites does not necessarily require the very high shock pressure (30–90 GPa) that was previously estimated on the basis of shock recovery experiments.
    Geophysical Research Letters 11/2010; 37(21). · 4.46 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 03/2010; 31(13).
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    ABSTRACT: Infrared absorption spectra of delta-AlOOH and its deuterated form (delta-AlOOD) were measured at high pressure using a diamond anvil cell under a quasi-hydrostatic pressure condition using helium as a pressure-transmitting medium. Two absorption bands at 1180 cm-1and 1330 cm-1 involving vibrations of hydrogen and oxygen atoms shifted to higher frequencies with increasing pressure up to 10 and 12 GPa for delta-AlOOH and delta-AlOOD, respectively. In contrast, at higher pressures the two bands did not shift so much. The pressure-response on the infrared spectra has a close relationship to the symmetrization of the hydrogen bonds and change in the compressibility which was observed from X-ray diffraction measurements.
    Journal of Physics Conference Series 03/2010; 215(1).
  • The Review of High Pressure Science and Technology 01/2010; 20(3):269-276.
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    ABSTRACT: High pressure and high temperature experiments on CaSiO3, FeSiO3, MnSiO3 and CoSiO3 using a laser-heated diamond anvil cell combined with synchrotron X-ray diffraction were conducted to explore the perovskite structure of these compounds and the transition to the post-perovskite structure. The experimental results revealed that MnSiO3 has a perovskite structure from relatively low pressure (ca. 20 GPa) similarly to CaSiO3, while the stable forms of FeSiO3 and CoSiO3 are mixtures of mono-oxide (NaCl structure) + high pressure polymorph of SiO2 even at very high pressure and temperature (149 GPa and 1800 K for FeSiO3 and 79 GPa and 2000 K for CoSiO3). This strongly suggests that the crystal field stabilization energy (CFSE) of Fe2+ with six 3d electrons and Co2+ with seven 3d electrons at the octahedral site of mono-oxides favors a mixture of mono-oxide + SiO2 over perovskite where Fe2+ and Co2+ would occupy the distorted dodecahedral sites having a smaller CFSE (Mn2+ has five 3d electrons and has no CFSE). The structural characteristics that the orthorhombic distortion of MnSiO3 perovskite decreases with pressure and the tolerance factor of CaSiO3 perovskite (0.99) is far from the orthorhombic range suggest that both MnSiO3 and CaSiO3 perovskites will not transform to the CaIrO3-type post-perovskite structure even at the Earth's core–mantle boundary conditions, although CaSiO3 perovskite has a potentiality to transform to the CaIrO3-type post-perovskite structure at still higher pressure as long as another type of transformation does not occur.
    Physics of The Earth and Planetary Interiors 12/2009; 177(s 3–4):147–151. · 2.40 Impact Factor
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    ABSTRACT: Material and Life Science experimental Facility (MLF) of Japan Proton Accelerator Research Complex (J-PARC) will be one of the most powerful spallation neutron facilities in the world. The pulsed neutron source with a liquid Hg target is designed to be running at 25 Hz with a power of 1 MW. We have started the construction of the powder diffractometer dedicated to high-pressure experiments (PLANET) on BL-11. PLANET aims to study structures of hydrogen-bearing materials including dense hydrous minerals of the Earth's deep interior, magmas and light element liquids. The instrument will realize diffraction and radiography experiments for powder and liquid/glass samples at high pressures up to 20 GPa and 2000 K using a large sized multi-anvil hydraulic press that can apply forces of ˜1500 ton. The instrument views a decoupled liquid H2 moderator with a cross section of 100 × 100 mm2. The primary and secondary fight paths are 25 m and 1.5 m, respectively. The 11.5-m-long supermirror guide with elliptical shape starts at a distance of 11.5 m from the moderator. Design of elliptical geometry is optimized by means of incorporating several different grade mirrors and linear approximation with planar guide in order to save cost for production without degradation of the intensity performance. The guide has a rectangular cross-section and consists of four walls coated with supermirror material. Sample is placed at 2 m from the guide exit. The 90° detectors will be installed at 1.5 m from the sample position. For the powder diffraction measurements using a multi-anvil press, an incident neutron beam passes through the vertical anvil gaps and irradiates the sample in the pressure medium. Diffracted neutrons go through the other anvil gaps at 90° direction. Half inch 3He linear position sensitive detectors with 600 mm length will be arranged horizontally and form these detector banks, which cover the scattering angle of 79°
    AGU Fall Meeting Abstracts. 12/2009;
  • Meteoritics and Planetary Science Supplement. 09/2009;
  • Geochmica et Cosmochimica Acta 06/2009;
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    ABSTRACT: It is expected that high-pressure material science and the investigation of the Earth's interior will progress greatly using the high-flux pulse neutrons of J-PARC. In this article, we introduce our plans for in situ neutron powder diffraction experiments under high pressure at J-PARC. The use of three different types of high-pressure devices is planned; a Paris–Edinburgh cell, a new opposed-anvil cell with a nano-polycrystalline diamond, and a cubic anvil high-pressure apparatus. These devices will be brought to the neutron powder diffraction beamlines to conduct a “day-one” high-pressure experiment. For the next stage of research, we propose construction of a dedicated beamline for high-pressure material science. Its conceptual designs are also introduced here.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 02/2009; 600(1):50-52. · 1.32 Impact Factor
  • The Review of High Pressure Science and Technology 01/2009; 19(1):15-23.
  • American Mineralogist - AMER MINERAL. 01/2009; 94:1255-1261.

Publication Stats

272 Citations
115.56 Total Impact Points

Institutions

  • 2005–2014
    • Hokkaido University
      • • Department of Natural History Sciences
      • • Division of Earth and Planetary Sciences
      • • Faculty of Science
      • • Graduate School of Science
      Sapporo, Hokkaidō, Japan
  • 1997–2010
    • Osaka University
      • • Department of Earth and Space Science
      • • Graduate School of Science
      Suika, Ōsaka, Japan
  • 2001
    • Stony Brook University
      • Department of Geosciences
      Stony Brook, NY, United States