Pressure-induced structural change in MgSiO3 glass at pressures near the Earth's core-mantle boundary.

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.

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

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.

Unsupervised machine learning combined with 4D scanning transmission electron microscopy for bimodal nanostructural analysis.

Unsupervised machine learning techniques have been combined with scanning transmission electron microscopy (STEM) to enable comprehensive crystal structure analysis with nanometer spatial resolution. In this study, we investigated large-scale data obtained by four-dimensional (4D) STEM using dimensionality reduction techniques such as non-negative matrix factorization (NMF) and hierarchical clustering with various optimization methods. We developed software scripts incorporating knowledge of electron diffraction and STEM imaging for data preprocessing, NMF, and hierarchical clustering. Hierarchical clustering was performed using cross-correlation instead of conventional Euclidean distances, resulting in rotation-corrected diffractions and shift-corrected maps of major components. An experimental analysis was conducted on a high-pressure-annealed metallic glass, Zr-Cu-Al, revealing an amorphous matrix and crystalline precipitates with an average diameter of approximately 7 nm, which were challenging to detect using conventional STEM techniques. Combining 4D-STEM and optimized unsupervised machine learning enables comprehensive bimodal (i.e., spatial and reciprocal) analyses of material nanostructures.

Alignment and use of microbeam with full-field x-ray microscopes.

Demonstration tests of the alignment of Fresnel zone plate focusing optics using a full-field x-ray microscope and microbeam x-ray diffraction measurements combined with the full-field x-ray microscope were performed. It was confirmed that the full-field x-ray microscope enables direct two-dimensional observation of a microbeam with sub-micrometer spatial resolution. This allowed visualization of the misalignment of the focusing optics, resulting in accurate alignment of the optics within a short time. In addition, the microscope could be used to observe the sample as well as the microbeam, which enabled clarification of the position and two-dimensional shape of the microbeam on the sample. This realized a measurement procedure that a 100-µm-size sample was imaged with sub-micrometer spatial resolution, and then, microbeam-use measurements were performed for only the region of interest determined by the microscope, which has been difficult with conventional microbeam applications. The combination of observations by a full-field x-ray microscope and measurements using a microbeam is expected to open a new style of measurement.

Siliceous zeolite-derived topology of amorphous silica.

The topology of amorphous materials can be affected by mechanical forces during compression or milling, which can induce material densification. Here, we show that densified amorphous silica (SiO2) fabricated by cold compression of siliceous zeolite (SZ) is permanently densified, unlike densified glassy SiO2 (GS) fabricated by cold compression although the X-ray diffraction data and density of the former are identical to those of the latter. Moreover, the topology of the densified amorphous SiO2 fabricated from SZ retains that of crystalline SZ, whereas the densified GS relaxes to pristine GS after thermal annealing. These results indicate that it is possible to design new functional amorphous materials by tuning the topology of the initial zeolitic crystalline phases.

X-ray zooming microscopy with two Fresnel zone plates.

We propose a variable-magnification full-field x-ray microscope using two Fresnel zone plates (FZPs). By moving the positions of the two FZPs, the magnification can be continuously changed even if the sample and camera positions are fixed. It was demonstrated that the magnification can be changed in the range of 25-150× using a hard x-ray beam at 14.4 keV. Using the first FZP as a convex lens and the second FZP as a concave lens, high magnification can be achieved at a short camera length. Even under the condition of a camera length of about 7 m, a magnification higher than 300× was achieved, and a line and space pattern with a pitch of 40 nm was observed at 10 keV. By inserting a knife edge at an appropriate position in the optical system, a phase-contrast image can be easily obtained, which is useful for soft-tissue observation of biological samples.

Breakdown of One-to-One Correspondence in Energy and Volume in a High-Pressure Heat-Treated Zr-Based Metallic Glass During Annealing.

The glassy state of a high-pressure heat-treated Zr50Cu40Al10 metallic glass was investigated from energetic and volumetric perspectives. The specific heat (Cp) data of initial heating and subsequent cooling from the supercooled liquid region indicates that the sample behaviour cannot be explained simply by the derivative of the well-known enthalpy (H)-temperature (T) curve. Unlike the Cp data, the thermal-expansion coefficient (α) value increased monotonically during the first heating step, which suggests the collapse of a one-to-one correspondence between the energy and volume during the process. The α data of the cooling process follow almost the same path as those of the as-cast process, whereas the corresponding Cp curves do not. This result implies that the volume appears to be more sensitive to obeying external heat compared with energy, which highlights the different time scale for achieving an equilibrium state in energy and volume. The second heating data of the Cp and α exhibit an inverse relationship against the corresponding first heating set, which confirms the breakdown of a one-to-one correspondence during annealing. The newly constructed energy-density diagram shows that the treated sample is rejuvenated volumetrically but is relaxed energetically during aging, which has never been observed experimentally previously.

Effect of sulfur on sound velocity of liquid iron under Martian core conditions.

Sulfur has been considered to be a predominant light element in the Martian core, and thus the sound velocity of Fe-S alloys at relevant high pressure and temperature is of great importance to interpret its seismological data. Here we measured the compressional sound velocity (VP) of liquid Fe, Fe80S20 and Fe57S43 using ultrasonic pulse-echo overlap method combined with a Kawai-type multi-anvil apparatus up to 20 GPa, likely corresponding to the condition at the uppermost core of Mars. The results demonstrate that the VP of liquid iron is least sensitive to its sulfur concentration in the Mars' whole core pressure range. The comparison of seismic wave speeds of Fe-S liquids with future observations will therefore tell whether the Martian core is molten and contains impurity elements other than sulfur.

Compressed glassy carbon maintaining graphite-like structure with linkage formation between graphene layers.

Amorphous diamond, formed by high-pressure compression of glassy carbon, is of interests for new carbon materials with unique properties such as high compressive strength. Previous studies attributed the ultrahigh strength of the compressed glassy carbon to structural transformation from graphite-like sp2-bonded structure to diamond-like sp3-bonded structure. However, there is no direct experimental determination of the bond structure of the compressed glassy carbon, because of experimental challenges. Here we succeeded to experimentally determine pair distribution functions of a glassy carbon at ultrahigh pressures up to 49.0 GPa by utilizing our recently developed double-stage large volume cell. Our results show that the C-C-C bond angle in the glassy carbon remains close to 120°, which is the ideal angle for the sp2-bonded honey-comb structure, up to 49.0 GPa. Our data clearly indicate that the glassy carbon maintains graphite-like structure up to 49.0 GPa. In contrast, graphene interlayer distance decreases sharply with increasing pressure, approaching values of the second neighbor C-C distance above 31.4 GPa. Linkages between the graphene layers may be formed with such a short distance, but not in the form of tetrahedral sp3 bond. The unique structure of the compressed glassy carbon may be the key to the ultrahigh strength.

Interstitial hydrogen atoms in face-centered cubic iron in the Earth's core.

Hydrogen is likely one of the light elements in the Earth's core. Despite its importance, no direct observation has been made of hydrogen in an iron lattice at high pressure. We made the first direct determination of site occupancy and volume of interstitial hydrogen in a face-centered cubic (fcc) iron lattice up to 12 GPa and 1200 K using the in situ neutron diffraction method. The transition temperatures from the body-centered cubic and the double-hexagonal close-packed phases to the fcc phase were higher than reported previously. At pressures <5 GPa, the hydrogen content in the fcc iron hydride lattice (x) was small at x < 0.3, but increased to x > 0.8 with increasing pressure. Hydrogen atoms occupy both octahedral (O) and tetrahedral (T) sites; typically 0.870(±0.047) in O-sites and 0.057(±0.035) in T-sites at 12 GPa and 1200 K. The fcc lattice expanded approximately linearly at a rate of 2.22(±0.36) Å3 per hydrogen atom, which is higher than previously estimated (1.9 Å3/H). The lattice expansion by hydrogen dissolution was negligibly dependent on pressure. The large lattice expansion by interstitial hydrogen reduced the estimated hydrogen content in the Earth's core that accounted for the density deficit of the core. The revised analyses indicate that whole core may contain hydrogen of 80(±31) times of the ocean mass with 79(±30) and 0.8(±0.3) ocean mass for the outer and inner cores, respectively.

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.

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.