Melting phase relations of mantle peridotite and basaltic oceanic crust are essentially important for understanding the chemical evolution of the Earth's mantle. Here we discuss melting phase relations of KLB-1 peridotite and mid-ocean ridge basalt (MORB) at the uppermost lower mantle based on our recent experimental results. The results show the narrow solidus-liquidus interval within 200 K and that MORB has lower solidus temperature than KLB-1 peridotite at 27 GPa. We also estimated the density of partial silicate melts at high pressures and found that partial melts of KLB-1 peridotite may be gravitationally unstable at the uppermost lower mantle conditions, whereas partial melts of MORB may be stable. This suggests that the distribution of partial melts at the deep Earth`s mantle is different between KLB-1 peridotite and MORB.

Viscosity is one of the most fundamental physical properties controlling migration processes of magmas and liquids in the Earth's interior. Here we introduce recent advances in viscosity measurement of liquids under high pressure and high temperature conditions. Capability of viscosity measurement using the falling sphere technique combined with large volume press predominantly depends on speed of X-ray imaging. Recent development of ultrafast X-ray imaging with more than 1,000 frames/second imaging rate opens a new way to investigate viscosity of low viscous melts such as carbonate melts and molten salts at in situ high pressure and high temperature conditions.

Knowledge of distribution and transport mechanism of melt in the Earth and planetary interiors is important for understanding formation process of stratified structure, estimation of melt fraction in partial molten region, and volatile recycling. In this article, advances in high pressure studies on melt distribution and its transport process were reviewed in terms of surface tension-driven process. Melt migration in solid media is controlled by melt connectivity. A traditional way to determine melt connectivity is determination of dihedral angle by quench experiments. Recently, in situ X-ray radiographic observation or electrical conductivity measurement are developing as effective ways to determine melt morphology. At the end of this article, an example of melt penetration experiments is reported. High pressure experiments on penetration of metallic iron alloy melt into lower mantle phases suggest absence of interaction between core and mantle by surface-tension driven penetration mechanism.

Planetary cores are considered to be composed of iron (Fe) and light elements. Knowledge of structure and properties of liquid Fe-light element alloys under high pressures is essential to discuss the nature and dynamics of planetary liquid cores such as Earth's outer core, while those remain poorly understood. In this article, we introduce our recent studies about the influence of possible light elements [silicon (Si), carbon (C), and sulfur (S)] on the structure of liquid Fe, and the relation between the structure and physical property of liquid Fe-light element alloys.

The Earth's core is mainly composed of iron. Since liquid outer core coexists with solid at the inner core boundary (ICB), the melting temperature of iron provides a key constraint on the temperature of the core. In this article, we will introduce our recent progress on the experimental approach to determine the melting temperature of iron at high-pressure by using internal-resistance-heated diamond-anvil cell. We determined the melting temperature of pure iron up to 290 GPa, for the first time above 200 GPa by static compression experiments. A small extrapolation of the present experimental results yields a melting point of 5,500±220 K at the ICB.

Hexagonal boron nitride (hBN) and cubic BN (cBN) are known as heat-resistant materials and super-hard materials respectively. Some progresses in the synthesis of high purity BN crystals were achieved by using Ba-BN as a growth solvent material exhibiting BN's band-edge natures (cBN E_g=6.2 eV and hBN E_g=6.4 eV). By reducing oxygen and carbon impurities, an attractive potential of hBN as a deep ultraviolet (DUV) light emitter and also superior properties as substrates of graphene devices were realized. Our experimental study on BN crystals with respect to their phase transformation behavior and impurity control is reviewed.

The permanent densification of SiO₂ glass has long attracted considerable attention in various fields such as condensed-matter physics, materials science, and geophysics. We have conducted isotropic- and uniaxial-compression experiments and ab-initio molecular-dynamics simulations to understand the permanent densification. It has been clarified that the permanent densification can be interpreted as a phase transition in the intermediate-range order (the network structure of SiO₄ tetrahedra) and it changes not only density but also plasticity. This article attempts to review the permanent densification of SiO₂ glass and introduce an application to estimate the behavior of magma in the Earth's deep upper mantle, based on our recent researches.

The hydrostatic pressure effect on multiferroic properties of rare-earth manganites RMnO₃ (R=Dy, Tb, and Gd) with the Pbnm orthorhombic structure was investigated by using a diamond anvil cell. Under high pressures, all of these manganites were found to exhibit ferroelectricity in which spontaneous polarization (≈10⁰ µC cm^－2) develops along the a axis. The application of a magnetic field B along the a axis to the high-pressure ferroelectric phase causes a drastic change in the electric polarization P. Especially in GdMnO₃, the B-induced change in P reached ΔP(B)=1.3 µC cm^－2. A possible origin of the observed R-dependent pressure effect is discussed.

GHz ultrasonic velocity measurement has been developed up to ～15 GPa in a diamond anvil cell (DAC). In order to perform the GHz ultrasonic measurement, both P and S wave buffer rods were manufactured through sputtering ZnO piezoelectric film on single crystal rods, such as sapphire, rutile (TiO₂), and YAG. Several technical breakthroughs have been achieved before observing stable signals from the specimen squeezed in DAC: (1) Wiping by aqueous ammonia on the contact surfaces enhances transmissibility of GHz waves from the buffer rod to the diamond anvil. (2) Cleaning with lathery detergent and water jet effectively removes grime on the rear surface of the diamond. (3) A steel ball supporter allows effortless adjustment of contact between the buffer rod and the diamond. As a result, we succeeded in demonstrating the GHz-DAC ultrasonic velocity measurement on KCl and natural olivine samples.