Nagayoshi Sata

Post-perovskite phase transition in MgSiO3

In situ x-ray diffraction measurements of MgSiO3 were performed at high pressure and temperature similar to the conditions at Earths core-mantle boundary. Results demonstrate that MgSiO3 perovskite transforms to a new high-pressure form with stacked SiO6-octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D″ seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D″ discontinuity.

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.

The Electrical Conductivity of Post-Perovskite in Earth's D'' Layer

Recent discovery of a phase transition from perovskite to post-perovskite suggests that the physical properties of Earths lowermost mantle, called the D″ layer, may be different from those of the overlying mantle. We report that the electrical conductivity of (Mg0.9Fe0.1)SiO3 post-perovskite is >102 siemens per meter and does not vary greatly with temperature at the conditions of the D″ layer. A post-perovskite layer above the core-mantle boundary would, by electromagnetic coupling, enhance the exchange of angular momentum between the fluid core and the solid mantle, which can explain the observed changes in the length of a day on decadal time scales. Heterogeneity in the conductivity of the lowermost mantle is likely to depend on changes in chemistry of the boundary region, not fluctuations in temperature.

Stability and equation of state of MgGeO3 post-perovskite phase

Abstract A phase transition of MgGeO3 perovskite was examined at high-pressure and -temperature using synchrotron X-ray diffraction measurements. The results demonstrate that it transforms to a CaIrO3- type post-perovskite phase above 63 GPa at 1800 K. The density increase is 1.5% at the transition pressure. These observations confirm that MgGeO3 is a low-pressure analogue to MgSiO3, for which a similar phase transition was recently found above 125 GPa and 2500 K. The unit-cell parameters of MgGeO3 post-perovskite phase obtained at 300 K during decompression from 79 to 6 GPa show that the b-axis is significantly more compressible than are the a- and c-axes, which could be due to the GeO6-octahedral sheet stacking structure along b. The bulk modulus was determined to be K0 = 192(±5) GPa with a fixed pressure derivative of the bulk modulus, K’, of 4.

Molar volumes of molten indium at high pressures measured in a diamond anvil cell

Molar volumes of molten indium have been measured in an isothermal compression up to 8.5 GPa at 710(3) K in an externally heated diamond anvil cell. The measurement is based on the x-ray diffraction and x-ray absorption of materials using a synchrotron monochromatic x-ray microbeam. The fit to the results with the Birch–Murnaghan equation of state gives parameters of V0=16.80 cm3, K0=23.9(6) GPa, assuming that K′=4. This method should be applicable for measuring molar volumes of liquids and other amorphous materials in the diamond anvil cell.

Dense yttria phase eclipsing the A-type sesquioxide structure: high-pressure experiments and ab initio calculations.

In situ X-ray diffraction experiments and ab initio calculations elucidated the high-pressure phase transition properties of yttrium sesquioxides. The C-, B-, and A-type sesquioxides structure sequence observed in the room-temperature compression does not coincide with the high-pressure phase sequence of yttrium sesquioxides at high temperature. A reconstructive-type transformation taking place at high temperature yields the Gd(2)S(3) structure around 8 GPa with a drastic change in cation-oxygen coordinations. Ab initio structural optimization suggests that a displacive-type transformation from B- to A-type sesquioxides structure metastably occurs under pressure at room temperature. The calculated density of states indicates that the transition to the Gd(2)S(3) structure causes a significant decrease in the band gap. The Gd(2)S(3) phase was also found to be partially recovered at ambient pressure. We briefly discuss the quenchability of the Gd(2)S(3) structure in sesquioxides on the basis of the enthalpy differences between the ambient phase and the recovered products.

Ferric iron content in (Mg,Fe)SiO3 perovskite and post-perovskite at deep lower mantle conditions

Abstract We have determined the Fe3+/ΣFe ratio of Al-free (Mg,Fe)SiO3 perovskite, post-perovskite, and (Mg,Fe)O ferropericlase synthesized at 99 to 187 GPa and 1830 to 3500 K based on the electron energyloss near-edge structure (ELNES) spectroscopy. The results demonstrate that post-perovskite includes minor amounts of ferric iron with Fe3+/ΣFe ratios of 0.11 to 0.21. These values are substantially lower than those of Al-rich post-perovskite (Fe3+/ΣFe = 0.59 to 0.69) reported in a previous study, suggesting that the Fe3+-Al3+ coupled substitution is important in post-perovskite, as in the case of perovskite. The Al-bearing post-perovskite in a pyrolitic mantle composition likely contains a considerable amount of ferric iron, which affects various physical properties in the lowermost mantle.

Pressure-induced spin transition in FeCO3-siderite studied by X-ray diffraction measurements

We have collected synchrotron X-ray diffraction patterns of FeCO3-siderite after or in-situ laser heating at high pressures to 66 GPa. Diffraction peaks of FeCO3 in all diffraction patterns obtained can be indexed as a trigonal cell. However, calculated cell volumes show an abrupt decrease (about 6.5%) between 47 and 50 GPa at room temperature. This abrupt change of the cell volume on FeCO3 is possibly due to a pressure-induced spin transition of ferrous Fe (HS: high-spin → LS: low-spin). Because cell parameters obtained at high temperature and at pressures above 50 GPa suggest HS state rather than LS state, the Clapeyron slope of the HS-to-LS transition of FeCO3 should be positive.

High-Pressure Phase Transition to the Gd2S3 Structure in Sc2O3: A New Trend in Dense Structures in Sesquioxides

In situ X-ray diffraction experiments using a laser-heated diamond anvil cell revealed a novel dense phase with the Gd(2)S(3) structure stabilizing in Sc(2)O(3) at pressures over 19 GPa. Although no phase transformation was induced during room-temperature compression up to 31 GPa, the C rare earth sesquioxide structure transformed into the B rare earth sesquioxide structure at 10 GPa after laser annealing and subsequently into the Gd(2)S(3) structure at 19 GPa. Neither the A rare earth sesquioxide structure nor the U(2)S(3) structure was found in Sc(2)O(3). Static density functional lattice energy calculations demonstrated that the C structure prefers Gd(2)S(3) over U(2)S(3) as the post phase. Sc(2)O(3) is the second sesquioxide, after In(2)O(3), to crystallize into a Gd(2)S(3) structure at high pressures and high temperatures.

New high-pressure B2 phase of FeS above 180 GPa

Abstract FeS exhibits extensive polymorphism at high pressure and temperature. All with NiAs-type (B8) or closely related structures. Here we report a new phase transition from FeS VI to CsCl-type (B2) phase (FeS VII) above 180 GPa based on the synchrotron X-ray diffraction (XRD) measurements. A significant volume reduction by 3.0% was observed at the phase transition, due to an increase in the coordination number from six to eight. Present results suggest that a substantial amount of sulfur may be incorporated into an Fe-Ni alloy with bcc structure in the Earth’s inner core.