Norimasa Nishiyama

High-Pressure Creep of Serpentine, Interseismic Deformation, and Initiation of Subduction

The supposed low viscosity of serpentine may strongly influence subduction-zone dynamics at all time scales, but until now its role could not be quantified because measurements relevant to intermediate-depth settings were lacking. Deformation experiments on the serpentine antigorite at high pressures and temperatures (1 to 4 gigapascals, 200° to 500°C) showed that the viscosity of serpentine is much lower than that of the major mantle-forming minerals. Regardless of the temperature, low-viscosity serpentinized mantle at the slab surface can localize deformation, impede stress buildup, and limit the downdip propagation of large earthquakes at subduction zones. Antigorite enables viscous relaxation with characteristic times comparable to those of long-term postseismic deformations after large earthquakes and slow earthquakes. Antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation.

Covalency-reinforced oxygen evolution reaction catalyst.

The oxygen evolution reaction that occurs during water oxidation is of considerable importance as an essential energy conversion reaction for rechargeable metal–air batteries and direct solar water splitting. Cost-efficient ABO3 perovskites have been studied extensively because of their high activity for the oxygen evolution reaction; however, they lack stability, and an effective solution to this problem has not yet been demonstrated. Here we report that the Fe4+-based quadruple perovskite CaCu3Fe4O12 has high activity, which is comparable to or exceeding those of state-of-the-art catalysts such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ and the gold standard RuO2. The covalent bonding network incorporating multiple Cu2+ and Fe4+ transition metal ions significantly enhances the structural stability of CaCu3Fe4O12, which is key to achieving highly active long-life catalysts.

High-pressure x-ray tomography microscope: Synchrotron computed microtomography at high pressure and temperature

A new apparatus has been developed for microtomography studies under high pressure. The pressure generation mechanism is based on the concept of the widely used Drickamer anvil apparatus, with two opposed anvils compressed inside a containment ring. Modifications are made with thin aluminum alloy containment rings to allow transmission of x rays. Pressures up to 8GPa have been generated with a hydraulic load of 25T. The modified Drickamer cell is supported by thrust bearings so that the entire pressure cell can be rotated under load. Spatial resolution of the high pressure tomography apparatus has been evaluated using a sample containing vitreous carbon spheres embedded in FeS matrix, with diameters ranging from 0.01to0.2mm. Spheres with diameters as small as 0.02mm were well resolved, with measured surface-to-volume ratios approaching theoretical values. The sample was then subject to a large shear strain field by twisting the top and bottom Drickamer anvils. Imaging analysis showed that detailed microst...

Development of the Multi-anvil Assembly 6-6 for DIA and D-DIA type high-pressure apparatuses

A newly designed anvil assembly, Multi-anvil Assembly 6-6 (MA6-6), was developed. This assembly consists of six small anvils with an anvil guide, and can be compressed by DIA-type and deformation-DIA (D-DIA) apparatuses. The use of this anvil assembly simplifies the process of replacing anvils with those having different truncated edge length and/or hardness. As a consequence, the time needed for anvil replacement is significantly shortened. This is a benefit to experiments at synchrotron facilities because anvil replacement has to be carried out within the limited beamtime. Using a combination of an MA6-6 and DIA-type apparatus, pressure above 12 GPa was generated. A deformation experiment of polycrystalline MgO was performed using an MA6-6 with D-DIA at 4 GPa and room temperature. Two-dimensional X-ray diffraction patterns and X-ray radiographic images were collected from the deforming sample using monochromatic X-rays. Quantitative deformation experiments can be carried out using this experimental setup.

Equation of state and phase transition in KAlSi3O8 hollandite at high pressure

Abstract The tetragonal hollandite structure (KAlSi3O8 hollandite) has been studied up to 32 GPa at room temperature using high-pressure in-situ X-ray diffraction techniques. A phase transformation from tetragonal I4/m phase to a new phase was found to occur at about 20 GPa. This transition is reversible on release of pressure without noticeable hysteresis and hence this new high-pressure phase is unquenchable to ambient conditions. The volume change associated with the transition is found to be small (not measurable), suggesting a second order transition. The diffraction pattern of the highpressure phase can be indexed in a monoclinic unit cell (space group I2/m), which is isostructual with BaMn8O16 hollandite. The γ angle of the monoclinic unit cell increases continuously above the transition. A Birch-Murnaghan equation of state fit to pressure-volume data obtained for KAlSi3O8 hollandite yields a bulk modulus K0 = 201.4 (7) GPa with K’0 = 4.0.

Pd2+-Incorporated Perovskite CaPd3B4O12 (B = Ti, V)

Novel A-site ordered perovskites CaPd(3)Ti(4)O(12) and CaPd(3)V(4)O(12) were synthesized under high-pressure and high-temperature of 15 GPa and 1000 °C. These compounds are the first example in which a crystallographic site in a perovskite-type structure is occupied by Pd(2+) ions with a 4d(8) low spin configuration. The ionic models for these compounds were determined to be Ca(2+)Pd(2+)(3)Ti(4+)(4)O(12) and Ca(2+)Pd(2+)(3)V(4+)(4)O(12) by structural refinement using synchrotron X-ray powder diffraction, hard X-ray photoemission, and soft X-ray absorption spectroscopy. Magnetic susceptibility, electrical resistivity, and specific heat measurements demonstrated diamagnetic insulating behavior for CaPd(3)Ti(4)O(12) in contrast to the Pauli-paramagnetic metallic nature of CaPd(3)V(4)O(12).

A new technique for angle-dispersive powder diffraction using an energy-dispersive setup and synchrotron radiation

A new diffraction technique for combined angle- and energy-dispersive structural analysis and refinement (CAESAR), by collecting angle-dispersive data using a solid-state detector (SSD) and white synchrotron radiation, is introduced. By step scanning a well calibrated SSD over a limited 2θ range, a series of one-dimensional energy-dispersive data (intensity versus energy) are obtained as a function of 2θ. The entire intensity (Int) data set consists of several thousand channels covering a range of photon energies, E (up to ∼150 keV), at each of the ∼1000 2θ steps, forming a 2–4 mega-element two-dimensional array, Int(E, 2θ). These intensity data are then regrouped according to photon energies, which are defined in the multichannel SSD as individual channels, yielding a large number of intensity versus 2θ (angle-dispersive) data sets, Int(E = const., 2θ), each of which corresponds to a given photon energy or wavelength. The entire data set, selected subsets or composite scans can be used for multiple data set Rietveld refinement. Data collected both on α-Al2O3 (a NIST diffraction standard) at ambient conditions and on a mixture of MgO and Au at high pressure were analyzed using the Rietveld technique, with varying schemes of data treatment. Furthermore, it is demonstrated that data within certain energy bands (ΔE/E = ±10%) may be binned together to improve counting statistics in a composite angle-dispersive scan, even when collected with much coarser scan steps of 0.1 or 0.2°. This technique is useful for high-pressure as well as general purpose powder diffraction studies that have limited X-ray access to the sample using synchrotron radiation. Several advantages are discussed.

Control of bond-strain-induced electronic phase transitions in iron perovskites.

Unusual electronic phase transitions in the A-site ordered perovskites LnCu3Fe4O12 (Ln: trivalent lanthanide ion) are investigated. All LnCu3Fe4O12 compounds are in identical valence states of Ln(3+)Cu(2+)3Fe(3.75+)4O12 at high temperature. LnCu3Fe4O12 with larger Ln ions (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb) show an intersite charge transfer transition (3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+)) in which the transition temperature decreases from 360 to 240 K with decreasing Ln ion size. In contrast, LnCu3Fe4O12 with smaller Ln ions (Ln = Dy, Ho, Er, Tm Yb, Lu) transform into a charge-disproportionated (8Fe(3.75+) → 5Fe(3+) + 3Fe(5+)) and charge-ordered phase below ∼250-260 K. The former series exhibits metal-to-insulator, antiferromagnetic, and isostructural volume expansion transitions simultaneously with intersite charge transfer. The latter shows metal-to-semiconductor, ferrimagnetic, and structural phase transitions simultaneously with charge disproportionation. Bond valence calculation reveals that the metal-oxygen bond strains in these compounds are classified into two types: overbonding or compression stress (underbonding or tensile stress) in the Ln-O (Fe-O) bond is dominant in the former series, while the opposite stresses or bond strains are found in the latter. Intersite charge transfer transition temperatures are strongly dependent upon the global instability indices that represent the structural instability calculated from the bond valence sum, whereas the charge disproportionation occurs at almost identical temperatures, regardless of the magnitude of structural instability. These findings provide a new aspect of the structure-property relationship in transition metal oxides and enable precise control of electronic states by bond strains.

Phase relations in boron at pressures up to 18 GPa and temperatures up to 2200 ∘ C

The phase relations in boron have been investigated at high pressure and high temperature using a multianvil apparatus, and the quenched sample has been analyzed by x-ray diffraction, Raman spectra, and transmission electron microscopy. We demonstrate that

Pressure generation to 25 GPa using a cubic anvil apparatus with a multi-anvil 6-6 assembly

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