Liquid alkali metals in the vicinity of the triple point behave as a simple monatomic metallic liquid. However, as the volume expands under high-pressure and temperature condition, they undergo a metal-nonmetal transition around the critical density. In order to investigate the correlation between the electronic and structural properties, we have performed x-ray diffraction and small angle x-ray scattering measurements for fluid rubidium and cesium up to supercritical conditions using synchrotron radiation at SPring-8. Experimental observation indicates that with volume expansion the nearest neighbor distance starts to decrease and the density fluctuation increases even in the density range where the fluids are metallic. The electronic density where such structural inhomogeneity appears corresponds to the theoretically calculated threshold density where the compressibility of the homogeneous interacting electron gas turns to negative. This finding suggests that the observed structural changes reflect instability of the electron gas.

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We have carried out x-ray diffraction, small-angle x-ray scattering and high-resolution inelastic x-ray scattering measurements for supercritical fluid Hg accompanying the metal-nonmetal (M-NM) transition using synchrotron radiation at SPring-8. In these structural studies we have found an increase in the short-range correlation length and the dynamical sound velocity much faster than the adiabatic sound velocity in the M-NM transition region. These findings strongly suggest the existence of a peculiar fluctuation intrinsic to the M-NM transition in the fluid, which may reflect a first-order M-NM transition predicted by Landau, the winner of the Nobel Prize in physics.

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In this article, a brief review of two different types of molecular fluids is given. One is hydrogen-bonded fluids such as water and lower alcohols, for which the molecules are preserved even in the supercritical condition. The other is covalently bonded fluid selenium, where the twofold coordinated chain structure is broken by thermal agitations and the fluid undergoes a semiconductor-metal transition near the critical point.

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In this article, we review some recent advances in the studies of the pressure-induced structural changes in liquid arsenic and liquid IV-VI compounds. For liquid As, the A7-like local structure has been experimentally observed and attributed to a “Peierls distortion in the liquid state”. Most recently the suppression of the distortion has been observed at high pressures. We also observed the similar pressure dependence in liquid GeS, GeSe, and GeTe, where their structures are distorted at low pressures. We describe the pressure-induced suppression of Peierls-type distortion in these liquids, and show the differences between liquid and crystalline states.

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In this article, recent advances in the theoretical study on structures of liquid metals at high temperatures and high pressures using ab initio molecular-dynamics simulations are reviewed. We are particularly concerned with the pressure-induced liquid-liquid structural change in liquid carbon for a wide range of pressure and the melting curve maximum of liquid sodium at high pressures.

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In situ high-pressure X-ray diffraction studies on the structure of hydrous Mg-silicate melts have been performed at superliquidus temperatures up to 7 GPa using a newly developed encapsulation method. The results of in situ experiments on hydrous silicate melts demonstrate the characteristic structural changes in the silicate network and local structures with pressure. This paper discusses the effect of water on the silicate network and the short-range order of a hydrous silicate melt at high pressures, based on the recent structural studies and previous works on silicate glasses.

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This is a review paper presented as a Norman L. Bowen Lecture at the 2007 AGU fall meeting in San Francisco in December 2007. Melting relations of the mantle minerals together with the physical properties of silicate melts at high pressure have been extensively studied during recent decades. Melting relations of minerals and equation of states of magmas are especially important for formation and differentiation of the Earth, such as nature of the terrestrial magma ocean, and subsequent formation of the core, mantle, and crust of the Earth. Since the magmas are compressible, we can expect that an olivine-magma density crossover played an important role for controlling the geochemical nature of the primitive mantle after the magma ocean of the primordial Earth. The crystal-magma density crossover is also expected at the base of the upper mantle in presence of volatiles, and at the base of the lower mantle. Existence of dense magmas in the present earth is consistent with seismological observations of low velocity regions existing at the base of the upper mantle beneath Japan, Europe, and US, and ultralow velocity zone (ULVZ) at the core mantle boundary of the present Earth.

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High-pressure processed food is in the stream of hundred years’ history of “high-pressure biology” which concerns high-pressure effects on biological systems. This review briefly describes the research histories of high-pressure effects on deep-sea organism, biological reaction, enzyme and protein, cell membrane and lipid, and muscle contraction, citing proceeding books of several symposia, which were mainly held in Europe and U.S.A. from 1950 to 1990. Burst of study on high-pressure food science in 1987 stimulated to found an international organization with high-pressure biology and food science, and led to the successful establishment of the International Conference of High Pressure Bioscience and Biotechnology (HPBB). The first conference was honorably held in Kyoto as HPBB-2000, promising the successive holding in every two years. This review also suggests artificial science in addition to natural science to give a logical background in applying unnaturally high compression of water to living systems.

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