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Journal Articles

Observation of dihydrogen bonds in high-pressure phases of ammonia borane by X-ray and neutron diffraction measurements

Nakano, Satoshi*; Sano, Asami; Hattori, Takanori; Machida, Shinichi*; Komatsu, Kazuki*; Fujihisa, Hiroshi*; Yamawaki, Hiroshi*; Goto, Yoshito*; Kikegawa, Takumi*

Inorganic Chemistry, 60(5), p.3065 - 3073, 2021/03

 Times Cited Count:0

X-ray and neutron diffraction analyses of ammonia borane were conducted at ambient and high pressures. The H-H distance in dihydrogen bonds was shorter than twice the van der Waals radius (2.4 ${AA}$). The half of the dihydrogen bonds were broken on phase transition from AP to the first high pressure phase (HP1) at approximately 1.2 GPa as revealed by an increase in the H-H distances. On further pressure increase, all of the H-H distances became shorter than 2.4 ${AA}$ again, implying the pressure-induced reformation of the dihydrogen bonds. Furthermore, the HP1 transformed to the second one with the structure of $$P2_1$$ (Z = 2) at about 11 GPa. In this phase transition, the inclination of the molecule axis became larger and the number of types of dihydrogen bonds increased from 6 to 11. Just before the third transition at 18.9 GPa, the shortest dihydrogen bond decreased to 1.65 ${AA}$. The present study experimentally first confirmed the breakage and reformation of the dihydrogen bonds by the structural change under pressure.

Journal Articles

Origin of magnetovolume effect in a cobaltite

Miao, P.*; Tan, Z.*; Lee, S. H.*; Ishikawa, Yoshihisa*; Torii, Shuki*; Yonemura, Masao*; Koda, Akihiro*; Komatsu, Kazuki*; Machida, Shinichi*; Sano, Asami; et al.

Physical Review B, 103(9), p.094302_1 - 094302_18, 2021/03

 Times Cited Count:0 Percentile:100(Materials Science, Multidisciplinary)

The layered perovskite PrBaCo$$_{2}$$O$$_{5.5}$$ demonstrates a strong negative thermal expansion (NTE) which holds potential for being fabricated into composites with zero thermal expansion. The NTE was found to be intimately associated with the spontaneous magnetic ordering, known as magneto-volume effect (MVE). Here we report with compelling evidences that the continuous-like MVE in PrBaCo$$_{2}$$O$$_{5.5}$$ is intrinsically of discontinuous character, originating from an magnetoelectric transition from an antiferromagnetic insulating large-volume (AFILV) phase to a ferromagnetic less-insulating small-volume (FLISV) phase. Furthermore, the magnetoelectric effect (ME) shows high sensitivity to multiple external stimuli such as temperature, carrier doping, hydrostatic pressure, magnetic field etc. In contrast to the well-known ME such as colossal magnetoresistance and multi-ferroic effect which involve symmetry breaking of crystal structure, the ME in the cobaltite is purely isostructural. Our discovery provides a new path way to realizing the ME as well as the NTE, which may find applications in new techniques.

Journal Articles

Experimental evidence for the existence of a second partially-ordered phase of ice VI

Yamane, Ryo*; Komatsu, Kazuki*; Gochi, Jun*; Uwatoko, Yoshiya*; Machida, Shinichi*; Hattori, Takanori; Ito, Hayate*; Kagi, Hiroyuki*

Nature Communications (Internet), 12, p.1129_1 - 1129_6, 2021/02

 Times Cited Count:1 Percentile:14.36(Multidisciplinary Sciences)

Ice exhibits extraordinary structural variety in its polymorphic structures. The existence of a new form of diversity in ice polymorphism has recently been debated in both experimental and theoretical studies, questioning whether hydrogen-disordered ice can transform into multiple hydrogen-ordered phases, contrary to the known one-to-one correspondence between disordered ice and its ordered phase. Here we report a new high-pressure phase, ice XIX, which is a second hydrogen-ordered phase of ice VI. This is the first discovery to demonstrate that disordered ice undergoes different manners of hydrogen ordering. Such multiplicity can appear in all disordered ice, and it widely provides a new research approach to deepen our knowledge, for example of the crucial issues of ice: the centrosymmetry of hydrogen-ordered configurations and potentially induced (anti-)ferroelectricity. Ultimately, this research opens up the possibility of completing the phase diagram of ice.

Journal Articles

Practical effects of pressure-transmitting media on neutron diffraction experiments using Paris-Edinburgh presses

Hattori, Takanori; Sano, Asami; Machida, Shinichi*; Ouchi, Keiichi*; Kira, Hiroshi*; Abe, Jun*; Funakoshi, Kenichi*

High Pressure Research, 40(3), p.325 - 338, 2020/09

 Times Cited Count:1 Percentile:100(Physics, Multidisciplinary)

To understand the practical effects of pressure-transmitting media (PTM) on neutron diffraction using Paris-Edinburgh presses, diffraction patterns of MgO were collected to approximately 20 GPa using PTMs of Pb, AgCl, 4:1 methano-ethanol (ME) mixture with and without heating, N$$_2$$, and Ar. Hydrostaticity in the sample chamber estimated from the MgO 220 peak width improves in the order of Pb, AgCl, Ar, ME mixture, N$$_2$$, and the heated ME mixture. Unlike previous results using a diamond anvil cell, the unheated ME mixture is superior to Ar even after freezing, probably due to the cup on the anvil face. Considering these results and the sizable coherent scattering of Ne, which would show good hydrostaticity, we conclude that the ME mixture (preferably the heated one) is the best PTM in neutron experiments up to 20 GPa, while Ar can be substituted when a sample is reactive to alcohols.

Journal Articles

Slow compression of crystalline ice at low temperature

Bauer, R.*; Tse, J. S.*; Komatsu, Kazuki*; Machida, Shinichi*; Hattori, Takanori

Nature, 585(7825), p.E9 - E10, 2020/09

 Times Cited Count:0 Percentile:100(Multidisciplinary Sciences)

Pressure-induced structural transformations in deuterated crystalline ice-Ih were studied in-situ at 100 K using neutron diffraction. Very long relaxation time was allowed between small pressure increments to promote transformations to the thermodynamic stable high pressure crystalline phases. The results contradict a recent report in which measurements under similar temperature and pressure environment show successive crystal-to-crystal transformations (Tulk, et.al., Nature 2019). Instead, ice Ih was found to transform partially to an amorphous form (high density amorphous, HDA) at 1.0 GPa and then ice VII started to emerge at 1.5 GPa, a pressure substantially lower than all earlier studies. During this pressure interval, crystalline ice Ih or ice VII co-exist with HDA. The ice VII formed is stable upon pressure release down to 0.1 GPa. The very low compression rate has a profound effect on the crystallinity in the amorphous regime. Gathering all the existing experimental evidences allows an unambiguous description of the phenomenon of pressure induced amorphization. The onset of the phase transition is triggered by a shear instability of the ice lattice. The co-existence ice VII with HDA, instead of the equilibrium thermodynamic stable and proton-ordered ice-VIII under the same pressure-temperature condition reveals at low temperature there is insufficient thermal energy to overcome the substantial geometrical rearrangement from a single proton disordered H-bond network to an interpenetrating proton ordered H-bond crystalline network. Thus, leaving the proton disordered H-network intact. The analysis shows unequivocally that the structure obtained from the compression of ice is controlled by kinetics and dependent on the temperature.

Journal Articles

Structure refinement of black phosphorus under high pressure

Akahama, Yuichi*; Miyakawa, Masashi*; Taniguchi, Takashi*; Sano, Asami; Machida, Shinichi*; Hattori, Takanori

Journal of Chemical Physics, 153(1), p.014704_1 - 014704_5, 2020/07

 Times Cited Count:2 Percentile:31.62(Chemistry, Physical)

The structure refinement of black phosphorus was performed at pressures of up to 3.2 GPa at room temperature by powder neutron diffraction techniques. The bond lengths and bond angles between the phosphorus atoms at pressures were precisely determined and confirmed to be consistent with those of the previous single crystal X-ray analysis [Brown and Randqvist, Acta Cryst. 19, 684 (1965)]. Although lattice parameters exhibited an anisotropic compressibility, the covalent P1-P2 and P1-P3 bond lengths were almost independent of pressure and only the P3-P1-P2 bond angle was reduced significantly. On the basis of our results, the significant discrepancy in the bond length reported by Cartz et al. [J. Chem. Phys. 71, 1718 (1979)] has been solved. Our structural data will contribute to the elucidation of the Dirac semimetal state of black phosphorus under high pressure.

Journal Articles

Neutron diffraction study on the deuterium composition of nickel deuteride at high temperatures and high pressures

Saito, Hiroyuki*; Machida, Akihiko*; Hattori, Takanori; Sano, Asami; Funakoshi, Kenichi*; Sato, Toyoto*; Orimo, Shinichi*; Aoki, Katsutoshi*

Physica B; Physics of Condensed Matter, 587, p.412153_1 - 412153_6, 2020/06

 Times Cited Count:0 Percentile:100(Physics, Condensed Matter)

The site occupancy of deuterium (D) atoms in face-centered-cubic nickel (fcc Ni) was measured along a cooling path from 1073 to 300 K at an initial pressure of 3.36 GPa via in situ neutron powder diffraction. Deuterium atoms predominantly occupy the octahedral (O) sites and slightly occupy the tetrahedral (T) sites of the fcc metal lattice. The O-site occupancy increases from 0.4 to 0.85 as the temperature is lowered from 1073 to 300 K. Meanwhile, the T-site occupancy remains c.a. 0.02. The temperature-independent behavior of the T-site occupancy is unusual, and its process is not yet understood. From the linear relation between the expanded lattice volume and D content, a D-induced volume expansion of 2.09(13) ${AA $^{3}$/D}$ atom was obtained. This value is in agreement with the values of 2.14-2.2 ${AA $^{3}$/D}$ atom previously reported for Ni and Ni$$_{0.8}$$ Fe$$_{0.2}$$ alloy.

Journal Articles

Crystal and magnetic structures of double hexagonal close-packed iron deuteride

Saito, Hiroyuki*; Machida, Akihiko*; Iizuka, Riko*; Hattori, Takanori; Sano, Asami; Funakoshi, Kenichi*; Sato, Toyoto*; Orimo, Shinichi*; Aoki, Katsutoshi*

Scientific Reports (Internet), 10, p.9934_1 - 9934_8, 2020/06

 Times Cited Count:0 Percentile:100(Multidisciplinary Sciences)

Neutron powder diffraction profiles were collected for iron deuteride (FeDx) while the temperature decreased from 1023 to 300 K for a pressure range of 4-6 GPa. The $$varepsilon$$' deuteride with a double hexagonal close-packed (dhcp) structure, which coexisted with other stable or metastable deutrides at each temperature and pressure condition, formed solid solutions with a composition of FeD$$_{0.68(1)}$$ at 673 K and 6.1 GPa and FeD$$_{0.74(1)}$$ at 603 K and 4.8 GPa. Upon stepwise cooling to 300 K, the D-content x increased to a stoichiometric value of 1.0 to form monodeuteride FeD$$_{1.0}$$. In the dhcp FeD$$_{1.0}$$ at 300 K and 4.2 GPa, dissolved D atoms fully occupied the octahedral interstitial sites, slightly displaced from the octahedral centers in the dhcp metal lattice, and the dhcp sequence of close-packed Fe planes contained hcp-stacking faults at 12%. Magnetic moments with 2.11 $$pm$$ 0.06 B/Fe-atom aligned ferromagnetically in parallel on the Fe planes.

Journal Articles

Anomalous hydrogen dynamics of the ice VII-VIII transition revealed by high-pressure neutron diffraction

Komatsu, Kazuki*; Klotz, S.*; Machida, Shinichi*; Sano, Asami; Hattori, Takanori; Kagi, Hiroyuki*

Proceedings of the National Academy of Sciences of the United States of America, 117(12), p.6356 - 6361, 2020/03

 Times Cited Count:2 Percentile:64.45(Multidisciplinary Sciences)

Above 2 GPa the phase diagram of water simplifies considerably and exhibits only two solid phases up to 60 GPa, ice VII and ice VIII. The two phases are related to each other by hydrogen ordering, with the oxygen sub-lattice being essentially the same. Here we present neutron diffraction data to 15 GPa which reveal that the rate of hydrogen-ordering at the ice VII-VIII transition decreases strongly with pressure to reach time scales of minutes at 10 GPa. Surprisingly, the ordering process becomes more rapid again upon further compression. We show that such an unusual change in transition rate can be explained by a slowing-down of the rotational dynamics of water molecules with a simultaneous increase of translational motion of hydrogen under pressure, as previously suspected. The observed crossover in the hydrogen dynamics in ice is likely the origin of various hitherto unexplained anomalies of ice VII in the 10-15 GPa range reported by Raman spectroscopy, X-ray diffraction, and proton conductivity.

Journal Articles

Developments of nano-polycrystalline diamond anvil cells for neutron diffraction experiments

Komatsu, Kazuki*; Klotz, S.*; Nakano, Satoshi*; Machida, Shinichi*; Hattori, Takanori; Sano, Asami; Yamashita, Keishiro*; Irifune, Tetsuo*

High Pressure Research, 40(1), p.184 - 193, 2020/02

 Times Cited Count:3 Percentile:27.55(Physics, Multidisciplinary)

A new high pressure cells for neutron diffraction experiments using nano-polycrystalline anvil is presented. The cell design, off-line pressure generation tests and a gas-loading procedure for this cell are described. The performance is illustrated by powder neutron diffraction patterns of ice VII to $$sim$$ 82 GPa. We also demonstrate the feasibility of single crystal neutron diffraction experiments of Fe$$_{3}$$O$$_{4}$$ at ambient conditions using this cell and discuss the current limitation and future developments.

Journal Articles

Ice I$$_{rm c}$$ without stacking disorder by evacuating hydrogen from hydrogen hydrate

Komatsu, Kazuki*; Machida, Shinichi*; Noritake, Fumiya*; Hattori, Takanori; Sano, Asami; Yamane, Ryo*; Yamashita, Keishiro*; Kagi, Hiroyuki*

Nature Communications (Internet), 11(1), p.464_1 - 464_5, 2020/02

 Times Cited Count:6 Percentile:13.29(Multidisciplinary Sciences)

Water freezes below 0$$^{circ}$$C at ambient pressure ordinarily to ice I$$_{rm h}$$, with hexagonal stacking sequence. Under certain conditions, ice with a cubic stacking sequence can also be formed, but ideal ice I$$_{rm c}$$ without stacking-disorder has never been formed until recently. Here we demonstrate a route to obtain ice I$$_{rm c}$$ without stacking-disorder by degassing hydrogen from the high-pressure form of hydrogen hydrate, C$$_{2}$$, which has a host framework isostructural with ice I$$_{rm c}$$. The stacking-disorder free ice I$$_{rm c}$$ is formed from C$$_{2}$$ via an intermediate amorphous or nano-crystalline form under decompression, unlike the direct transformations occurring in ice XVI from neon hydrate, or ice XVII from hydrogen hydrate. The obtained ice I$$_{rm c}$$ shows remarkable thermal stability, until the phase transition to ice I$$_{rm h}$$ at 250 K, originating from the lack of dislocations. This discovery of ideal ice I$$_{rm c}$$ will promote understanding of the role of stacking-disorder on the physical properties of ice as a counter end-member of ice I$$_{rm h}$$.

Journal Articles

Crystal structure and magnetism of MnO under pressure

Klotz, S.*; Komatsu, Kazuki*; Polian, A.*; Machida, Shinichi*; Sano, Asami; Iti$'e$, J.-P.*; Hattori, Takanori

Physical Review B, 101(6), p.064105_1 - 064105_6, 2020/02

 Times Cited Count:0 Percentile:100(Materials Science, Multidisciplinary)

Manganese oxide (MnO) is a prototype of an antiferromagnetic Mott-insulator. Here we investigate the interplay of magnetic ordering and lattice distortion across the N$'e$el temperature $$T_{N}$$ under pressure using neutron and X-ray diffraction. We find an increase of $$T_{N}$$ with a rate of $$T_{N}/dP$$ = +4.5(5) K/GPa, an increase of the rhombohedral distortion $$alpha$$ by $$dalpha/dP$$ = +0.018$$^{circ}$$/GPa, as well as a volume striction which is insensitive to pressure. These results allow to retrieve the dependence of the coupling constants $$J_{1}$$ and $$J_{2}$$ on interatomic distances and compare it to first-principles predictions. Antiferromagnetic diffuse scattering was observed up to $$sim$$1.2 $$T_{N}$$, and long-range magnetic order appears at room temperature at 42 GPa.

Journal Articles

Crystal structure of a high-pressure phase of magnesium chloride hexahydrate determined by ${it in-situ}$ X-ray and neutron diffraction methods

Yamashita, Keishiro*; Komatsu, Kazuki*; Hattori, Takanori; Machida, Shinichi*; Kagi, Hiroyuki*

Acta Crystallographica Section C; Structural Chemistry (Internet), 75(12), p.1605 - 1612, 2019/12

 Times Cited Count:1 Percentile:72.8(Chemistry, Multidisciplinary)

A crystal structure of a high-pressure phase of magnesium chloride hexahydrate (MgCl$$_{2}$$ $$cdot$$ 6H$$_{2}$$O-II) and its deuterated counterpart (MgCl$$_{2}$$ $$cdot$$ 6D$$_{2}$$O-II) have been identified for the first time by in-situ single-crystal X-ray and powder neutron diffraction. The crystal structure was analyzed by the Rietveld method for the neutron diffraction pattern based on the initial structure determined by single-crystal X-ray diffraction. This high-pressure phase has a similar framework to that in the known ambient-pressure phase, but exhibits some structural changes with symmetry reduction caused by a subtle modification in the hydrogen-bond network around the Mg(H$$_{2}$$O)$$_{6}$$ octahedra. These structural features reflect the strain in the high-pressure phases of MgCl$$_{2}$$ hydrates.

Journal Articles

Hexagonal close-packed iron hydride behind the conventional phase diagram

Machida, Akihiko*; Saito, Hiroyuki*; Hattori, Takanori; Sano, Asami; Funakoshi, Kenichi*; Sato, Toyoto*; Orimo, Shinichi*; Aoki, Katsutoshi*

Scientific Reports (Internet), 9(1), p.12290_1 - 12290_9, 2019/08

 Times Cited Count:3 Percentile:48.72(Multidisciplinary Sciences)

Hexagonal close-packed iron hydride, hcp FeHx, is absent from the conventional phase diagram of the Fe-H system, although hcp metallic Fe exists stably over extensive temperature ($$T$$) and pressure ($$P$$) conditions, including those corresponding to the Earth's inner core. ${{it In situ}}$ X-ray and neutron diffraction measurements at temperatures ranging from 298 to 1073 K and H pressures ranging from 4 to 7 GPa revealed that the hcp hydride was formed for FeH$$_{x}$$ compositions when $$x < 0.6$$. Hydrogen atoms occupied the octahedral interstitial sites of the host metal lattice both partially and randomly. The hcp hydride exhibited a H-induced volume expansion of 2.48(5) $AA $^{3}$$/H-atom, which was larger than that of the face-centered cubic (fcc) hydride. The hcp hydride showed an increase in $$x$$ with $$T$$, whereas the fcc hydride showed a corresponding decrease. The present study provides guidance for further investigations of the Fe-H system over an extensive $$x$$-$$T$$-$$P$$ region.

Journal Articles

Research and development on membrane IS process for hydrogen production using solar heat

Myagmarjav, O.; Iwatsuki, Jin; Tanaka, Nobuyuki; Noguchi, Hiroki; Kamiji, Yu; Ioka, Ikuo; Kubo, Shinji; Nomura, Mikihiro*; Yamaki, Tetsuya*; Sawada, Shinichi*; et al.

International Journal of Hydrogen Energy, 44(35), p.19141 - 19152, 2019/07

 Times Cited Count:9 Percentile:31.13(Chemistry, Physical)

Journal Articles

High-pressure structure and electronic properties of YbD$$_{2}$$ to 34 GPa

Klotz, S.*; Casula, M.*; Komatsu, Kazuki*; Machida, Shinichi*; Hattori, Takanori

Physical Review B, 100(2), p.020101_1 - 020101_5, 2019/07

 Times Cited Count:5 Percentile:36.84(Materials Science, Multidisciplinary)

Ytterbium dihydride (YbH$$_{2}$$) shows a well-known transition at $$approx$$16 GPa from a CaH$$_{2}$$-type structure to a high-pressure (high-$$P$$) phase with Yb at hcp sites and unknown H-positions. Here, we report its complete structure determination by neutron diffraction at 34 GPa. Hydrogen(deuterium) is located at 2$$a$$ and 2$$d$$ positions of space group $$P6_{3}/mmc$$, thus forming a high-symmetry "collapsed" close-packed lattice. The transition is sluggish and can be seen as a transfer of 1/2 of the hydrogen atoms from strongly corrugated H-layers to interstitial sites of the Yb-lattice. We demonstrate by first-principles calculations that the transition is related to a change from a completely filled $$f$$-electron configuration to a fractional $$f$$-hole ($$approx$$0.25 h) occupation in the high-$$P$$ phase. The $$f$$ $$to$$ $$d$$ charge transfer closes the gap at the transition and leads to a metallic ground state with sizeable electron-phonon interaction involving out-of-plane vibrational modes of interstitial hydrogen.

Journal Articles

Development of a technique for high pressure neutron diffraction at 40 GPa with a Paris-Edinburgh press

Hattori, Takanori; Sano, Asami; Machida, Shinichi*; Abe, Jun*; Funakoshi, Kenichi*; Arima, Hiroshi*; Okazaki, Nobuo*

High Pressure Research, 39(3), p.417 - 425, 2019/06

 Times Cited Count:7 Percentile:23.62(Physics, Multidisciplinary)

We have developed a technique for neutron diffraction experiments at pressures up to 40 GPa using a Paris-Edinburgh press at the PLANET beamline in J-PARC. To increase the maximum accessible pressure, the diameter of the dimple for sample chamber at the top of the sintered diamond anvils is sequentially reduced from 4.0 mm to 1.0 mm. As a result, the maximum pressure increased and finally reached 40 GPa. By combining this technique with the beam optics which defines the gauge volume, diffraction patterns sufficient for full-structure refinements are obtainable at such pressures.

Journal Articles

Crystal structure change of katoite, Ca$$_{3}$$Al$$_{2}$$(O$$_{4}$$D$$_{4}$$)$$_{3}$$, with temperature at high pressure

Kyono, Atsushi*; Kato, Masato*; Sano, Asami; Machida, Shinichi*; Hattori, Takanori

Physics and Chemistry of Minerals, 46(5), p.459 - 469, 2019/05

 Times Cited Count:1 Percentile:81.42(Materials Science, Multidisciplinary)

To reveal the decomposition mechanism with temperature under high-pressure, crystal structure of a hydrogrossular, katoite Ca$$_{3}$$Al$$_{2}$$(O$$_{4}$$D$$_{4}$$)$$_{3}$$ has been studied by in-situ neutron diffraction at 8 GPa. Although unusual expansion behavior was discerned at 200-400$$^circ$$C, the unit cell was continuously expanded up to 850$$^circ$$C. At 900$$^circ$$C, katoite was decomposed, indicating that pressure strongly increases dehydration temperature from 300$$^circ$$C to 900$$^circ$$C. On release of pressure, the katoite reappear together with corundum and portlandite. At 8 GPa, CaO$$_{8}$$ and AlO$$_{6}$$ polyhedra expand with temperature up to 850$$^circ$$C by about 8% and 13%, respectively. On the other hand, tetrahedral interstices are isotopically squeezed by about 10%: due to the expansion of above polyhedra. The neighboring D-D distance remains almost unchanged in this temperature range, while the O-D bond distance shrinks drastically just before decomposition. This finding suggests that the shortening of O-D distance caused by the D-D repulsion destabilizes the O-D bond, which induces the thermal decomposition of katoite.

Journal Articles

Experimental investigation on dependence of decontamination factor on aerosol number concentration in pool scrubbing under normal temperature and pressure

Sun, Haomin; Machida, Shinichi*; Shibamoto, Yasuteru; Okagaki, Yuria; Yonomoto, Taisuke

Proceedings of 26th International Conference on Nuclear Engineering (ICONE-26) (Internet), 7 Pages, 2018/07

Journal Articles

Pressure-induced stacking disorder in boehmite

Ishii, Yusuke*; Komatsu, Kazuki*; Nakano, Satoshi*; Machida, Shinichi*; Hattori, Takanori; Sano, Asami; Kagi, Hiroyuki*

Physical Chemistry Chemical Physics, 20(24), p.16650 - 16656, 2018/06

 Times Cited Count:2 Percentile:83(Chemistry, Physical)

The structure of an aluminum layered hydroxide, boehmite ($$gamma$$-AlOOH), as a function of pressure was studied by using ${it in situ}$ synchrotron X-ray and neutron diffraction. Peak broadening and subsequent splitting, which are only found for hkl (h $$neq$$ 0) peaks in the X-ray diffraction patterns above 25 GPa, are explained by stacking disorder accompanied with a continuously increasing displacement of the AlO$$_{6}$$ octahedral layer along a-axis. This finding could be the first experimental result for the pressure-induced stacking disorder driven by the continuous layer displacement. The magnitude of the layer displacement was estimated from the X-ray scattering profile calculation based on the stacking disordered structure model. Hydrogen bond geometries of boehmite, obtained by structure refinements on the observed neutron diffraction patterns for deuterated sample up to 10 GPa, show linearly approaching O-D covalent and D$$cdots$$O hydrogen bond distances and they could merge below 26 GPa. The pressure-induced stacking disorder would make the electrostatic potential of hydrogen bonds asymmetric, yielding less chance for the proton-tunnelling.

116 (Records 1-20 displayed on this page)