Naohisa Hirao

A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data.

The determination of the chemical composition of Earth’s lower mantle is a long-standing challenge in earth science. Accurate knowledge of sound velocities in the lower-mantle minerals under relevant high-pressure, high-temperature conditions is essential in constraining the mineralogy and chemical composition using seismological observations, but previous acoustic measurements were limited to a range of low pressures and temperatures. Here we determine the shear-wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle using Brillouin scattering spectroscopy. The mineralogical model that provides the best fit to a global seismic velocity profile indicates that perovskite constitutes more than 93 per cent by volume of the lower mantle, which is a much higher proportion than that predicted by the conventional peridotitic mantle model. It suggests that the lower mantle is enriched in silicon relative to the upper mantle, which is consistent with the chondritic Earth model. Such chemical stratification implies layered-mantle convection with limited mass transport between the upper and the lower mantle.

Highly intense monochromatic X-ray diffraction facility for high-pressure research at SPring-8

Beamline BL10XU at SPring-8, designed for X-ray diffraction experiments using diamond anvil cells at high pressure and low/high temperature, is continuously upgraded. The X-ray source, optics, and attractive experimental equipment such as simultaneous measurement systems have been optimized over the past years. The high energy and high intensity monochromatic X-ray beams emitted by an undulator source, focused using a characteristic X-ray refractive lens, have enabled us to obtain excellent counting statistics and high-resolution X-ray diffraction data even from a highly compressed sample under multi-megabar pressure. At BL10XU, low- and high-temperature conditions are achieved using a cryostat (10–300 K) and double-sided laser-heating system (1000–4000 K), respectively. Numerous results have been obtained in the fields of high-pressure materials science and mineral physics: for instance, the structural information on novel materials under pressure, including new pressure-induced structural phase transitions, the measurements of equations of states, and the phase-equilibrium data of Earth interior minerals.

Crystal structure of the superconducting phase of sulfur hydride

A superconducting critical temperature above 200 K has recently been discovered in H2S (or D2S) under high hydrostatic pressure1, 2. These measurements were interpreted in terms of a decomposition of these materials into elemental sulfur and a hydrogen-rich hydride that is responsible for the superconductivity, although direct experimental evidence for this mechanism has so far been lacking. Here we report the crystal structure of the superconducting phase of hydrogen sulfide (and deuterium sulfide) in the normal and superconducting states obtained by means of synchrotron X-ray diffraction measurements, combined with electrical resistance measurements at both room and low temperatures. We find that the superconducting phase is mostly in good agreement with theoretically predicted body-centered cubic (bcc) structure for H3S (Ref.3). The presence of elemental sulfur is also manifest in the X-ray diffraction patterns, thus proving the decomposition mechanism of H2S to H3S + S under pressure4–6.

Fe-Mg partitioning between perovskite and ferropericlase in the lower mantle

Abstract Fe-Mg partitioning between perovskite and ferropericlase in the MgO-FeO-SiO2 system has been studied up to about 100 GPa at around 2000 K using a laser-heated diamond anvil cell (LHDAC). The compositions of both phases were determined by using analytical transmission electron microscopy (ATEM) on the recovered samples. Present results reveal that the Fe-Mg apparent partition coefficient between perovskite and ferropericlase [KDPv/Fp = (XFePv XMgFp)/(XMgPv XFeFp)] decreases with increasing pressure for a constant FeO of the system, and it decreases with increasing FeO content of ferropericlase. The gradual decrease of KDPv/Fp with increasing pressure is consistent with the spin transition in ferropericlase occurring in the broad pressure range from 50 to 100 GPa at around 2000 K.

Phase transition of FeO and stratification in earth's outer core

Stratified convection in the outer core may influence Earth’s magnetic field. Light elements such as oxygen in Earth’s core influence the physical properties of the iron alloys that exist in this region. Describing the high-pressure behavior of these materials at core conditions constrains models of core structure and dynamics. From x-ray diffraction measurements of iron monoxide (FeO) at high pressure and temperature, we show that sodium chloride (NaCl)–type (B1) FeO transforms to a cesium chloride (CsCl)–type (B2) phase above 240 gigapascals at 4000 kelvin with 2% density increase. The oxygen-bearing liquid in the middle of the outer core therefore has a modified Fe–O bonding environment that, according to our numerical simulations, suppresses convection. The phase-induced stratification is seismologically invisible but strongly affects the geodynamo.

Development of an energy-domain 57Fe-Mössbauer spectrometer using synchrotron radiation and its application to ultrahigh-pressure studies with a diamond anvil cell.

An energy-domain (57)Fe-Mössbauer spectrometer using synchrotron radiation (SR) with a diamond anvil cell (DAC) has been developed for ultrahigh-pressure measurements. The main optical system consists of a single-line pure nuclear Bragg reflection from an oscillating (57)FeBO(3) single crystal near the Néel temperature and an X-ray focusing device. The developed spectrometer can filter the Doppler-shifted single-line (57)Fe-Mössbauer radiation with a narrow bandwidth of neV order from a broadband SR source. The focused incident X-rays make it easy to measure a small specimen in the DAC. The present paper introduces the design and performance of the SR (57)Fe-Mössbauer spectrometer and its demonstrative applications including the newly discovered result of a pressure-induced magnetic phase transition of polycrystalline (57)Fe(3)BO(6) and an unknown high-pressure phase of Gd(57)Fe(2) alloy placed in a DAC under high pressures up to 302 GPa. The achievement of Mössbauer spectroscopy in the multimegabar range is of particular interest to researchers studying the nature of the Earths core.

Natural dissociation of olivine to (Mg,Fe)SiO3 perovskite and magnesiowüstite in a shocked Martian meteorite

We report evidence for the natural dissociation of olivine in a shergottite at high-pressure and high-temperature conditions induced by a dynamic event on Mars. Olivine (Fa34-41) adjacent to or entrained in the shock melt vein and melt pockets of Martian meteorite olivine-phyric shergottite Dar al Gani 735 dissociated into (Mg,Fe)SiO3 perovskite (Pv)+magnesiowüstite (Mw), whereby perovskite partially vitrified during decompression. Transmission electron microscopy observations reveal that microtexture of olivine dissociation products evolves from lamellar to equigranular with increasing temperature at the same pressure condition. This is in accord with the observations of synthetic samples recovered from high-pressure and high-temperature experiments. Equigranular (Mg,Fe)SiO3 Pv and Mw have 50–100 nm in diameter, and lamellar (Mg,Fe)SiO3 Pv and Mw have approximately 20 and approximately 10 nm in thickness, respectively. Partitioning coefficient, KPv/Mw = [FeO/MgO]/[FeO/MgO]Mw, between (Mg,Fe)SiO3 Pv and Mw in equigranular and lamellar textures are approximately 0.15 and approximately 0.78, respectively. The dissociation of olivine implies that the pressure and temperature conditions recorded in the shock melt vein and melt pockets during the dynamic event were approximately 25 GPa but 700 °C at least.

Peculiar High-Pressure Behavior of BiMnO3

High-pressure structural properties of perovskite-type BiMnO(3) have been investigated by synchrotron X-ray powder diffraction at room temperature. A new monoclinic phase having P2(1)/c symmetry was found between about 1.5 and 5.5 GPa. Above 8 GPa, the orthorhombic GdFeO(3)-type phase (space group Pnma) is stable. The crystal structure of BiMnO(3) at 8.6 GPa and room temperature was investigated (a = 5.5132(3) A, b = 7.5752(3) A, c = 5.4535(3) A). The orthorhombic phase of BiMnO(3) has an orbital order similar to LaMnO(3) but with a different arrangement of orbitals in the ac plane. High-pressure room-temperature behavior of BiMnO(3) differs from high-temperature behavior at ambient pressure in comparison with BiCrO(3) and BiScO(3). These findings may open new directions in investigation of BiMnO(3).

Stability of hydrous phase H MgSiO4H2 under lower mantle conditions

We report the stability field of a new high-pressure hydrous phase, phase H MgSiO4H2, and its implications for water transport into the deep lower mantle. We observed the existence of hydrous phase H at pressures around 50 GPa, and this phase was stable up to 60 GPa. Our results, together with those of previous works, indicate that pure phase H MgSiO4H2 has a very narrow stability field in the pressure range 35 < P < 60 GPa, equivalent to the uppermost part of the lower mantle. The stability field expands significantly toward higher pressures and temperatures on dissolution of the hydrous AlOOH component. The hydrous phase H-phase δ solid solution (aluminous phase H), (MgSi,Al2)O4H2, is potentially the most important hydrous phase present under the deep lower mantle conditions.

Generation and Application of Ultrahigh Monochromatic X-ray Using High-Quality 57FeBO3 Single Crystal

Ultrahigh monochromatic 14.4 keV X-rays with a narrow bandwidth of 15.4 neV were generated successfully with a high counting rate of 12,000 counts/s at the undulator beamline (BL11XU) of SPring-8. It was achieved by combining an intense X-ray from the third generation synchrotron radiation facility SPring-8 and pure nuclear Bragg scattering of a very high-quality 57FeBO3 perfect single crystal at the Neel temperature. We describe the detailed study of the beam characteristics and some performance test experiments of energy-domain synchrotron radiation Mossbauer spectroscopy, including a high-pressure experiment using a diamond anvil cel.