Hirokazu Kadobayashi

High pressure X-ray diffraction and Raman spectroscopic studies of the phase change of D2O ice VII at approximately 11 GPa

High pressure experiments were performed on D2O ice VII using a diamond anvil cell in a pressure range of 2.0–60 GPa at room temperature. In situ X-ray diffractometry revealed that the structure changed from cubic to a low symmetry phase at approximately 11 GPa, based on the observed splitting of the cubic structures diffraction lines. Heating treatments were added for the samples to reduce the effect of non-hydrostatic stress. After heating, splitting diffraction lines became sharp and the splitting was clearly retained. Although symmetry and structure of the transformed phase have not been determined, change in volumes vs. pressure was calculated, assuming that the low-symmetry phase had a tetragonal structure. The bulk modulus calculated for the low-symmetry phase was slightly larger than that for the cubic structure. In Raman spectroscopy, the squared vibrational frequencies of ν1 (A1g), as a function of pressure, showed a clear change in the slope at 11–13 GPa. The full width at half maxima of the O-D modes decreased with increasing pressure, reaching a minimum at approximately 11 GPa, and increased again above 11 GPa. These results evidently support the existence of phase change at approximately 11 GPa for D2O ice VII.

High pressure generation using double-stage diamond anvil technique: problems and equations of state of rhenium

ABSTRACT We have developed a double stage diamond anvil cell (ds-DAC) technique for reproducible pressure by precisely fabricating 2nd stage anvils using a focused ion beam system. We used 2nd stage micro-anvils made of ultra-fine (< 10 nm) nano-polycrystalline diamond with various shapes and dimensions synthesized from glassy carbon at high pressure and temperature. The X-ray diffraction patterns from the rhenium sample always showed very broad peaks due to large pressure gradients in the culet of the micro-anvils. Deconvolution of the broad 101 diffraction peak results in compression of rhenium to V/V0 = 0.633 for the smallest d-spacing. The calculated pressure for this minimum volume varies from 430 to 630 GPa, depending on the choice of the equation of state of rhenium. We conclude that the most likely pressure achieved for the minimum volume of rhenium is in a range of 430–460 GPa based on a calibration using the platinum pressure scale to 280 GPa and the latter value of 630 GPa is unreasonably high, suggesting that the pressures in an earlier study for the equation of state of rhenium would have been significantly overestimated.

High-pressure rotational deformation apparatus to 135 GPa

A large-strain, torsional deformation apparatus has been developed based on diamond anvil cells at high pressures, up to 135 GPa with a help of hard nano-polycrystalline diamond anvils. These pressure conditions correspond to the base of the Earths mantle. An X-ray laminography technique is introduced for high-pressure in situ 3D observations of the strain markers. The technique developed in this study introduces the possibility of the in situ rheological measurements of the deep Earth materials under ultrahigh-pressure conditions.

Time-resolved x-ray diffraction and Raman studies of the phase transition mechanisms of methane hydrate

The mechanisms by which methane hydrate transforms from an sI to sH structure and from an sH to filled-ice Ih structure were examined using time-resolved X-ray diffractometry (XRD) and Raman spectroscopy in conjunction with charge-coupled device camera observation under fixed pressure conditions. The XRD data obtained for the sI-sH transition at 0.8 GPa revealed an inverse correlation between sI and sH, suggesting that the sI structure is replaced by sH. Meanwhile, the Raman analysis demonstrated that although the 12-hedra of sI are retained, the 14-hedra are replaced sequentially by additional 12-hedra, modified 12-hedra, and 20-hedra cages of sH. With the sH to filled-ice Ih transition at 1.8 GPa, both the XRD and Raman data showed that this occurs through a sudden collapse of the sH structure and subsequent release of solid and fluid methane that is gradually incorporated into the filled-ice Ih to complete its structure. This therefore represents a typical reconstructive transition mechanism.

Transition mechanism of sH to filled-ice Ih structure of methane hydrate under fixed pressure condition

The phase transition mechanism of methane hydrate from sH to filled-ice Ih structure was examined using a combination of time-resolved X-ray diffractometry (XRD) and Raman spectroscopy in conjunction with charge-coupled device (CCD) camera observation under fixed pressure conditions. Prior to time-resolved Raman experiments, the typical C-H vibration modes and their pressure dependence of three methane hydrate structures, fluid methane and solid methane were measured using Raman spectroscopy to distinguish the phase transitions of methane hydrates from decomposition to solid methane and ice VI or VII. Experimental results by XRD, Raman spectroscopy and CCD camera observation revealed that the structural transition of sH to filled-ice Ih occurs through a collapse of the sH framework followed by the release of fluid methane that is then gradually incorporated into the filled-ice Ih to reconstruct its structure. These observations suggest that the phase transition of sH to filled-ice Ih takes place by a typical reconstructive mechanism.

A possible existence of phase change of deuterated ice VII at about 11 GPa by X-ray and Raman studies

High pressure experiments were performed with deuterated ice VII using diamond anvil cell in a pressure range of 0.1 MPa to 60 GPa at room temperature in order to further understanding a long-argued issue of phase change in ice VII at approximately 11 GPa. In-situ X-ray diffractometry revealed splitting of diffraction lines of cubic ice VII above 11GPa, which were indexed as a tetragonal structure. The tetragonal structure survived at least up to 60 GPa. Pressure versus volume curves of cubic and tetragonal structures were obtained and the bulk moduli were calculated. Raman spectroscopy showed that the full width at half maximum of O-D vibrational mode decreased with increasing pressure, and showed minimum value at 11 GPa, and then it became broader again above this pressure. The squared vibrational frequency changed linearly with pressure, and the slope changed at about 14 GPa, indicating existence of phase change. All experimental results evidently supported the existence of phase change of ice VII at approximately 11 GPa.

In situ Raman and X-ray diffraction studies on the high pressure and temperature stability of methane hydrate up to 55 GPa

High-temperature and high-pressure experiments were performed under 2-55 GPa and 298-653 K using in situ Raman spectroscopy and X-ray diffraction combined with externally heated diamond anvil cells to investigate the stability of methane hydrate. Prior to in situ experiments, the typical C-H vibration modes of methane hydrate and their pressure dependence were measured at room temperature using Raman spectroscopy to make a clear discrimination between methane hydrate and solid methane which forms through the decomposition of methane hydrate at high temperature. The sequential in situ Raman spectroscopy and X-ray diffraction revealed that methane hydrate survives up to 633 K and 40.3 GPa and then decomposes into solid methane and ice VII above the conditions. The decomposition curve of methane hydrate estimated by the present experiments is >200 K lower than the melting curves of solid methane and ice VII, and moderately increases with increasing pressure. Our result suggests that although methane hydrate may be an important candidate for major constituents of cool exoplanets and other icy bodies, it is unlikely to be present in the ice mantle of Neptune and Uranus, where the temperature is expected to be far beyond the decomposition temperatures.

A step toward better understanding of behavior of organic materials at simultaneous high pressures and high temperatures

ABSTRACT Using an external heating system, specifically designed for the lever-type diamond anvil cell, we investigated for the first time the phase relationships in the C2H2O4 system at temperatures exceeding 850°C and pressures up to 6.5 GPa. In situ Raman spectroscopy was applied for the characterization of structural features of observed high-temperature phases, which transformed to the black carbon-rich material upon quenching and decompression. Results of this work give insights on the pressure-induced polymerization process of organic compounds at high temperatures.