Yuki Shibazaki

Ultrahigh-pressure polyamorphism in GeO2 glass with coordination number >6

Significance A new double-stage large-volume cell was developed to compress large GeO2 glass samples to near 100 GPa and to conduct multiangle energy-dispersive X-ray diffraction measurement for in situ structure measurements. We find new experimental evidence of ultrahigh-pressure polyamorphism in GeO2 glass with coordination number (CN) significantly >6. The structural change to CN higher than 6 is closely associated with the change in oxygen-packing fraction. Our results provide direct structural evidence for ultradense network-forming glasses and liquids. The observed ultrahigh-pressure polyamorphism may also exist in other network-forming glasses and liquids as well, such as SiO2 and other silicate and germanate systems. Knowledge of pressure-induced structural changes in glasses is important in various scientific fields as well as in engineering and industry. However, polyamorphism in glasses under high pressure remains poorly understood because of experimental challenges. Here we report new experimental findings of ultrahigh-pressure polyamorphism in GeO2 glass, investigated using a newly developed double-stage large-volume cell. The Ge–O coordination number (CN) is found to remain constant at ∼6 between 22.6 and 37.9 GPa. At higher pressures, CN begins to increase rapidly and reaches 7.4 at 91.7 GPa. This transformation begins when the oxygen-packing fraction in GeO2 glass is close to the maximal dense-packing state (the Kepler conjecture = ∼0.74), which provides new insights into structural changes in network-forming glasses and liquids with CN higher than 6 at ultrahigh-pressure conditions.

Icosahedral AlCuFe quasicrystal at high pressure and temperature and its implications for the stability of icosahedrite.

The first natural-occurring quasicrystal, icosahedrite, was recently discovered in the Khatyrka meteorite, a new CV3 carbonaceous chondrite. Its finding raised fundamental questions regarding the effects of pressure and temperature on the kinetic and thermodynamic stability of the quasicrystal structure relative to possible isochemical crystalline or amorphous phases. Although several studies showed the stability at ambient temperature of synthetic icosahedral AlCuFe up to ~35 GPa, the simultaneous effect of temperature and pressure relevant for the formation of icosahedrite has been never investigated so far. Here we present in situ synchrotron X-ray diffraction experiments on synthetic icosahedral AlCuFe using multianvil device to explore possible temperature-induced phase transformations at pressures of 5 GPa and temperature up to 1773 K. Results show the structural stability of i-AlCuFe phase with a negligible effect of pressure on the volumetric thermal expansion properties. In addition, the structural analysis of the recovered sample excludes the transformation of AlCuFe quasicrystalline phase to possible approximant phases, which is in contrast with previous predictions at ambient pressure. Results from this study extend our knowledge on the stability of icosahedral AlCuFe at higher temperature and pressure than previously examined, and provide a new constraint on the stability of icosahedrite.

Microscopic structural change in a liquid Fe‐C alloy of ~5 GPa

The structure of a liquid Fe-3.5 wt % C alloy is examined for up to 7.2 GPa via multiangle energy-dispersive X-ray diffraction using a Paris-Edinburgh type large-volume press. X-ray diffraction data show clear changes in the pressure-dependent peak positions of structure factor and reduced pair distribution function at 5 GPa. These results suggest that the liquid Fe-3.5 wt % C alloys change structurally at approximately 5 GPa. This finding serves as a microscopic explanation for the alloys previously observed density change at the same pressure. The pressure dependencies of the nearest and second neighbor distances of the liquid Fe-3.5 wt % C alloy are similar to those of liquid Fe which exhibits a structural change near the bcc-fcc-liquid triple point (5.2 GPa and 1991 K). Similarities between Fe-3.5 wt % C and Fe suggest that a density change also occurs in liquid Fe and that this structural change extends to other Fe-light element alloys.

Thermal equation of state of hcp-iron: Constraint on the density deficit of Earth's solid inner core

We conducted high-pressure experiments on hexagonal close packed iron (hcp-Fe) in MgO, NaCl, and Ne pressure-transmitting media and found general agreement among the experimental data at 300 K that yield the best fitted values of the bulk modulus K0 = 172.7(±1.4) GPa and its pressure derivative K0′ = 4.79(±0.05) for hcp-Fe, using the third-order Birch-Murnaghan equation of state. Using the derived thermal pressures for hcp-Fe up to 100 GPa and 1800 K and previous shockwave Hugoniot data, we developed a thermal equation of state of hcp-Fe. The thermal equation of state of hcp-Fe is further used to calculate the densities of iron along adiabatic geotherms to define the density deficit of the inner core, which serves as the basis for developing quantitative composition models of the Earths inner core. We determine the density deficit at the inner core boundary to be 3.6%, assuming an inner core boundary temperature of 6000 K.

Hydrogenation of FeSi under high pressure

Abstract Hydrogen is the most abundant element in the solar system, suggesting that hydrogen is one of the plausible light elements in the planetary cores. To investigate the solubility of hydrogen into FeSi and phase relations of the FeSi-H system under high pressure, we performed in situ X-ray diffraction experiments on the FeSi-H and FeSi systems at high pressure and high temperature. Hydrogen starts to dissolve in FeSi (hydrogenation) and form FeSiHx with cubic B20 structure above 10 GPa. Hydrogen content (x), estimated from the volume difference between the FeSi-H and FeSi systems, increases from 0.07 to 0.22 with increasing pressure for P > 10 GPa. Comparing the present results with hydrogenation pressure of Fe, presence of Si in metal increases the minimal pressure for H incorporation. Hydrogen, therefore, can only incorporate into the Fe-Si core at the deeper part (P > 10 GPa) in the planetary interior.

Interfacial tension measurement of Ni-S liquid using high-pressure X-ray micro-tomography

High-pressure, high-temperature X-ray tomography experiments have been carried out using a large volume toroidal cell, which is optimized for interfacial tension measurements. A wide anvil gap, which corresponds to a field of view in the radiography imaging, was successively maintained to high pressures and temperatures using a composite plastic gasket. Obtained interfacial tensions of Ni-S liquid against Na, K-disilicate melt, were 414 and 336 mN/m at 1253 and 1293 K, respectively. Three-dimensional tomo-graphy images revealed that the sample had an irregular shape at the early stage of melting, suggesting either non-equilibrium in sample texture and force balance or partial melting of surrounding silicate. This information cannot always be obtained from two-dimensional radiographic imaging techniques. Therefore, a three-dimensional tomography measurement is appropriate for the precise interfacial measurements.

X-ray imaging for studying behavior of liquids at high pressures and high temperatures using Paris-Edinburgh press

Several X-ray techniques for studying structure, elastic properties, viscosity, and immiscibility of liquids at high pressures have been integrated using a Paris-Edinburgh press at the 16-BM-B beamline of the Advanced Photon Source. Here, we report the development of X-ray imaging techniques suitable for studying behavior of liquids at high pressures and high temperatures. White X-ray radiography allows for imaging phase separation and immiscibility of melts at high pressures, identified not only by density contrast but also by phase contrast imaging in particular for low density contrast liquids such as silicate and carbonate melts. In addition, ultrafast X-ray imaging, at frame rates up to ∼10(5) frames/second (fps) in air and up to ∼10(4) fps in Paris-Edinburgh press, enables us to investigate dynamics of liquids at high pressures. Very low viscosities of melts similar to that of water can be reliably measured. These high-pressure X-ray imaging techniques provide useful tools for understanding behavior of liquids or melts at high pressures and high temperatures.

Pressure-induced structural change in MgSiO 3 glass at pressures near the Earth’s core–mantle boundary

Significance Knowledge of the structure of MgSiO3 melt at pressures near the Earth’s core–mantle boundary is important in understanding geochemical and geophysical processes at the region. However, there is no structural determination under such ultrahigh pressures. A double-stage Paris–Edinburgh press combined with multiangle energy dispersive X-ray diffraction enabled in situ structure measurements on MgSiO3 glass up to 111 GPa. We report direct experimental evidence of a structural change in this glass at pressures greater than 88 GPa, which is shallower than the pressure of the Earth’s core–mantle boundary. Considering similarities in pressure-induced structural changes between silicate melts and glasses, a similar ultrahigh-pressure structural change may occur in MgSiO3 melts in the deep lower mantle. Knowledge of the structure and properties of silicate magma under extreme pressure plays an important role in understanding the nature and evolution of Earth’s deep interior. Here we report the structure of MgSiO3 glass, considered an analog of silicate melts, up to 111 GPa. The first (r1) and second (r2) neighbor distances in the pair distribution function change rapidly, with r1 increasing and r2 decreasing with pressure. At 53–62 GPa, the observed r1 and r2 distances are similar to the Si-O and Si-Si distances, respectively, of crystalline MgSiO3 akimotoite with edge-sharing SiO6 structural motifs. Above 62 GPa, r1 decreases, and r2 remains constant, with increasing pressure until 88 GPa. Above this pressure, r1 remains more or less constant, and r2 begins decreasing again. These observations suggest an ultrahigh-pressure structural change around 88 GPa. The structure above 88 GPa is interpreted as having the closest edge-shared SiO6 structural motifs similar to those of the crystalline postperovskite, with densely packed oxygen atoms. The pressure of the structural change is broadly consistent with or slightly lower than that of the bridgmanite-to-postperovskite transition in crystalline MgSiO3. These results suggest that a structural change may occur in MgSiO3 melt under pressure conditions corresponding to the deep lower mantle.

Compressional and shear wave velocities for polycrystalline bcc-Fe up to 6.3 GPa and 800 K

Abstract The cores of the Earth and other differentiated bodies are believed to be comprised of iron and various amounts of light elements. Measuring the densities and sound velocities of iron and its alloys at high pressures and high temperatures is crucial for understanding the structure and composition of these cores. In this study, the sound velocities (vP and vS) and density measurements of body-centered cubic (bcc)-Fe were determined experimentally up to 6.3 GPa and 800 K using ultrasonic and X-ray diffraction methods. Based on the measured vP, vS, and density, we obtained the following parameters regarding the adiabatic bulk KS and shear G moduli of bcc-Fe: KS0 = 163.2(15) GPa, ∂KS/̸P = 6.75(33), ∂KS/∂T = –0.038(3) GPa/K, G0 = 81.4(6) GPa, ∂G/̸P = 1.66(14), and ∂G/∂T = –0.029(1) GPa/K. Moreover, we observed that the sound velocity–density relationship for bcc-Fe depended on temperature in the pressure and temperature ranges analyzed in this study and the effect of temperature on vS was stronger than that on vP at a constant density, e.g., 6.0% and 2.7% depression for vS and vP, respectively, from 300 to 800 K at 8000 kg/m3. Furthermore, the effects of temperature on both vP and vS at a constant density were much greater for bcc-Fe than for ε-FeSi (cubic B20 structure), according to previously obtained measurements, which may be attributable to differences in the degree of thermal pressure. These results suggest that the effects of temperature on the sound velocity–density relationship for Fe alloys strongly depend on their crystal structures and light element contents in the range of pressure and temperature studied.