Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
; Raman, infrared, and visible absorption spectroscopyKume, Tetsuji*; Oura, Hiroyuki*; Takeichi, Tomoo*; Omura, Ayako*; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Sasaki, Shigeo*; Shimizu, Hiroyasu*; Takemura, Kenichi*
Physical Review B, 84(6), p.064132_1 - 064132_7, 2011/08
Times Cited Count：19 Percentile：61.17(Materials Science, Multidisciplinary)Raman, IR, and visible absorption spectra of scandium trihydride (ScH) have been measured at high pressures up to 50 GPa at room temperature. The structural and electronic phase transitions were investigated. Successive hcp-intermediate-fcc phase transitions took place at 25 and 46 GPa. It was suggested that the intermediate phase of ScH has the same structure as that of YH. The energy gap of ScH is 1.7 eV at the ambient condition and is closed around 50 GPa, at which the crystal structure transforms to fcc.
Machida, Akihiko; Watanuki, Tetsu; Omura, Ayako*; Ikeda, Tomohiro*; Aoki, Katsutoshi; Nakano, Satoshi*; Takemura, Kenichi*
Solid State Communications, 151(5), p.341 - 345, 2011/03
Times Cited Count：10 Percentile：41.92(Physics, Condensed Matter)The compressibility of lanthanum (La) metal and its hydrides were measured at room temperature by high pressure synchrotron X-ray diffraction. La metal pressurized in a hydrogen medium forms a hydride with an fcc metal lattice, which likely contains hydrogen at a concentration close to 3.0. Equations of state have been determined by helium compression experiments for LaH
with tetrahedral interstitial sites fully occupied with hydrogen atoms and for LaH
with octahedral interstitial sites partially occupied with hydrogen atoms and tetrahedral sites fully occupied. Both hydrides possess fcc metal lattices. These values are three times larger than that of La metal and are very close to each other despite the difference in hydrogen occupation states. The hardening of the metal lattice by hydrogenation is attributed predominantly to hydrogen-metal interactions at the tetrahedral sites and is most pronounced for La, which has the largest ionic radius among rare-earth metals.
Takemura, Kenichi*; Watanuki, Tetsu; Owada, Kenji; Machida, Akihiko; Omura, Ayako*; Aoki, Katsutoshi
Journal of Physics; Conference Series, 215, p.012017_1 - 012017_4, 2010/03
Times Cited Count：18 Percentile：97.26(Instruments & Instrumentation)Powder X-ray diffraction experiments have been done on Ne up to 237 GPa at room temperature. Ne remains in the fcc phase up to the highest pressure. In order to observe the weak 200 reflection at ultrahigh pressure we have used the tube-cross slit at the exit side of X-rays, which effectively reduced the scattered X-rays by the diamond anvil. The lattice parameters obtained from the 111 and 200 reflections significantly differ each other, implying the existence of sizable uniaxial stress in Ne at ultrahigh pressure.
Aoki, Katsutoshi; Machida, Akihiko; Omura, Ayako*; Watanuki, Tetsu
Koatsuryoku No Kagaku To Gijutsu, 18(3), p.273 - 278, 2008/08
The highly compressed state of metal hydride has been investigated to explore structural and electronic transitions characteristic to hydride. The hydrogen atoms play the lead in the transitions, showing displacement in or transfer between the interstitial sites of a metal lattice. One example is the band gap closing observed for a long period lattice of yttrium tri-hydride, appearing at pressures above 12 GPa. Another example is photochromic event observed again for yttrium hydride. The former is likely accompanied by the in-site displacement of hydrogen atoms and the latter by the site-to-site transfer.
Omura, Ayako*; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Nakano, Satoshi*; Takemura, Kenichi*
Applied Physics Letters, 91(15), p.151904_1 - 151904_3, 2007/10
Times Cited Count：34 Percentile：74.97(Physics, Applied)Transparent orange yttrium hydride turns to black when illuminated by visible laser light at pressures of several gigapascals at room temperature. The marked reduction in optical transmittance extends over the infrared region, suggesting that illumination creates persistent free carriers. The opaque black sample returns to the transparent orange hydride during room-temperature annealing for a few hours. Photochromism is pronounced for the coexistent state of the metallic fcc-YH
and the insulating hexagonal-YH state, but is depressed for the single phase of hexagonal-YH. The results indicate that light illumination can modify the optical and possibly electronic properties during a certain period of times.
Omura, Ayako; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Nakano, Satoshi*; Takemura, Kenichi*
Journal of Alloys and Compounds, 446-447, p.598 - 602, 2007/10
Times Cited Count：19 Percentile：68.7(Chemistry, Physical)Pressure-induced structural change from hcp to fcc-like metal lattice was investigated for scandium hydride, ScHx, by X-ray diffraction measurement up to 60 GPa at room temperature. Hydride was prepared in a diamond anvil cell (DAC) by compressing the metal foil with fluid hydrogen. The metal foil reacted with fluid hydrogen to form ScH with an fcc metal lattice at about 4 GPa. Up to 5 GPa, fcc ScH changed for the most part to ScH with a hexagonal metal lattice, which persisted to about 25 GPa together with unreacted ScH. On further compression, the hcp lattice began to transform towards an fcc-like lattice. The transition completed at about 46 GPa. The successive change in diffraction pattern during the transition was quite similar to that observed for YHx, the transient state was not explained by the coexistence phase of ScH and ScH but able to be interpreted in terms of the gradual change in the sequential stacking of the metal layers.
Machida, Akihiko; Omura, Ayako*; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
Physical Review B, 76(5), p.052101_1 - 052101_4, 2007/08
Times Cited Count：34 Percentile：76.74(Materials Science, Multidisciplinary)The pressure induced structural transition of yttrium tri-hydride YH
has been investigated by X-ray diffraction experiments under hydrostatic pressure at room temperature. The hexagonal metal lattice transforms to a face-centered cubic (fcc) one through an intermediate state, which appears between 12-22 GPa. The diffraction patterns of the intermediate state are interpreted in terms of long-period rhombohedral polytypes with the periodic arrangements of hexagonal-type (
) and fcc-type (
) stacking layers of the yttrium metals. These polytypes gradually transforms toward the fcc metal lattice with a successive increase in the fcc-type component upon compression. The hydrogen-hydrogen repulsive interactions and hydrogen-metal bonding likely play a dominant role in the gradual hexagonal-fcc lattice conversion via the long-period polytypic structures.
by Raman and visible absorption spectroscopyKume, Tetsuji*; Oura, Hiroyuki*; Sasaki, Shigeo*; Shimizu, Hiroyasu*; Omura, Ayako*; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
Physical Review B, 76(2), p.024107_1 - 024107_5, 2007/07
Times Cited Count：38 Percentile：79.26(Materials Science, Multidisciplinary)Raman and visible absorption spectra of YH were measured at high pressures, in order to clarify the structural and electronic phase transitions. The Raman results revealed the presence of an intermediate phase at 9-24 GPa between the low-pressure hexagonal and high-pressure fcc phases. The Y-framework structure and the positions of interstitial H atoms are changed on the transition to the intermediate phase. The results of the absorption measurements demonstrate that the optical band gap begins to close in response to the phase transition.
Watanuki, Tetsu; Machida, Akihiko; Ikeda, Tomohiro*; Omura, Ayako*; Kaneko, Hiroshi; Aoki, Katsutoshi; Sato, Taku*; Tsai, A. P.*
Philosophical Magazine, 87(18-21), p.2905 - 2911, 2007/06
Times Cited Count：46 Percentile：87.07(Materials Science, Multidisciplinary)We have constructed a single-crystal synchrotron X-ray diffraction system to precisely study the structure under hydrostatic pressure conditions at low temperatures and applied it to a study on the phase transition phenomena of a Cd-Yb periodic approximant and a Cd-Yb quasicrystal. Four phases were newly observed for the 1/1 approximant crystal in a 
span up to 5.2 GPa and down to 10 K. The innermost part of the atomic clusters of Cd
tetrahedra exhibited various orientational ordering sensitively depending on pressure and temperature. High pressure diffraction measurements using a highly parallel synchrotron X-ray beam and a hydrostatically compressed single crystal enabled us to detect the weak diffractions due to the subtle structural changes.
Aoki, Katsutoshi; Watanuki, Tetsu; Machida, Akihiko; Omura, Ayako
Kotai Butsuri, 41(11), p.84 - 92, 2006/11
Compression of rare-earth metal, Y, with hydrogen fluid produced insulating trihysdide YH, which showed a structural transition from hexagonal to fcc metal lattice at about 10 GPa and successively an electronic transition with band gap closure at 23 GPa on further compression.
Aso, Naofumi*; Owada, Kenji; Watanuki, Tetsu; Machida, Akihiko; Omura, Ayako; Inami, Toshiya; Homma, Yoshiya*; Shiokawa, Yoshinobu*; Hirota, Kazuma*; Sato, Noriaki*
Journal of the Physical Society of Japan, 75(Suppl.), p.88 - 90, 2006/08
no abstracts in English
Machida, Akihiko; Omura, Ayako; Watanuki, Tetsu; Ikeda, Takashi; Aoki, Katsutoshi; Nakano, Satoshi*; Takemura, Kenichi*
Solid State Communications, 138(9), p.436 - 440, 2006/06
Times Cited Count：52 Percentile：85.91(Physics, Condensed Matter)no abstracts in English
at high pressureOmura, Ayako; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Nakano, Satoshi*; Takemura, Kenichi*
Physical Review B, 73(10), p.104105_1 - 104105_7, 2006/03
Times Cited Count：56 Percentile：87.2(Materials Science, Multidisciplinary)Infrared vibrational absorption spectra are measured for yttrium trihydride at room temperature and pressures up to 30 GPa. The spectral change that begins near 12 GPa is interpreted in terms of a hcp-fcc structural transition, which agrees with previous X-ray diffraction measurements. For the hydrogen vibrations in the low-pressure hcp phase, the mode Gr
neisen parameters are derived from the observed peak frequency shifts with pressure and the reported bulk modulus. The value of 1.91 for the octahedral-site vibration is three times larger than those for the tetrahedral-site vibrations, suggesting hybridization between the hydrogen-1
and yttrium-4
orbitals at the octahedral site. The infrared transmission spectra collapse when the high-pressure fcc phase is compressed beyond 23 GPa. The band gap abruptly closes without a structural change in the fcc metal lattice. The experimental results are contrast to the previous theoretical calculations predicting the electronic transition either with hcp-fcc structural change or in the hcp low-pressure phase without structural change. The transition mechanism is still inconclusive.
Omura, Ayako; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Nakano, Satoshi*; Takemura, Kenichi*
Proceedings of Joint 20th AIRAPT - 43rd EHPRG International Conference on High Pressure Science and Technology (CD-ROM), 3 Pages, 2005/06
Yttrium hydride shows a structural change from a cubic dihydride to a hexagonal trihydride by hydrogenation. This is accompanied by the metal-insulator transition. The theoretical calculation of yttrium hydride speculates that the hybridization between 4d-Y and 1s-H leads to the opening of a large energy gap. They also predict that a transition from insulator to metal occurs when the volume is reduced to about 85% of the equilibrium volume in yttrium trihydride. Recently, we performed an infrared spectroscopy of yttrium trihydride to investigate pressure-induced metallization under high pressure. The insulator-metal transition occurred with an abrupt disappearance of the optical gap of 
1 eV at 23 GPa. There is a possibility of the electronic transition because no structural changes occur above 20 GPa. The electronic transition can be attributed to the delocalization of the 1s electrons bound to the H+ core ions or the rearrangement of hydrogen atoms.
Fuchizaki, Kazuhiro*; Fujii, Yasuhiko*; Oishi, Yasuo*; Omura, Ayako*; Hamaya, Nozomu*; Katayama, Yoshinori; Okada, Taku
Journal of Chemical Physics, 120(23), p.11196 - 11199, 2004/06
Times Cited Count：22 Percentile：58.16(Chemistry, Physical)The location of the liquidus in the low-pressure crystalline phase of SnI was determined utilizing 
X-ray diffraction measurements under pressures up to approximately 
GPa. The liquidus is not well fitted to a monotonically increasing curve such as Simon's equation, but breaks near 
GPa and then becomes almost flat. The results are compared to those from molecular dynamics simulations. Ways to improve the model potential adopted in the simulations are discussed.
Watanuki, Tetsu; Machida, Akihiko; Omura, Ayako; Aoki, Katsutoshi; Owada, Kenji; Sato, Taku*; Tsai, A. P.*
no journal, ,
no abstracts in English
Machida, Akihiko; Omura, Ayako; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
no journal, ,
no abstracts in English
Omura, Ayako; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
no journal, ,
no abstracts in English
Watanuki, Tetsu; Machida, Akihiko; Omura, Ayako; Aoki, Katsutoshi; Owada, Kenji; Sato, Taku*; Tsai, A. P.*
no journal, ,
no abstracts in English
Watanuki, Tetsu; Machida, Akihiko; Omura, Ayako; Aoki, Katsutoshi; Owada, Kenji; Sato, Taku*; Tsai, A. P.*
no journal, ,
no abstracts in English
