Density deficit of Earth's core revealed by a multimegabar primary pressure scale.

An accurate pressure scale is a fundamental requirement to understand planetary interiors. Here, we establish a primary pressure scale extending to the multimegabar pressures of Earth's core, by combined measurement of the acoustic velocities and the density from a rhenium sample in a diamond anvil cell using inelastic x-ray scattering and x-ray diffraction. Our scale agrees well with previous primary scales and shock Hugoniots in each experimental pressure range and reveals that previous scales have overestimated laboratory pressures by at least 20% at 230 gigapascals. It suggests that the light element content in Earth's inner core (the density deficit relative to iron) is likely to be double what was previously estimated, or Earth's inner core temperature is much higher than expected, or some combination thereof.

Sound velocity of hexagonal close-packed iron to the Earth's inner core pressure.

Here we determine the compressional and shear wave velocities (vp and vs) of hexagonal close-packed iron, a candidate for the main constituent of the Earth's inner core, to pressures above 300 gigapascals using a newly designed diamond anvil cell and inelastic X-ray scattering combined with X-ray diffraction. The present results reveal that the vp and vs of the Preliminary reference Earth model (PREM) inner core are 4(±2)% and 36(±17)% slower than those of the pure iron, respectively at the centre of the core. The density and sound velocity of the PREM inner core can be explained by addition of 3(±1) wt% silicon and 3(±2) wt% sulphur to iron‒5 wt% nickel alloy. Our suggested inner core composition is consistent with the existing outer core model with oxygen, as the growth of the inner core may have created a secular enrichment of the element in the outer core.

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.

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.

New developments in micro-X-ray diffraction and X-ray absorption spectroscopy for high-pressure research at 16-BM-D at the Advanced Photon Source.

The monochromator and focusing mirrors of the 16-BM-D beamline, which is dedicated to high-pressure research with micro-X-ray diffraction (micro-XRD) and X-ray absorption near edge structure (XANES) (6-45 keV) spectroscopy, have been recently upgraded. Monochromatic X-rays are selected by a Si (111) double-crystal monochromator operated in an artificial channel-cut mode and focused to 5 µm × 5 µm (FWHM) by table-top Kirkpatrick-Baez type mirrors located near the sample stage. The typical X-ray flux is ∼5 × 10(8) photons/s at 30 keV. The instrumental resolution, Δq/qmax, reaches to 2 × 10(-3) and is tunable through adjustments of the detector distance and X-ray energy. The setup is stable and reproducible, which allows versatile application to various types of experiments including resistive heating and cryogenic cooling as well as ambient temperature compression. Transmission XANES is readily combined with micro-XRD utilizing the fixed-exit feature of the monochromator, which allows combined XRD-XANES measurements at a given sample condition.

High-pressure studies of Bi2S3.

The high-pressure structural and vibrational properties of Bi2S3 have been probed up to 65 GPa with a combination of experimental and theoretical methods. The ambient-pressure Pnma structure is found to persist up to 50 GPa; further compression leads to structural disorder. Closer inspection of our structural and Raman spectroscopic results reveals notable compressibility changes in specific structural parameters of the Pnma phase beyond 4-6 GPa. By taking the available literature into account, we speculate that a second-order isostructural transition is realized near that pressure, originating probably from a topological modification of the Bi2S3 electronic structure near that pressure. Finally, the Bi(3+) lone-electron pair (LEP) stereochemical activity decreases against pressure increase; an utter vanishing, however, is not expected until 1 Mbar. This persistence of the Bi(3+) LEP activity in Bi2S3 can explain the absence of any structural transitions toward higher crystalline symmetries in the investigated pressure range.

Direct observation of a pressure-induced precursor lattice in silicon.

Detailed knowledge of atomic-scale structural change is essential for understanding the process and mechanism of phase transitions in solids. We present the direct experimental evidence of a precursor lattice in silicon at high pressures. The precursor lattice may appear to coexist dynamically with the host lattice over a large pressure range through rapid lattice fluctuations. The first-principles calculations are used to elucidate a dynamic lattice-fluctuation mechanism that accounts for the experimental observations. This precursor lattice-fluctuation mechanism for the phase transition goes beyond previously considered reconstructive or displacive processes and provides a novel picture of the underlying dynamics.