Takuo Okuchi

X-ray Raman scattering study of MgSiO3 glass at high pressure: implication for triclustered MgSiO3 melt in Earth's mantle.

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earths interior. MgSiO3-rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earths interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earths interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO3 glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; [3]O) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO3 melt toward deeper part of the Earths lower mantle.

Efficient storage of hydrogen fuel into leaky cages of clathrate hydrate

We demonstrate an alternative principle to efficiently store molecular hydrogen fuel into clathrate hydrate medium. Hydrogen-free hydrate powders quickly absorb the hydrogen gas at moderate pressure appropriate for industrial applications. The absorption kinetics was observed in situ by nuclear magnetic resonance (NMR) spectroscopy in a pressurized tube. The diffusion of hydrogen through the solid hydrate medium was directly measured by pulsed field gradient NMR. At temperatures down to 250K, the stored hydrogen is still mobile so that the hydrate storage should work well even in cold environments.

Shock Hugoniot and temperature data for polystyrene obtained with quartz standard

Equation-of-state data, not only pressure and density but also temperature, for polystyrene (CH) are obtained up to 510 GPa. The region investigated in this work corresponds to an intermediate region, bridging a large gap between available gas-gun data below 60 GPa and laser shock data above 500 GPa. The Hugoniot parameters and shock temperature were simultaneously determined by using optical velocimeters and pyrometers as the diagnostic tools and the α-quartz as a new standard material. The CH Hugoniot obtained tends to become stiffer than a semiempirical chemical theoretical model predictions at ultrahigh pressures but is consistent with other models and available experimental data.

The melting temperature of iron hydride at high pressures and its implications for the temperature of the Earth's core

The Earths core is about 10% less dense than pure iron under the relevant pressure and temperature conditions, so elements lighter than iron should exist in it. Recent experiments support the hypothesis that hydrogen dominates this light component. The hydrogen dissolved in metallic iron has a large potential to reduce its melting temperature. To estimate the melting temperature of the iron-hydrogen core, experiments were carried out at pressures up to 10 GPa by means of a new technique to determine the melting temperature and chemical composition from the textures of rapidly decompressed iron hydride grains. The rate of reduction of the melting temperature induced by adding hydrogen to iron was obtained as per unit mole fraction. If this behaviour of hydrogen persists to the much higher pressure deep inside the Earth, the isentrope in the iron-hydrogen core is about 600 K lower than the previous estimates based on experiments on the iron-sulphur-oxygen system.

Elasticity of Ferropericlase across the Spin Crossover in the Earth's Lower Mantle.

Knowing the elasticity of ferropericlase across the spin transition can help explain seismic and mineralogical models of the lower-mantle including the origin of seismic heterogeneities in the middle to lowermost parts of the lower mantle1234. However, the effects of spin transition on full elastic constants of ferropericlase remain experimentally controversial due to technical challenges in directly measuring sound velocities under lower-mantle conditions12345. Here we have reliably measured both VP and VS of a single-crystal ferropericlase ((Mg0.92,Fe0.08)O) using complementary Brillouin Light Scattering and Impulsive Stimulated Light Scattering coupled with a diamond anvil cell up to 96 GPa. The derived elastic constants show drastically softened C11 and C12 within the spin transition at 40–60 GPa while C44 is not affected. The spin transition is associated with a significant reduction of the aggregate VP/VS via the aggregate VP softening because VS softening does not visibly occur within the transition. Based on thermoelastic modelling along an expected geotherm, the spin crossover in ferropericlase can contribute to 2% reduction in VP/VS in a pyrolite mineralogical model in mid lower-mantle. Our results imply that the middle to lowermost parts of the lower-mantle would exhibit enhanced seismic heterogeneities due to the occurrence of the mixed-spin and low-spin ferropericlase.

Resonant X-ray emission study of the lower-mantle ferropericlase at high pressures

Abstract Electronic states of iron in Earth’s mantle minerals including ferropericlase, silicate perovskite, and post-perovskite have been previously investigated at high pressures and/or temperatures using various experimental techniques, including X-ray emission and Mössbauer spectroscopies. Although such methods have been used to infer changes in the electronic spin and valence states of iron in lower mantle minerals, they do not directly probe the 3d electronic states quantitatively. Here we use 1s2p resonant X-ray emission spectroscopy (RXES) at the Fe K pre-edge to directly probe and assess the 3d electronic states and the crystal-field splittings of Fe2+ in the lower-mantle ferropericlase [(Mg0.75,Fe0.25)O] at pressures up to 90 GPa. The pre-edge features from X-ray absorption spectroscopy in the partial fluorescence yield (PFY-XAS) and RXES results explicitly show three excited states for high-spin Fe2+ (a lower-energy 4T1g state, a 4T2g state, and a higher-energy 4T1g state) and a single 2Eg state for low-spin Fe2+, attributed to the (t2g)0(eg)3 excited configuration. This latter feature begins to appear at 48 GPa and grows with pressure, while the peaks related to high-spin Fe2+ vanish above 80 GPa. The observed pre-edge features are consistent with purely quadrupolar transitions resulting from the centrosymmetric character of the Fe2+ site. The K pre-edge RXES spectra at the incident energy of 7112 eV, which are similar to the Fe L-edge spectra, are also used successfully to quantitatively obtain consistent results on the spin transition of Fe2+ in ferropericlase under high pressures. Owing to the superior sensitivity of the RXES technique, the observed electronic states and their energy separations provide direct information on the local electronic structures and crystal-field splitting energies of the 3d electronic shells of Fe2+ in ferropericlase at relevant pressures of the Earth’s lower mantle.

Hydrogen bonding and dynamics of methanol by high-pressure diamond-anvil cell NMR

Liquid methanol at densities up to ρ∕ρ0=1.7 was studied by NMR in a specially designed diamond-anvil cell. Methyl and hydroxyl resonances have been separately observed at pressures to 43kbars which exceeds equilibrium freezing pressure of methanol. The chemical shift difference between methyl and hydroxyl protons increases nonlinearly with increasing density, indicating a noticeable decrease in hydrogen bond length. The analyses of spin-lattice relaxation rates of both hydroxyl and methyl protons indicate that compression enhances intermolecular proton exchange and selectively reduces motion of the hydroxyl protons. Collectively these observations reveal that hydrogen bonding interaction in liquid methanol noticeably increases with compression, inhibiting the liquid-solid transition even above the freezing pressure.

Laser-shock compression and Hugoniot measurements of liquid hydrogen to 55 GPa

KYOKUGEN, Center for Quantum Science and Technology under Extreme Conditions,Osaka University, Toyonaka, Osaka 560-8531, Japan(Dated: January 7, 2011)The principal Hugoniot for liquid hydrogen was obtained up to 55 GPa under laser-driven shockloading. Pressure and density of compressed hydrogen were determined by impedance-matching toa quartz standard. The shock temperature was independently measured from the brightness of theshock front. Hugoniot data of hydrogen provide a good benchmark to modern theories of condensedmatter. The initial number density of liquid hydrogen is lower than that for liquid deuterium, andthis results in shock compressed hydrogen having a higher compression and higher temperature thandeuterium at the same shock pressure.

High‐spin Fe2+ and Fe3+ in single‐crystal aluminous bridgmanite in the lower mantle

Spin and valence states of iron in single-crystal bridgmanite (Mg0.89Fe0.12Al0.11Si0.89O3) are investigated using X-ray emission and Mossbauer spectroscopies with laser annealing up to 115 GPa. The results show that Fe predominantly substitutes for Mg2+ in the pseudo-dodecahedral A site, in which 80% of the iron is Fe3+ that enters the lattice via the charge-coupled substitution with Al3+ in the octahedral B site. The total spin momentum and hyperfine parameters indicate that these ions remain in the high-spin state with Fe2+ having extremely high quadrupole splitting due to lattice distortion. (Al,Fe)-bearing bridgmanite is expected to contain mostly high-spin, A-site Fe3+, together with a smaller amount of A-site Fe2+, that remains stable throughout the region. Even though the spin transition of B-site Fe3+ in bridgmanite was reported to cause changes in its elasticity at high pressures, (Fe,Al)-bearing bridgmanite with predominantly A-site Fe will not exhibit elastic anomalies associated with the spin transition.

Radio frequency probe with improved sensitivity for diamond anvil cell nuclear magnetic resonance

An alternative type of single-solenoid radio frequency (rf) probe for diamond anvil cell (DAC) NMR is described. The rf probe provides a higher signal-to-noise ratio (S∕N) than previous designs for DAC-NMR of liquid samples. It induces a practically homogeneous rf field that enables straightforward measurements even at a marginal S∕N. It can be installed and removed without changing the sample pressure, which enhances the efficiency of experimentation beyond that attainable with previous designs. This rf probe will facilitate the application of NMR to liquids at extreme pressures.