In this article, recent researches on ionic hydration in subcritical and supercritical water with neutron diffraction (ND) and X-ray absorption spectroscopy (XAS) were introduced. ND with an isotopic substitution technique is one of the powerful methods to reveal the hydration structure of ions in electrolyte solutions. XAS can also observe the hydration structure of ions and ion-pair formation at a lower concentration of the ions. The fundamental information on ionic hydration is important to understand the underlying mechanism of formation of metal oxide nanoparticles from electrolyte solutions in subcritical and supercritical water, which is expected as a new fabrication technique of nanoparticles in the engineering field.

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The bilayer phase transitions of double-chain cationic surfactants, dialkyldimethylammonium bromides (2C_nBr), at ambient and high pressures were investigated. The 2C_nBr bilayers underwent a single or multiple phase transitions depending on the alkyl-chain length. The temperature-pressure phase diagrams and thermodynamic quantities of the 2C_nBr bilayers for the phase transitions were determined from their phase-transition data. We could comprehensively interpret the complicated phase behavior of the 2C_nBr bilayers, which has not been revealed by conventional experimental methods at ambient pressure.

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This article reports the original high-pressure apparatuses for in-situ observation of functional solvents under high-pressure conditions. First, we describe the instruments to measure gas solubility and volume expansivity over the wide ranges of temperature and pressure, and discuss the high-pressure gas solubilities in the ionic liquid chemical absorbents. Next, the experimental setup for the high-pressure NMR spectroscopy and electrochemical measurements is presented with the chemical shifts and transport properties in the CO₂ dissolved liquids.

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The solution-state NMR measurement of a synthetic polymer in sub-critical fluids has been accomplished by taking advantage of the specially-designed high-pressure and high-temperature NMR probe. The purpose here is to overcome the signal broadening typically occurring in conventional solution-state NMR due to the slow dynamics of polymers at room temperature. A remarkable sharpening of the ¹H NMR signal was observed for poly (N-vinyl-2-pyrrolidone) dissolved in D₂O at 250°C. This result has encouraged us to apply high-temperature NMR methods to polymers, such as copolymers, whose spectra were too complicated to be deconvoluted by means of conventional procedures.

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The thermodynamic and kinetic properties of clathrate hydrates have been studied for scientific interest and various potential industrial applications. This article highlights recent advances in the hydrate researches using molecular simulations with a focus on formation processes and thermodynamic properties. Subsets of molecular simulations, Molecular Dynamics (MD), Monte Carlo (MC) and ab initio MD simulations have been applied to hydrate systems and provide the fundamental insights at molecular level.

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Some bacteria such as the Bacillus and Clostridium species can form spores, which are the dormant form of vegetative bacteria and are highly resistant to physical and chemical stimuli, the sterilization of which requires heating at ～121°C for 30 min. A crucial factor that maintains this high-resistivity is the high concentration (～10% in dry weight of spore) of dipicolinic acid (DPA), which is spore-specific. We have applied ¹H high-resolution high-pressure NMR spectroscopy, for the first time, to the spore suspension, and observed, directly and in real-time, the specific release of DPA molecules from the spore into the bulk water phase in response to a mild hydrostatic pressure (200 MPa) at 20°C. After the release of DPA, heat-treatment below ～100°C has proven to be sufficient to cause the inactivation of the spores. Direct observation of molecular processes in bacterial spores with high-pressure NMR in response to pressure and temperature perturbations shown here opens a new avenue for studying spores and vegetative cells directly at the molecular level.

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Advances in crystal structure analysis techniques by powder diffraction experiments and Rietveld analysis over the past 30 years have made a great contribution to high pressure physics and earth science. By incorporating density functional theory (DFT) calculation, it has become possible to analyze the atomic coordinates of hydrogen and complicated crystal structures. In addition, it compensated the uncertainty of the analysis result due to various constraints of high pressure experiment, and improved the reliability of the structural analysis result. In this article, the latest techniques on a crystal structure analysis for high pressure powder diffraction are briefly reviewed.

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Laser, different from other light sources in that it emits light with a single wavelength coherently, spatially and temporally, is one of the most applicable tools for high-pressure experiments. Here I introduce three experimental techniques using the laser; (1) both-sided CO₂-laser heated DAC system, (2) laser-shock compression technique coupling with DAC, and (3) Brillouin and Raman spectroscopies combined with CO₂-laser heated DAC system. The newly-developed techniques were proposed as promising ways to access new material conditions, to determine temperature accurately, or to identify phase transitions including melting, toward explorations of new material phase regimes.

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Knowledge of rheological properties of bridgmanite, which is the most abundant mineral at the lower mantle, is one of the most important parameters to understand mantle dynamics. We developed deformation experimental technique up to the top of lower mantle conditions using D-DIA apparatus as Kawai-type (6-8 type) Press. Using this technique, crystallographic-preferred-orientation (CPO) of bridgmanite induced by shear deformation was determined. The seismic shear wave anisotropy observed around subduction zone at the lower mantle, can be explained in terms of the CPO of bridgmanite with mantle flow parallel to the direction of subduction zone. We also succeeded to measure the viscosity of bridgmanite in dislocation creep regime by in-situ deformation experiments.

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