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  2. Hiroshi Kojitani

Hiroshi Kojitani

  (糀谷 浩)

Profile Information

Affiliation
Professor, Faculty of Science Department of Chemistry, Gakushuin University
Degree
Ph.D(Gakushuin University)
Ph.D(Gakushuin University)
Researcher number
60291522
J-GLOBAL ID
200901050139239240
researchmap Member ID
5000032247

Research Interests

 6

Research Areas

 1

Research History

 13
More

Committee Memberships

 4

Awards

 1

Papers

 36
  • Hiroshi Kojitani, Mei Gonai, Yoshiyuki Inaguma, Masaki Akaogi
    Physics and Chemistry of Minerals, 51(1), Feb 9, 2024  Peer-reviewedLead author
  • Hiroaki Hayashi, Yuichi Shirako, Lei Xing, Alexei A. Belik, Masao Arai, Masanori Kohno, Taichi Terashima, Hiroshi Kojitani, Masaki Akaogi, Kazunari Yamaura
    Physical Review B, 108(7), Aug 17, 2023  Peer-reviewed
  • Masaki Akaogi, Natsuki Miyazaki, Taisuke Tajima, Hiroshi Kojitani
    Physics and Chemistry of Minerals, 50(3), Jul 14, 2023  
  • Hiroshi Kojitani, Monami Yamazaki, Yuki Tsunekawa, Shiho Katsuragi, Masamichi Noda, Toru Inoue, Yoshiyuki Inaguma, Masaki Akaogi
    Physics of the Earth and Planetary Interiors, 333 106937-106937, Dec, 2022  Peer-reviewedLead author
  • Takayuki Ishii, Giacomo Criniti, Elena Bykova, Leonid Dubrovinsky, Tomoo Katsura, Hidekazu Arii, Hiroshi Kojitani, Masaki Akaogi
    American Mineralogist, 106(7) 1105-1112, Jul 1, 2021  Peer-reviewed
    Abstract Three single crystals of CaTi2O4 (CT)-type, CaFe2O4 (CF)-type, and new low-density CaFe2O4 (LD-CF) related MgAl2O4 were synthesized at 27 GPa and 2500 °C and also CT-type MgAl2O4 at 45 GPa and 1727 °C using conventional and advanced multi-anvil technologies, respectively. The structures of CT-type and LD-CF related MgAl2O4 were analyzed by single-crystal X-ray diffraction. The lattice parameters of the CT-type phases synthesized at 27 and 45 GPa were a = 2.7903(4), b = 9.2132(10), and c = 9.3968(12) Å, and a = 2.7982(6), b = 9.2532(15), and c = 9.4461(16) Å, respectively, (Z = 4, space group: Cmcm) at ambient conditions. This phase has an AlO6 octahedral site and an MgO8 bicapped trigonal prism with two longer cation-oxygen bonds. The LD-CF related phase has a novel structure with orthorhombic symmetry (space group: Pnma), and lattice parameters of a = 9.207(2), b = 3.0118(6), and c = 9.739(2) Å (Z = 4). The structural framework comprises tunnel-shaped spaces constructed by edge- and corner-sharing of AlO6 and a 4+1 AlO5 trigonal bipyramid, in which MgO5 trigonal bipyramids are accommodated. The CF-type MgAl2O4 also has the same space group of Pnma but a slightly different atomic arrangement, with Mg and Al coordination numbers of 8 and 6, respectively. The LD-CF related phase has the lowest density of 3.50 g/cm3 among MgAl2O4 polymorphs, despite its high-pressure synthesis from the spinel-type phase (3.58 g/cm3), indicating that the LD-CF related phase formed via back-transformation from a high-pressure phase during the recovery. Combined with the previously determined phase relations, the phase transition between CF-and CT-type MgAl2O4 is expected to have a steep Clapeyron slope. Therefore, CT-type phase may be stable in basaltic- and continental-crust compositions at higher temperatures than the average mantle geotherm in the wide pressure range of the lower mantle. The LD-CF related phase could be found in shocked meteorites and used for estimating shock conditions.
  • Kevin Lemoine, Agnieszka Wizner, Sandy Auguste, Jean-Marc Grenèche, Hiroshi Kojitani, Masaki Akaogi, Yoshiyuki Inaguma
    Open Ceramics, 6 100123-100123, Jun, 2021  Peer-reviewed
  • Fumiya Kimura, Hiroshi Kojitani, Masaki Akaogi
    Physics of the Earth and Planetary Interiors, 310, Jan 1, 2021  
    Phase relations in the system KAlSiO4-MgAl2O4 were determined up to 28 GPa and 1500 °C. A hexagonal aluminous (NAL) phase is stable above 16 GPa with a narrow compositional range of (1-x)KAlSiO4·xMgAl2O4, x ≈ 0.61–0.65. The stability field of NAL phase in the CaAl2O4-MgAl2O4 system was also determined up to 29 GPa and 1200 °C, resulting in a compositional range of NAL phase of (1-x)CaAl2O4·xMgAl2O4, x ≈ 0.67–0.74. Compared with the above compositional widths of NAL phases, the stability field of NAL phase in the NaAlSiO4-MgAl2O4 system is much wider, (1-x)NaAlSiO4·xMgAl2O4, x ≈ 0.47–0.70 (Ono et al., 2009). The difference may be caused by the fact that both of Na+ and Mg2+ with similar cation sizes enter 6-fold trigonal prism sites in the NAL structure. In the system KAlSiO4-MgAl2O4, KAlSiO4 kalsilite and MgAl2O4 spinel coexist below ~8 GPa, above which an assemblage of MgAl2O4 spinel, corundum, K2Mg2Si2O7-rich phase X and pyrope becomes stable. The assemblage further changes into NAL phase at 16 GPa. These results and compositional similarity suggest that composite inclusions consisting of MgAl2O4-rich spinel and kalsilite-nepheline solid solution found in diamonds in the Juina-5 kimberlite, Brazil, were formed below ~8 GPa presumably by decomposition of K-rich NAL phase trapped in diamonds in the lower mantle conditions.
  • Takayuki Ishii, Nobuyoshi Miyajima, Ryosuke Sinmyo, Hiroshi Kojitani, Daisuke Mori, Yoshiyuki Inaguma, Masaki Akaogi
    Geophysical Research Letters, 47(6), Mar 28, 2020  Peer-reviewed
  • M. Akaogi, T. Tajima, M. Okano, H. Kojitani
    Minerals, 9(614) 1-12, 2019  Peer-reviewed
  • M. Tokuda, A. Yoshiasa, H. Kojitani, S. Hashimoto, S. Uehara, T. Mashimo, T. Tobase, M. Akaogi
    Mineral. Mag., 83(4) 561-567, 2019  Peer-reviewed
    Single crystals of synthetic reidite and natural radiation-damaged zircon from Okueyama, Japan were investigated using X-ray diffraction. The pressure-induced zircon-reidite transition is described by the twisting and translations of SiO4 tetrahedra with disappearance of the SiO4-ZrO8 shared edges. The lattice of radiation-damaged zircons expands mainly from alpha-decays of radioactive elements such as U and Th. Although old radiation-damaged zircons show anomolous lattice distortion, young radiation-damaged zircons do not show such distortions. These distortions are caused by thermal recovery that suppresses the Si4+-Zr4+ repulsion between the SiO4 tetrahedron and ZrO8 dodecahedron. These changes in structure can be understood by considering the cation-cation repulsion between the SiO4-ZrO8 shared edges.
  • T. Ishii, H. Kojitani, M. Akaogi
    J. Geophys. Res., 124(4) 3491-3507, 2019  Peer-reviewed
  • H. Kojitani, M. Yamazaki, M. Kojima, Y. Inaguma, D. Mori, M. Akaogi
    Physics and Chemistry of Minerals, Nov, 2018  Peer-reviewed
  • Takayuki Ishii, Tsubasa Sakai, Hiroshi Kojitani, Daisuke Mori, Yoshiyuki Inaguma, Yoshitaka Matsushita, Kazunari Yamaura, Masaki Akaogi
    Inorganic Chemistry, 57(11) 6648-6657, Jun, 2018  Peer-reviewed
  • M. Akaogi, A. Kawahara, H. Kojitani, K. Yoshida, Y. Anegawa, T. Ishii
    Am. Mineral., 103(1) 161-170, 2018  Peer-reviewed
  • M. Akaogi, S. Hashimoto, H. Kojitani
    Phys. Earth Planet. Inter., 281 1-7, 2018  Peer-reviewed
  • K. Soda, D. Kobayashi, T. Mizui, M. Kato, Y. Shirako, K. Niwa, M. Hasegawa, M. Akaogi, H. Kojitani, E. Ikenaga, T. MuroAKAOGI Masaki
    J. Phys. Soc. Japan, 87(044701) 1-5, 2018  Peer-reviewed
  • T. Ishii, H. Kojitani, M. Akaogi
    Phys. Earth Planet. Inter., 274 127-137, 2018  Peer-reviewed
  • Hiroshi Kojitani, Saki Terata, Maki Ohsawa, Daisuke Mori, Yoshiyuki Inaguma, Masaki Akaogi
    AMERICAN MINERALOGIST, 102(10) 2032-2044, Oct, 2017  Peer-reviewed
    High-pressure high-temperature phase relation experiments in Mg14Si5O24 were performed using a 6-8 multi-anvil high-pressure apparatus in the pressure range of 12-22 GPa and temperature range of 1673-2173 K. We first found that Mg14Si5O24 anhydrous phase B (Anh-B) dissociates to Mg2SiO4 wadsleysite (Wd) and MgO periclase (Per) at about 18 GPa and 1873 K. From the results of the high-pressure experiments, the phase boundaries of 5 Mg2SiO4 forsterite (Fo) + 4 Per = Anh-B and Anh-B = 5 Wd + 4 Per were determined. In addition, the isobaric heat capacity (CP) of Anh-B was measured by differential scanning calorimetry in the temperature range of 300-770 K and the thermal relaxation method using a Physical Property Measurement System (PPMS) in the range of 2-303 K. From the measured low-temperature C-P, the standard entropy (S-298.15 degrees) of Anh-B was determined to be 544.4(2) J/(mol . K). We also performed high-temperature X-ray diffraction measurements in the range 303-773 K to determine the thermal expansivity (a) of Anh-B. The obtained CP and a were theoretically extrapolated to higher temperature region using a lattice vibrational model calculation partly based on Raman spectroscopic data. Thermodynamic calculations by adopting the thermochemical and thermoelastic data for Anh-B obtained in this study and the estimated formation enthalpy for Anh-B of -13 208 kJ/mol gave phase equilibrium boundaries for 5 Fo + 4 Per = Anh-B and Anh-B = 5 Wd + 4 Per that were consistent with those determined by the present high-pressure high-temperature experiments. The results clarified that, in the Mg14Si5O24 system, Anh-B is stable between 12 and 18 GPa at the expected temperatures of the Earth's mantle.
  • Kazuo Soda, Tatsuya Mizui, Mai Komabuchi, Masahiko Kato, Toshiki Terabe, Kentaro Suzuki, Ken Niwa, Yuichi Shirako, Masashi Hasegawa, Masaki Akaogi, Hiroshi Kojitani, Eiji Ikenaga
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 86(6), Jun, 2017  Peer-reviewed
    Using microbeam hard X-ray photoelectron spectroscopy, we clarified the valence-band electronic structures and chemical states of platinum-group metal (Ru, Ir, and Pt) pernitrides, which have been synthesized in supercritical nitrogen fluid under extremely high pressures and temperatures. Their nitrogen contents relative to the platinum-group metal are estimated to be 2 from the photoemission intensity, which is consistent with the studies reported to date. The observed valence-band structures agree quite well with theoretically predicted structures for the pyrite-type PtN2, arsenopyrite-type IrN2, and marcasite-type RuN2. The origin of their extremely large bulk moduli is discussed based on the current results of the valence-band structures and core-level chemical shifts.
  • M. Akaogi, K. Abe, H. Yusa, T. Ishii, T. Tajima, H. Kojitani, D. Mori, Y. Inaguma
    PHYSICS AND CHEMISTRY OF MINERALS, 44(1) 63-73, Jan, 2017  Peer-reviewed
    Phase relations in FeTiO3 were precisely determined at 25-35 GPa and 600-1600 A degrees C using multianvil high-pressure experiments with tungsten carbide anvils. Pressure generation up to about 36 GPa at 1600 A degrees C was evaluated using Al2O3 solubility in MgSiO3 perovskite (Pv) in the system MgSiO3-Al2O3. At about 28 GPa, FeTiO3 Pv dissociates into an assemblage of calcium titanate (CT)-type Fe2TiO4 + orthorhombic-I (OI)-type TiO2 below 1200 A degrees C. However, above 1200 A degrees C at 28 GPa, FeTiO3 Pv decomposes into a new, denser phase assemblage of CT-type Fe2TiO4 + a new compound of FeTi2O5. The new phase FeTi2O5 was recovered as an amorphous phase at 1 atm. In situ X-ray diffraction experiments at 35.1 GPa indicated that the new phase (N-p) FeTi2O5 has orthorhombic symmetry with cell parameters a = 8.567(2) a"<<, b = 5.753(1) a"<< and c = 5.257(1) a"<<. In addition, the assemblage of CT-type Fe2TiO4 + OI-type TiO2 changes to FeO wustite (Wu) + OI-type TiO2 at about 33 GPa below 1000 A degrees C. The phase assemblages in FeTiO3 are denser in the order: FeTiO3 (Pv) -> 1/2Fe(2)TiO(4) (CT) + 1/2TiO(2) (OI) -> 1/3Fe(2)TiO(4) (CT) + 1/3FeTi(2)O(5) (N-p) -> FeO (Wu) + TiO2 (OI). Our results indicate that the upper stability limit of FeTiO3 Pv is about 28 GPa at 600-1600 A degrees C. This puts a constraint on peak shock pressure for formation of naturally discovered lithium niobate-type FeTiO3 which was interpreted to be retrograde transition product of FeTiO3 Pv on release of shock pressure.
  • Kojitani, H, T. Inoue, M. Akaogi
    J. Geophys. Res. Solid Earth, 121 729-742, 2016  Peer-reviewed
  • Soda K., Kobayashi D., Mizui T., Kato M., Shirako Y., Niwa K., Hasegawa M., Akaogi M., Kojitani H.
    Meeting Abstracts of the Physical Society of Japan, 70 1239-1239, 2015  
  • Fujino Kiyoshi, Ishii Takayuki, Kunimoto Takehiro, Kojitani Hiroshi, Akaogi Masaki
    Abstracts for Annual Meeting of Japan Association of Mineralogical Sciences, 2015 70-70, 2015  Peer-reviewed
    XRD and TEM observations of recovered high-pressure phase Mg2Cr2O5 revealed split spots around the position of h = n + 1/2 (n: integer) in the diffraction patterns of the basic structure of this phase. It was proved that these split spots were formed by the periodic arrangement of the anti-phase boundaries in the structure. Then, the relation between the formation of these periodic anti-phase boundaries and the high-pressure phase transformation of Mg2Cr2O5 is discussed.
  • Akaogi, M. Abe, K. Yusa, H. Kojitani, H. Mori, D. Inaguma, Y
    Physics and Chemistry of Minerals, 42(6) 421-429, 2015  Peer-reviewed
    High-pressure high-temperature phase transitions of ZnTiO3 ilmenite were examined using multianvil apparatus up to 25.5 GPa and 1,500 °C and diamond anvil cell to 26.5 GPa and about 2,000 °C. Combined results of the multianvil quench experiments and in situ diamond anvil cell experiments indicated that at about 10 GPa and 1,200 °C ZnTiO3 ilmenite transforms to orthorhombic perovskite which is converted to lithium niobate phase on release of pressure. The boundary of the ilmenite–provskite transition is expressed by P(GPa) = 15.9 − 0.005T (°C). The high-pressure experiments also indicated that at 20–24 GPa and 1,000–1,400 °C ZnTiO3 orthorhombic perovskite dissociates into rocksalt-type ZnO + baddeleyite-type TiO2 which are recovered, respectively, as wurtzite-type ZnO and α-PbO2-type TiO2 at 1 atm. The boundary of the perovskite dissociation is expressed by P(GPa) = 8.7 + 0.011T (°C). Molar volume changes of ZnTiO3 at ambient conditions were estimated as −4.7 % for the ilmenite–perovskite transition and −3.5 % for the perovskite decomposition into the oxides. The absence of CaIrO3-type postperovskite in ZnTiO3 is consistent with that dissociation of ZnTiO3 perovskite into the oxides has the larger molar volume change than −1 to −2 % of the perovskite–postperovskite transition in various ABO3 compounds and with previous data that ABO3 perovskites with relatively ionic B–O bonds do not transform to the postperovskite. The transition behaviors of ZnTiO3 are similar to those of MnTiO3 and FeTiO3, but ZnTiO3 perovskite dissociates into the constituent oxides.
  • Ishii Takayuki, Kojitani Hiroshi, Fujino Kiyoshi, Yusa Hitoshi, Mori Daisuke, Inaguma Yoshiyuki, Matsushita Yoshitaka, Yamaura Kazunari, Akaogi Masaki
    American Mineralogist, 100(1) 59-65, 2015  Peer-reviewed
    We determined phase relations in MgCr2O4 at 12-28 GPa and 1000-1600 °C using a multi-anvil apparatus. At 12-15 GPa, spinel-type MgCr2O4 (magnesiochromite) first decomposes into a mixture of new Mg2Cr2O5 phase + corundum-type Cr2O3 at 1100-1600 °C, but it dissociates first into MgO periclase + corundum-type Cr2O3 at 1000 °C. At about 17-19 GPa, the mixture of Mg2Cr2O5 phase + corundum-type Cr2O3 transforms to a single MgCr2O4 phase. Structure refinements using synchrotron X-ray powder diffraction data indicated that the high-pressure MgCr2O4 phase has a CaTi2O4-type structure (Cmcm), and that the basic structure of the Mg2Cr2O5 phase is the same as that of recently found modified ludwigite-type Mg2Al2O5 and Fe2Cr2O5 (Pbam). The phase relations in this study may suggest that natural chromitites in the Luobusa ophiolite regarded as the deep-mantle origin were derived from the mantle shallower than the depths corresponding to pressure of 12-15 GPa because of absence of the assemblage of (Mg, Fe)2Cr2O5 + Cr2O3 in the chromitites.
  • Hitoshi Yusa, Taku Tsuchiya, Masaki Akaogi, Hiroshi Kojitani, Daisuke Yamazaki, Naohisa Hirao, Yasuo Ohishi, Takumi Kikegawa
    INORGANIC CHEMISTRY, 53(21) 11732-11739, Nov, 2014  Peer-reviewed
    The postperovskite phase of ZnGeO3 was confirmed by laser heating experiments of the perovskite phase under 110-130 GPa at high temperature. Ab initio calculations indicated that the phase transition occurs at 133 GPa at 0 K. This postperovskite transition pressure is significantly higher than those reported for other germanates, such as MnGeO3 and MgGeO3. The comparative crystal chemistry of the perovskite-to-postperovskite transition suggests that a relatively elongated b-axis in the low-pressure range resulted in the delay in the transition to the postperovskite phase. Similar to most GdFeO3-type perovskites that transform to the CaIrO3-type postperovskite phase, ZnGeO3 perovskite eventually transformed to the CaIrO3-type postperovskite phase at a critical rotational angle of the GeO6 octahedron. The formation of the postperovskite structure at a very low critical rotational angle for MnGeO3 suggests that relatively large divalent cations likely break down the corner-sharing GeO6 frameworks without a large rotation of GeO6 to form the postperovskite phase.
  • Yuichi Shirako, Xia Wang, Yoshihiro Tsujimoto, Kie Tanaka, Yanfeng Guo, Yoshitaka Matsushita, Yoshihiro Nemoto, Yoshio Katsuya, Youguo Shi, Daisuke Mori, Hiroshi Kojitani, Kazunari Yamaura, Yoshiyuki Inaguma, Masaki Akaogi
    INORGANIC CHEMISTRY, 53(21) 11616-11625, Nov, 2014  Peer-reviewed
    The polycrystalline MO2's (HP-PdF2-type MO2, M = Rh, Os, Pt) with high-pressure PdF2 compounds were successfully synthesized under high-pressure conditions for the first time, to the best of our knowledge. The crystal structures and electromagnetic properties were studied. Previously unreported electronic properties of the polycrystalline HP-PdF2-type RuO2 and IrO2 were also studied. The refined structures clearly indicated that all compounds crystallized into the HP-PdF2-type structure, M4+O22-, rather than the pyrite-type structure, Mn+(O-2)(n-) (n < 4). The MO2 compounds (M = Ru, Rh, Os, Ir) exhibited metallic conduction, while PtO2 was highly insulating, probably because of the fully occupied t2g band. Neither superconductivity nor a magnetic transition was detected down to a temperature of 2 K, unlike the case of 3d transition metal chalcogenide pyrites.
  • 糀谷浩, 白子 雄一, 遊佐斉, 赤荻正樹
    高圧力の科学と技術, 24(3) 204-2011, 2014  Peer-reviewed
    Compounds with the postperovskite (PPv)-type crystal structure were searched using a Kawai-type multi-anvil high-pressure apparatus. We succeeded in synthesizing PPv-type CaRuO3, CaRhO3, NaNiF3, NaCoF3, and NaZnF3and found that perovskite (Pv)-type NaMnF3decomposes into two phases above about 10 GPa instead of phase transition to a PPv phase. The PPv-type CaRuO3, CaRhO3and NaNiF3are quenchable without partial back-transformation to the Pv-type structure. The structure refinements of the synthesized compounds suggest that PPv-type NaNiF3and NaZnF3are preferable to the other PPv-type compounds as analogue materials of PPv-type MgSiO3which is accepted as a major constituent mineral in the Earth's lowermost mantle.
  • Y. Shirako, H. Satsukawa, X. X. Wang, J. J. Li, Y. F. Guo, M. Arai, K. Yamaura, M. Yoshida, H. Kojitani, T. Katsumata, Y. Inaguma, K. Hiraki, T. Takahashi, M. Akaogi
    PHYSICAL REVIEW B, 83(17), May, 2011  Peer-reviewed
    A quasi-one-dimensional magnetism was discovered in the post-perovskite CaRuO3 (Ru4+: 4d(4), Cmcm), which is isocompositional with the perovskite CaRuO3 (Pbnm). An antiferromagnetic (AFM) spin-chain function with -J/k(B) = 350 K reproduces the experimental curve of the magnetic susceptibility vs temperature well, suggesting long-range AFM correlations. The anisotropic magnetism is probably owed to the d(yz)-2p(pi)-d(zx) and d(zx)-2p(pi)-d(yz) superexchange bonds along the a axis. The Sommerfeld coefficient of the specific heat is fairly small, 0.16(2) mJ mol(-1) K-2, indicating that the magnetism reflects the localized nature of the 4d electrons. This is an observation of an integer (S = 1) spin-chain AFM in the 4d electron system.
  • Mori, Daisuke, Sakaebe, Hikari, Shikano, Masahiro, Kojitani, Hiroshi, Tatsumi, Kuniaki, Inaguma, Yoshiyuki
    Journal of Power Sources, 196(16) 6934-6938, 2011  Peer-reviewed
  • Kojitani Hiroshi, Enomoto Akito, Akaogi Masaki, Miura Hiroyuki
    Abstracts for Annual Meeting of Japan Association of Mineralogical Sciences, 2008 128-128, 2008  
    A new high-pressure phase with the composition of Mg2Al2O5 was found. A crystal structure model of the Mg2Al2O5 phase was constructed based on that of ludwigite because the X-ray diffraction pattern of the former is very similar to that of the latter. In the model, (Mg, Al)O6 octahedra connected by edge-sharing and corner-sharing form triangular tunnels in which Mg ions are accommodated. Mg ions in the tunnels have a coordinate environment with trigonal prism-type MgO6. Systematic absences of reflections indicate that the space group of the structure is Pba2 or Pbam. The Rietveld analysis showed that R factor in the case of Pbam was smaller than that in the case of Pba2. This result suggests that the space group of the Ma2Al2O5 phase is Pbam.
  • Akaogi Masaki, Haraguchi Maiko, Yaguchi Masato, Kojitani Hiroshi
    Abstracts for Annual Meeting of Japan Association of Mineralogical Sciences, 2008 185-185, 2008  
    High pressure experiments and calorimetric measurements were performed to clarify the stability field of CaAl4Si2O11 CAS-phase. It was shown that CAS-phase is stable above about 13 GPa and 1100°C. Combining the measured enthalpy data with published ones, dissociation boundary of CAS-phase into a mixture of Ca-perovskite, corundum and stishovite was calculated. Assuming reasonable geotherms, it was concluded that CAS-phase is stable in the transition zone and upper part of the lower mantle.
  • Kuribayashi T, Nagase T, Kojitani H, Akaogi M, Hemley R, Mao H-k, Kudoh Y
    Acta Crystallographica, A64 C90, 2008  Peer-reviewed
  • Akaogi Masaki, Ajiro Hideaki, Kojitani Hiroshi
    Abstracts for Annual Meeting of the Mineralogical Society of Japan, 2005 3-3, 2005  
    Using a multianvil apparatus, high pressure phase relations in NaAlSi3O8 and in the system NaAlSi3O8-KAlSi3O8 were examined up to 24 GPa. Synthesized samples were examined using microfocused and powder X-ray diffraction, and were analyzed with SEM-EDS. At 21-22 GPa and 800-2000 C, NaAlSi2O6 jadeite and stishovite changed to NaAlSiO4 calcium ferrite and stishovite, but NaAlSi3O8 holladite was not observed. Maximum solubility of NaAlSi3O8 in KAlSi3O8 holladite was 40-50 mol% at 1400 C, which was almost the same as 40 mol% at 1000 C. Above results suggest that NaAlSi3O8 hollandite would be stable at higher temperature than 2000 C. This interpretation is consistent with natural observation of NaAlSi3O8-rich hollandite in shocked meteorites.
  • Kojitani Hiroshi, Akaogi Masaki
    Abstracts for Annual Meeting of the Mineralogical Society of Japan, 2005 5-5, 2005  
  • M Akaogi, H Kojitani, K Matsuzaka, T Suzuki
    PROPERTIES OF EARTH AND PLANETARY MATERIALS AT HIGH PRESSURE AND TEMPERATURE, 101 373-384, 1998  Peer-reviewed
    Compositions of coexisting spinel and magnesiowustite in the system Mg2SiO4-Fe2SiO4 have been experimentally determined at 18.5 and 20.4 GPa at 1873 K to constrain equilibrium boundaries of the postspinel transitions in relatively Fe2SiO4-rich composition. Calorimetric measurements of pyroxene and perovskite solid solutions in the system Mg2SiO3-Fe2SiO3 have been performed by a differential drop-solution method in a controlled atmosphere. Using the above data with published thermodynamic data on the high-pressure phases in the system MgO-FeO-SiO2, phase relations of the postspinel transitions have been thermodynamically calculated. The calculated boundaries are generally consistent with the experiments by Ito and Takahashi. The calculated transition interval of spinel to perovskite + magnesiowustite is about 0.01-0.18 GPa for mantel spinel composition. The perovskite-magnesiowustite field expands to the Fe2SiO4-rich side with increasing temperature, in contrast to previous thermodynamic calculations. The present thermodynamic data show stability of an assemblage of magnesiowustite-stishovite in the Mg2SiO4-rich composition of the Mg2SiO4-Fe2SiO4 system between the fields of spinel and of perovskite + magnesiowustite at relatively low temperatures.

Misc.

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  • Takayuki Ishii, Noriyoshi Tsujino, Hidekazu Arii, Kiyoshi Fujino, Nobuyoshi Miyajima, Hiroshi Kojitani, Takehiro Kunimoto, Masaki Akaogi
    AMERICAN MINERALOGIST, 102(10) 2113-2118, Oct, 2017  
    The crystal structure of the high-pressure Mg2Cr2O5 phase was studied by single-crystal X-ray diffraction (XRD) analysis for the recovered samples. The 61 parameters including anisotropic displacement parameters of each atom and site occupancies of Mg and Cr in cation sites were refined with R-1 = 1.26%, wR(2) = 4.33%, and S-fit = 1.265 for 470 unique reflections. The results show that the structure of the recovered Mg2Cr2O5 phase is the same as modified ludwigite (mLd)-type Mg2Al2O5 [space group: Pbam (no. 55)], and the lattice parameters are a = 9.6091(2), b = 12.4324(2), c = 2.8498(1) angstrom (Z = 4). The refined structure of the Mg2Cr2O5 phase has four (Mg, Cr)O-6 octahedral sites and a MgO6 trigonal prism site, and is similar to but distinct from that of CaFe3O5-type Mg2Fe2O5 phase, which has two octahedral sites and a bicapped trigonal prism site with two longer cation-oxygen bonds. The isotropic atomic displacement parameter of the trigonal prism site cation in mLd-type Mg2Cr2O5 is relatively small compared with that of CaFe3O5-type Mg2Fe2O5, suggesting that the trigonal prism site is less flexible for cation substitution than that of CaFe3O5-type structure. To stabilize mLd-type A(2)(2+)B(2)(3+)O(5) phase, it would be an important factor for the B3+ cation to have high octahedral-site preference, resulting in only A(2+) cation being accommodated in the tight trigonal prism site. Our study also suggests that mLd-type phase with (Mg, Fe2+)(2)Cr2O5 composition would crystallize as one of decomposed phases of chromitites, when the chromitites were possibly subducted into the mantle transition zone.
  • Takayuki Ishii, Hiroshi Kojitani, Shoichi Tsukamoto, Kiyoshi Fujino, Daisuke Mori, Yoshiyuki Inaguma, Noriyoshi Tsujino, Takashi Yoshino, Daisuke Yamazaki, Yuji Higo, Kenichi Funakoshi, Masaki Akaogi
    AMERICAN MINERALOGIST, 99(8-9) 1788-1797, Aug, 2014  
    We determined phase relations in FeCr2O4 at 12-28 GPa and 800-1600 degrees C using a multi-anvil apparatus. At 12-16 GPa, FeCr2O4 spinel (chromite) first dissociates into two phases: a new Fe2Cr2O5 phase + Cr2O3 with the corundum structure. At 17-18 GPa, the two phases combine into CaFe2O4-type and CaTi2O4-type FeCr2O4 below and above 1300 degrees C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi2O4-structured FeCr2O4 (Cmcm), and indicated that the Fe2Cr2O5 phase is isostructural to a modified ludwigite-type Mg2Al2O5 (Pbam). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe2O4-type FeCr2O4 is unquenchable and is converted into another FeCr2O4 phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr2O4 phase has a new structure related to CaFe2O4-type. The high-pressure phase relations in FeCr2O4 reveal that natural FeCr2O4-rich phases of CaFe2O4- and CaTi2O4-type structures found in the shocked Suizhou meteorite were formed above about 18 GPa at temperature below and above 1300 degrees C, respectively. The phase relations also suggest that the natural chromitites in the Luobusa ophiolite previously interpreted as formed in the deep-mantle were formed at pressure below 12-16 GPa.
  • Masaki Akaogi, Yuichi Shirako, Hiroshi Kojitani, Takayuki Nagakari, Hitoshi Yusa, Kazunari Yamaura
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS, 228 160-169, Mar, 2014  
    Phase transitions in NaZnF3 and NaMnF3 were examined up to 24 GPa and 1100 degrees C using a multianvil apparatus. NaZnE3 perovskite transforms to postperovskite above 11-16 GPa at 600-1000 degrees C, and the postperovskite is quenchable at ambient conditions. The NaZnE3 perovskite-postperovskite transition boundary is expressed as P (GPa) = 4.9 + 0.011T (degrees C). At 8-11 GPa and 900-1100 degrees C, NaMnF3 perovskite dissociates into two phases of Na3Mn2F7 and MnF2. The latter phase is suggested to have the structure of orthorhombic-I type ZrO2 or cotunnite. Using available experimental data on the perovskite-postperovskite transitions in thirteen compounds of A(2+)B(4+)O(3) and A(+)B(2+)F(3), several crystal-chemical characteristics of the transition are elucidated as follows. In the transition, the volume change is between -1% and -2%, and the Clapeyron slope of the boundary is 10-17 MPa/degrees C. These support reliability of recently determined Clapeyron slope of 13 MPa/degrees C in MgSiO3 which suggests that the perovskite-postperovskite boundary intersects the temperature profile twice in the D" layer. Postperovskites of ABX(3) whose enthalpies are higher by more than 70 kj/mol relative to the phase stable at I atm are unquenchable, while those by less than 15 kj/mol are quenchable to ambient conditions. Structure refinements indicate that A(+)B(2+)F(3) postperovskites quenched at 1 atm are more similar to that of MgSiO3 postperovskite at high pressure, than those of quenched A(2+)B(4+)O(3) postperovskites. With increasing pressure, octahedral tilt angles of both A(2+)B(4+)O(3) and A(+)B(2+)F(3) perovskites increase, resulting in transition to postperovskite at the angle of about 26 degrees, and fluoride perovskites are more rapidly distorted with pressure than oxide perovskites. Covalent character of B-X bonds of ABX(3) postperovskite is suggested to be favorable for stabilization of the postperovskite structure. All these features suggest that NaNiF3 is a good quenchable, low-pressure analogue compound to MgSiO3 to investigate the perovskite-postperovskite transition. (C) 2013 Elsevier B.V. All rights reserved.
  • Shuangmeng Zhai, Masaki Akaogi, Hiroshi Kojitani, Weihong Xue, Eiji Ito
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS, 228 144-149, Mar, 2014  
    gamma-Ca-3(PO4)(2), naturally known as tuite, is regarded as an important potential reservoir for rare earth elements and large ion lithophile elements. It is a high-pressure polymorph of beta-Ca-3(PO4)(2) whitlockite and a decomposed product of apatite under high-pressure and temperature. Drop-solution enthalpies of beta- and gamma-Ca-3(PO4)(2) were obtained as 298.59 +/- 3.02 and 278.74 +/- 2.98 kJ/mol, respectively, by the drop-solution calorimetry with 2PbO center dot B2O3 solvent at 978 K. Thus the enthalpy of transition from beta- to gamma-Ca-3(PO4)(2) at 298 K (Delta H-tr,298(o)) was 19.85 +/- 4.24 kJ/mol. The isobaric heat capacities of beta- and gamma-Ca-3(PO4)(2) were measured at temperature range of 300-770 K by differential scanning calorimetry, and compared with the results calculated from the Kieffer model. The equilibrium phase boundary between beta- and gamma-Ca-3(PO4)(2) was calculated using present measured data combined with other available thermochemical and thermoelastic data. The calculated boundary gave a phase transition boundary with a dP/dT slope of 4.7 +/- 0.2 MPa/K in the temperature range of 900-2000 K. Based on the phase relationship, the occurrences of tuite and whitlockite in meteorites are discussed. (C) 2013 Elsevier B.V. All rights reserved.
  • Yoshiyuki Inaguma, Akihisa Aimi, Yuichi Shirako, Daichi Sakurai, Daisuke Mori, Hiroshi Kojitani, Masaki Akaogi, Masanobu Nakayama
    JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 136(7) 2748-2756, Feb, 2014  
    A polar LiNbO3-type (LN-type) titanate ZnTiO3 has been successfully synthesized using ilmenite-type (IL-type) ZnTiO3 under high pressure and high temperature. The first principles calculation indicates that LN-type ZnTiO3 is a metastable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement using synchrotron powder X-ray diffraction data reveals that LN-type ZnTiO3 crystallizes into a hexagonal structure with a polar space group R3c and exhibits greater intradistortion of the TiO6 octahedron in LN-type ZnTiO3 than that of the SnO6 octahedron in LN-type ZnSnO3. The estimated spontaneous polarization (75 mu C/cm(2), 88 mu C/cm(2)) using the nominal charge and the Born effective charge (BEC) derived from density functional perturbation theory, respectively, are greater than those of ZnSnO3 (59 mu C/cm(2), 65 mu C/cm(2)), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the c-axis and the fact that the BEC of Ti (+6.1) is greater than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-type ZnTiO3 is greater than that of LiNbO3 (62 mu C/cm(2), 76 mu C/cm(2)), indicating that LN-type ZnTiO3, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-type ZnTiO3 is 24 times greater than that of LN-type ZnSnO3. The findings indicate that the intraoctahedral distortion, spontaneous polarization, and the accompanying SHG response are caused by the stabilization of the polar LiNbO3-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d(0) ions such as Ti4+.
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