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; 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：61.3(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：42.01(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：17 Percentile：97.27(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.
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：75.01(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：68.81(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：76.14(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：79.32(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.
(
3) under high pressureMatsuoka, Takehiro*; Kitayama, Takayasu*; Shimizu, Katsuya*; Nakamoto, Yuki*; Kagayama, Tomoko*; Aoki, Katsutoshi; Oishi, Yasuo*; Takemura, Kenichi*
High Pressure Research, 26(4), p.391 - 394, 2006/12
Times Cited Count：3 Percentile：26.57(Physics, Multidisciplinary)The electrical and structural properties of yttrium hydride YHx was studied by the in-situ measurements of the electrical resistance and X-ray diffraction up to 86 GPa. The electrical resistivity increased with applying pressure up to 10 GPa and showed a maximum which accompanied with the structural transition of metal lattice from hcp to fcc structure. In fcc phase the resistivity decreased significantly, however the value still remained larger by 4 orders of magnitude that that of pure yttrium metal. We could not confirm the insulator to metal transition up to 23 GPa. The resistivity showed another maximum in the large pressure region 40 60 GPa without the structural phase transition of metal lattice.
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：85.96(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：87.26(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.
Machida, Akihiko; Omura, Ayako; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
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Omura, Ayako; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
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Machida, Akihiko; Omura, Ayako; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
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Omura, Ayako; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
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Machida, Akihiko; Omura, Ayako; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
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Omura, Ayako; Machida, Akihiko; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
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Machida, Akihiko; Omura, Ayako*; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
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under high pressureMachida, Akihiko; Omura, Ayako*; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
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Machida, Akihiko; Omura, Ayako*; Watanuki, Tetsu; Aoki, Katsutoshi; Takemura, Kenichi*
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no abstracts in English
