Ultrasound velocities of He and Ne were measured up to 3.0 and 3.7 GPa, respectively, at 295 K. Ultrasound velocities of Ar, Kr and Xe were measured up to solidification pressures at room temperature. The phenomena of phase separation and solidification in the systems of He-Kr, He-Ar, Ne-Ar and He-CO₂ were investigated by sound velocities and optical methods. Phase diagrams of He-Kr and He-CO₂ were represented by those methods. Solidification curves of He-Ar and Ne-Ar were obtained at 295 K.

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An 850 MPa type internally-heated gas pressure vessel, which adopts a double-stage compression system, was newly installed at the Tokyo Institute of Technology. At second compressing stage of this instrument, pressure gauge is difficult to use and pressure is estimated from the loaded oil pressure. In order to acquire reliable pressure conditions, a manganin string was placed in the sample chamber and its electrical resistance was measured as a pressure sensor. By using this technique, uncertainty of pressure value for the sample chamber is decreased to a few MPa.

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The effect of pressure and temperature on the dihedral angles of aqueous fluid in silicate rock matrix was reviewed. A connectivity of aqueous fluid at high-pressures and high-temperatures is important to understand the distribution and migration of super critical water in the Earth's mantle. The change of dihedral angles is due to both the P-T conditions and the solubility of silicate components into aqueous fluid.

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A series of direct observation of complete miscibility between aqueous fluids and island arc andesite/rhyolite magmas has been conducted using modified Bassett's type externally heated diamond anvil cell. The critical phenomena are observed between aqueous fluids and Fuji 1707 magmas with chemical compositions from andesite (62 wt.% SiO₂) to rhyolite (69% SiO₂). Such critical phenomena between aqueous fluids and magmas equilibrated with mantle peridotite system remain uncertain. This study is the first observation of critical phenomena of natural andesite and rhyolite with aqueous fluids, suggesting a second critical point between magmas and aqueous fluids in the depths of the Earth. Such a supercritical fluid can be separated into a melt and a fluid eventually along its migration to the surface. Under such circumstances elemental fractionation between the aqueous fluid and the melt should take place.

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Water is the most abundant volatile component on the Earth's surface, and it is important to clarify the water distribution in the Earth's interior because water influences the physical properties and melting temperature of minerals. Here the author reviews our recent works about the H₂O partitioning among high-pressure polymorphs of olivine. Based on the partitioning results combining with the seismological constraint, the author discusses the amount of water distribution in the Earth's interior.

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Nitridation of III-group elements is performed by high-pressure nitrogen fluid in a laser-heated diamond anvil cell to synthesize III-nitrides up to 15 GPa. In this article, we approach the nitridation process of elements through a close-up on the recovered products by high resolution scanning electron microscopy, Raman microscopy and micro-focused X-ray diffractometry. Gallium nitride (GaN) and aluminum nitride (AlN) are recrystallized as wruzite type structure followed by dissolution of gallium and aluminum into oversaturated nitrogen fluid. On the other hand, boron can be changed to cubic boron nitride (BN) without an extensive dissolution into nitrogen fluid. It would be considered that the difference of crystal habits is derived from the degree of dissolubility which would be partly related to the melting point of elements.

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Synthesis of metal nitrides has been tried in a supercritical nitrogen fluid at high pressures (about 10 GPa) and high temperatures yield using diamond anvil cell and YAG laser heating system. Transition-metal nitrides such as TiN, VN, Fe₂N, ZrN, NbN, MoN and HfN can be easily synthesized by a direct nitriding reaction between metal and fluid nitrogen. Superconducting transition of TiN, VN, ZrN and NbN can be observed using one synthesized sample by a SQUID magnetometer. Colorless transparent fine crystals of GaN in the wultz-rock type structure can be also grown from gallium and fluid nitrogen. They have hexagonal platelet or prismatic morphology, indicating a solution growth using supercritical nitrogen fluid as a solvent. These results in this study open us a new way of synthesis and crystal growth of various kinds of nitrides in a supercritical nitrogen fluid under high temperatures and high pressures yield using diamond anvil cell and YAG laser heating.

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In this article, we introduce recent attempts on our application in high-strength plastic materials for high pressure apparatus. Several plastic materials have enough strength in order to use for structural parts of high-pressure apparatus instead of ordinary metal parts. They have attractive features in magnetism, transparency to light or X-ray, etc. According to the circumstances we may have great advantages by utilizing these interesting characteristics of high-strength plastic materials.

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MgSiO₃ perovskite is believed to be a dominant mineral at least in the upper part of the Earth's lower mantle, but its stability and possible phase transition in deeper levels were not known. Recently we discovered the phase transition from MgSiO₃ perovskite to a new high-pressure form (space group: Cmcm) above 125 GPa and 2500 K on the basis of in-situ x-ray diffraction measurements [1]. This phase transition is most likely responsible for the origin of the D” seismic discontinuity observed around 2700 km depth, and the MgSiO₃ post-perovskite phase is a main constituent mineral in the D” region. Here we introduce the details of high-pressure experiment and crystal structure determination, and discuss the seismic anomalies in the lowermost mantle.

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