Yoshinori Katayama

High-pressure science with a multi-anvil apparatus at SPring-8

Since first opening its doors to public research in 1997, SPring-8 has seen the accomplishment of many important studies in a wide variety of fields through its stable operation and cutting edge technology. High-pressure experiments have been carried out on a number of beamlines using a diamond anvil cell or a multi-anvil press. Here, we review the multi-anvil presses installed on the SPring-8 beamlines and a few research projects currently utilizing this technology. The significant difference in post-spinel boundary between multi-anvil experiments and diamond anvil studies will also be discussed.

X-ray structural studies on elemental liquids under high pressures

X-ray structural studies on several elemental liquids under high pressure are reviewed. Combination of synchrotron radiation sources and large-volume presses enables us to carry out in situ structural measurements on liquids at high pressures up to several gigapascals. The measurements have revealed that compressions of liquid alkali metals are almost uniform, whereas those of liquids that have covalent components in bonding are mostly anisotropic. For covalent liquids, the volume dependence of the nearest-neighbour distance deviates from (V/V0)1/3 behaviour (V being the molar volume and V0 being the molar volume at zero pressure) and changes in coordination play important roles. In some elements, different types of volume dependence of the nearest-neighbour distances are observed in different pressure ranges. This behaviour suggests that the liquid phase can be divided into regions. Although most of the observed structural changes are continuous, a discovery of an abrupt structural change in liquid phosphorus, which is completed over a pressure range of less than 0.05 GPa around 1 GPa and 1050°C, supports the existence of a first-order liquid–liquid phase transition.

In situ observation of a first-order liquid–liquid transition in phosphorus

Abstract Recently, we have found an abrupt, pressure-induced structural change between two distinct forms of liquid phosphorus at about 1 GPa by an in situ X-ray diffraction method. X-ray diffraction of liquid phosphorus was measured up to 4.6 GPa and that of red amorphous phosphorus was measured at atmospheric pressure and 3.0 GPa. A model liquid of uncorrelated P4 molecules roughly reproduces the structure factor of the low-pressure form. On the other hand, there are similarities in the structure factors of the high-pressure form, red phosphorus and liquid arsenic, indicating that the high-pressure form has a polymeric nature.

X-Ray Diffraction Study on Structural Change in Liquid Selenium under High Pressure

X-ray diffraction experiments on liquid selenium have been performed near the melting temperatures in a pressure range from 2.6 to 4.9 GPa. At atmospheric pressure, liquid Se consists of long-chain molecules. The structure factor at 2.6 GPa was similar to that at ambient pressure. Above 3.5 GPa, however, the shape of the structure factor started to change. The ratio of the height of the first peak to that of the second peak increased with increasing pressure. The change of the structure of liquid Se is discussed in connection with a proposed semiconductor-metal transition in liquid Se under high pressure.

Structural transformations in liquid, crystalline, and glassy B2O3 under high pressure

We present in situ (x-ray diffraction) and ex situ (quenching) structural studies of crystalline, liquid, and glassy B2O3 up to 9 GPa and 1700 K, drawing equilibrium and nonequilibrium phase diagrams of B2O3. Particularly, we have determined the melting curve, the stability regions for crystalline B2O3 I and B2O3 II modifications, the regions of transformations, such as densification or crystallization, for both the liquid and glassy states, including the region of sharp first-order-like transition in liquid B2O3 to a high-density phase near 7 GPa. Quenching experiments also show that the transition to the high-density liquid can occur at much lower pressures in nonstoichiometric melts with an excess of boron. B2O3 is the first glassformer whose transformations in the disordered state have been comparatively studied for both liquid and glassy phases.

XAFS study on liquid selenium under high pressure.

Using large-volume presses, it is now possible to measure X-ray absorption spectra under a wide range of pressure-temperature conditions. X-ray absorption fine structure (XAFS) has successfully been measured at the K edge of crystalline and liquid Se at temperatures up to 1023 K and pressures of about 5 and 8 GPa, and at temperatures up to 1173 K at about 2.5 GPa. Crystalline Se consists of infinite chain molecules. At atmospheric pressure, it is known that the chain structure of Se is largely preserved upon melting. The temperature dependence of the extended X-ray absorption fine structure (EXAFS) at 2.5 GPa indicates that twofold covalent bonds remain upon melting, as at atmospheric pressure. On the other hand, the decrease of EXAFS oscillation upon melting at 8 GPa is larger than that expected from the temperature dependence of EXAFS in the crystalline state, indicating that the covalent bonds are modified in the liquid state. The change of structure of liquid Se is discussed in relation to a proposed semiconductor-metal transition under high pressure.

EXAFS study on liquid selenium and liquid tellurium under high pressure

Extended X-ray absorption fine structure (EXAFS) measurements were performed on crystalline and liquid Se at 2.5 GPa. A decrease of amplitude of the EXAFS oscillation for liquid Se was observed at temperatures >1050 K. The decrease was larger than that expected from the normal thermal effect and it indicates that the two-fold chain structure in liquid Se is modified at temperatures >1050 K. The change occurs near the boundary of a semiconductor-to-metal transition. This coincidence confirms that the transition is accompanied by a structural change. EXAFS for crystalline and liquid Te at 6 GPa were also measured. The peak amplitude in radial distribution function for liquid Te at 6 GPa is smaller than that observed at the similar temperature and atmospheric pressure. We suggest that the decrease is due to the modification of the covalent bonds.

Synchrotron radiation studies on pressure-induced structural changes in liquids and glasses

Combination of synchrotron radiation sources and large-volume presses enables us to carry out in situ observations of structural change in liquids and glasses at high pressures up to several gigaPascals and high temperatures above 1200oC. In this report, we present two examples: liquid Sn and SiO2 glass. X-ray diffraction measurements were carried out on liquid Sn at 2.0 and 5.3 GPa. With increasing pressure, a shoulder of the first peak in the structure factor became less prominent and the ratio of the position of the second peak to that of the first peak decreased. These changes are attributed to a diminishing of the covalent structures remaining in the liquid state. Data on the temperature dependence of the x-ray diffraction by SiO2 glass were measured up to 560oC at 17 GPa. The sample crystallized to stishovite above 560oC. While no drastic change was observed in the short-range order, a shift of the position of the first sharp diffraction peak to higher wavenumber and a sharpening of it were observed with increasing temperature. The shift and the sharpening suggest some relaxation in the intermediate-range order.

Pressure Dependence of Effective Pair Potentials in AgBr Determined by Extended X-Ray Absorption Fine Structure

The pressure dependence of extended X-ray absorption fine structure (EXAFS) Debye-Waller factors in AgBr has been investigated using the cumulant expansion method. The Br K-edge EXAFS spectra were measured in the transmission mode under high pressure (≤9.1 GPa) at room temperature using a cubic anvil type apparatus (MAX90) and synchrotron radiation from the Photon Factory, Tsukuba. The effective pair potentials, V(u)=au2/2+bu3/3!, were evaluated and the potential coefficient a at 2.1, 4.2 and 6.1 GPa are 1.59(4), 1.75(4) and 1.91(4) eV/A-2, respectively. The energies of the third-order anharmonic potential coefficient b maintain nearly constant values with pressure though the third-order cumulant σ3 decreases with increasing in pressure.