Naoki Noguchi

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.

OH group behavior and pressure-induced amorphization of antigorite examined under high pressure and temperature using synchrotron infrared spectroscopy

Abstract Infrared (IR) absorption spectra of antigorite were measured up to 27 GPa and 320 °C using synchrotron IR radiation to elucidate OH group behavior under high-pressure (HP) and high-temperature (HT) conditions. The absorption bands attributable to the OH stretching modes of outer OH groups (OHouter) and inner OH groups (OHinner) show positive pressure dependencies. The shift rate of the OHinner band is almost constant at all pressure ranges. In contrast, that of the OHouter band increases slightly at about 6 GPa. This discontinuous change of the shift rate is consistent with the anomalous behavior of the OHouter upon compression, which was predicted in the previous first-principle calculation study. Specifically, the pressure dependence of the OHouter band shows that the hydrogen ion of an OHouter interacts not only with the nearest basal oxygen ion of the SiO4 tetrahedron but also with the second nearest two basal oxygen ions upon compression. The latter interaction becomes dominant over the former interaction at about 6 GPa. Pressure-induced amorphization was indicated from IR spectra measured at 300 °C and 25.6 GPa. This P-T condition is out of the thermodynamic stability field of antigorite. A broad absorption band, which is close to the broad band attributable to natural hydrous silicate glass, appeared after amorphization, which suggests that the pressure-induced amorphization of antigorite does not induce dehydration. Hydrogen atoms are retained in amorphized antigorite as OH groups.

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.

An induction heating diamond anvil cell for high pressure and temperature micro-Raman spectroscopic measurements

A new external heating configuration is presented for high-temperature diamond anvil cell instruments. The supporting rockers are thermally excited by induction from an externally mounted copper coil passing a 30 kHz alternating current. The inductive heating configuration therefore avoids the use of breakable wires, yet is capable of cell temperatures of 1100 K or higher. The diamond anvil cell has no resistive heaters, but uses a single-turn induction coil for elevating the temperature. The induction coil is placed near the diamonds and directly heats the tungsten carbide rockers that support the diamond. The temperature in the cell is determined from a temperature-power curve calibrated by the ratio between the intensities of the Stokes and anti-Stokes Raman lines of silicon. The high-pressure transformation of quartz to coesite is successfully observed by micro-Raman spectroscopy using this apparatus. The induction heating diamond anvil cell is thus a useful alternative to resistively heated diamond anvil cells.

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.