Ayako Shinozaki

Pressure-induced oligomerization of benzene at room temperature as a precursory reaction of amorphization

Oligomerization of benzene at high pressures up to 16 GPa was investigated at room temperature using an opposed-anvil type pressure apparatus. The recovered samples were analyzed using GC-MS to identify and quantify the products after the high-pressure experiments. Some structural isomers of benzene dimer as well as biphenyl, naphthalene, and terphenyl isomers were detected at pressures higher than 13 GPa. The molar yield of the polycyclic aromatic hydrocarbons increased concomitantly with increasing pressure, although benzene still remained. The oligomerization is likely to occur when the neighbor distance of the benzene molecules exceeds the threshold of the reaction distance. The oligomerization is regarded as a precursory phenomenon of the amorphization that occurs at higher pressure.

Crystal structure of magnesium dichloride decahydrate determined by X-ray and neutron diffraction under high pressure

Magnesium dichloride decahydrate (MgCl2·10H2O) and its deuterated counterpart (MgCl2·10D2O) are identified for the first time by in-situ powder synchrotron X-ray and spallation neutron diffraction. These substances are crystallized from a previously unidentified nanocrystalline compound, which originates from an amorphous state at low temperature. A combination of a recently developed autoindexing procedure and the charge-flipping method reveals that the crystal structure of MgCl2·10H2O consists of an ABCABC··· sequence of Mg(H2O)6 octahedra. The Cl(-) anions and remaining water molecules unconnected to the Mg(2+) cations bind the octahedra, similar to other water-rich magnesium dichloride hydrates. The D positions in MgCl2·10D2O, determined by the difference Fourier methods using the neutron powder diffraction patterns at 2.5 GPa, show the features such as bifurcated hydrogen bonds and tetrahedrally coordinated O atoms, which were not found in other forms of magnesium chloride hydrates.

Performance of ceramic anvils for high pressure neutron scattering

Three kinds of ceramics, zirconia-toughened alumina (ZTA), alumina-toughened zirconia (ATZ) and yttria-stabilized zirconia (YSZ), were tested as anvil materials, mainly for the purpose of neutron scattering study under high pressure. ZTA with non-toroidal anvil profile, having the same sample volume as conventionally used double toroidal anvils, sustained pressures up to 11.9 GPa. This is comparable to anvils made of tungsten carbide (TC) with Ni binder with the same dimensions. ATZ would also be an alternative material to TC with pressure performance comparable to ZTA, whereas YSZ is much weaker than the other two ceramics. The attenuation coefficient for YSZ is significantly smaller than that of TC and similar to ZTA and ATZ, the latter being estimated by attenuation calculations. Neutron diffraction on a sample of lead in YSZ anvils as well as quasi-elastic neutron scattering on liquid water in ZTA also demonstrate the outstanding neutron transparency of these ceramics. The gain factor in count rate is up to one order of magnitude.

Influence of H2 fluid on the stability and dissolution of Mg2SiO4 forsterite under high pressure and high temperature

Abstract High-pressure and high-temperature experiments were carried out in a Mg2SiO4-H2 system using laser-heated diamond-anvil cells to understand the influence of H2 fluid on the stability of forsterite. In situ X‑ray diffraction experiments and Raman spectroscopic measurements showed the decomposition of forsterite, and formation of periclase (MgO) and stishovite/quartz (SiO2) in the presence of H2 after being heated in the range between 2.5 GPa, 1400 K and 15.0 GPa, 1500 K. Transmission electron microscopic observation of the samples recovered from 15.0 GPa and 1500 K showed that the granular to columnar periclase grains maintained the original grain shape of forsterite, indicating that the periclase crystals crystallized under high temperature. On the other hand, euhedral columnar stishovite crystals were found at the boundaries between residual forsterite grains and reacted periclase. This implies that the SiO2 component was dissolved in H2 fluid, and that stishovite was considered to have crystallized when the solubility of the SiO2 component became reduced with decreasing temperature. Additional experiment on a SiO2-H2 system clearly showed the dissolution of quartz in H2 fluid, while those on a MgO-H2 system, periclase was hardly dissolved. These lines of evidence indicate that forsterite was incongruently dissolved in H2 fluid to form periclase crystals in the Mg2SiO4-H2 system, which is different from what was observed in the Mg2SiO4-H2O system. The results indicate that the stability of forsterite is strongly affected by the composition of coexisting C-O-H fluid.

Formation of SiH4 and H2O by the dissolution of quartz in H2 fluid under high pressure and temperature

Abstract Species dissolved in H2 fluid were investigated in a SiO2-H2 system. Raman and infrared (IR) spectra were measured at high pressure and room temperature after heating experiments were conducted at two pressure and temperature conditions: 2.0 GPa, 1700 K and 3.0 GPa, 1500 K. With the dissolution of quartz, a SiH vibration mode assignable to SiH4 was detected from Raman spectra of the fluid phase. Furthermore, an OH vibration mode was observed at 3260 cm-1 from the IR spectra at 3.0 GPa. With decreasing pressure, the OH vibration frequencies observed between 3.0 and 2.1 GPa correspond to that of ice VII, and those observed at 1.4 and 1.1 GPa correspond to that of ice VI. These results indicate that the chemical reaction between dissolved SiO2 components and H2 fluid caused the formation of H2O and SiH4, which was contrastive to that observed in SiO2-H2O fluid. Results imply that a part of H2 is oxidized to form H2O when SiO2 components of mantle minerals dissolve in H2 fluid, even in an iron-free system.

Behavior of intermolecular interactions in α-glycine under high pressure

Pressure-response on the crystal structure of deuterated α-glycine was investigated at room temperature, using powder and single-crystal X-ray diffraction, and powder neutron diffraction measurements under high pressure. No phase change was observed up to 8.7 GPa, although anisotropy of the lattice compressibility was found. No significant changes in the compressibility and the intramolecular distance between non-deuterated α-glycine and deuterated α-glycine were observed. Neutron diffraction measurements indicated the distance of the intermolecular D⋯O bond along with the c-axis increased with compression up to 6.4 GPa. The distance of another D⋯O bond along with the a-axis decreased with increasing pressure and became the shortest intermolecular hydrogen bond above 3 GPa. In contrast, the lengths of the bifurcated N-D⋯O and C-D⋯O hydrogen bonds, which are formed between the layers of the α-glycine molecules along the b-axis, decreased significantly with increasing pressure. The decrease of the intermolecular distances resulted in the largest compressibility of the b-axis, compared to the other two axes. The Hirshfeld analysis suggested that the reduction of the void region size, rather than shrinkage of the strong N-D⋯O hydrogen bonds, occurred with compression.

Polymerization of methane molecules and phase transition of san carlos olivine under the Earth's mantle conditions

High-pressure and high-temperature experiments of the olivine-methane-water system were performed using a laser-heated diamond anvil cell at pressure range from 5.8 GPa to 29.4 GPa and temperatures up to 2000K. The samples were examined by X-ray diffractometry and Raman spectroscopy under high pressures and room temperature. The heated areas of the samples changed to black color. Raman spectroscopy revealed the existence of ethane, heavier hydrocarbons, graphite and glassy carbon besides methane molecules. X-ray diffractometry showed that olivine was remained in the sample heated at 5. 8 GPa, 2000K. Wadsleyite and ringwoodite were observed in the samples heated at 14.5 GPa and 19.5 GPa, respectively. At 29.4 GPa, the diffraction line of Mg-perovskite and magnesiowustite were observed. The observed phase changes were similar to those observed in anhydrous and hydrous conditions. The present results suggest that polymerization of methane molecules occurred and that phase transition of olivine occurred even under the existence of methane-water fluid in the deeper part of the mantle.

Near-infrared spectra of H2O under high pressure and high temperature: Implications for a transition from proton tunneling to hopping states

The nature of protons in ice VII up to 368°C and 16GPa was investigated with synchrotron near-infrared spectroscopy. The absorption band of the first OH stretching overtone mode divided into doublet peaks above 5GPa at room temperature, suggesting that proton tunneling occurs at the overtone level. As the temperature increased, the doublet peaks gradually reduced to a singlet. This result implies that thermally activated protons hop between the two potential minima along the oxygen-oxygen axis. A pressure-temperature diagram for the proton state was constructed from the changing band shape of the overtone mode.