Akifumi Nozawa

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.

High-pressure generation in the Kawai-type apparatus equipped with sintered diamond anvils: application to the wurtzite–rocksalt transformation in GaN

Abstract Technical development of the Kawai-type apparatus has briefly been described from the original single-staged split-sphere to the recent 6–8 double-staged devise in which sintered diamond (SD) is equipped as the anvil material. In this article, high-pressure generation using sintered diamond cubes with 14.0 mm edge length and 1.5 mm corner truncation has been reported together with results for the wurtzite–rocksalt transformation in GaN at high pressure. Octahedral specimen assembly with 5.0 mm edge length containing GaN powder in an MgO capsule (pressure marker) was compressed in a DIA-type press SPEED MkII recently installed at SPring-8, Japan. Pressure value was determined from compression of the MgO marker and the transformation of GaN was monitored by means of in situ X-ray diffraction. The maximum pressure of 63.3 ± 0.4 GPa was reached based on the MgO scale by Jamieson et al. at 300 K. Onset of the wurtzite–rocksalt transformation in GaN was observed at 54 GPa and 300 K and at 51.4 GPa and 750 K, suggesting a negative Clapeyron slope of −170 K/GPa. The rocksalt type of GaN is judged to be quenchable to the ambient conditions because no reverse transformation was observed. Noticeable change in electric resistance may not be accompanied with the transformation. Moreover, the sluggishness also prevents the phase transformation in GaN from being used as a pressure fixed point.

Determination of high-pressure phase equilibria of Fe2O3 using the Kawai-type apparatus equipped with sintered diamond anvils

Abstract Phase equilibria of Fe2O3 have been studied up to 58 GPa and 1400 K using the Kawai-type multi anvil apparatus equipped with sintered diamond anvils. Identification of phases and pressure determination has been carried out by means of in situ X-ray observation using synchrotron radiation at SPring-8. Hematite (phase I) successively transforms to the Rh2O3(II)-type structure (phase II) and then to an orthorhombic structure (phase III) with increasing pressure. The transformations of hematite into high-pressure phases have been observed only at temperatures higher than 500 K, which is not concordant with previous results obtained by using the diamond anvil cell. Volume changes accompanied by the I-II and II-III transformations are calculated to be -2.8 and -5.0%, respectively. The phase boundary between I and II phases and that between II and III have been proposed to be P (GPa) = -0.015 T (K) + 44.2 and P (GPa) = -0.005 T (K) + 48.7, respectively. Possible correlation between a Mott transition and the phase stabilities may be concealed at room temperature due to slow reaction kinetics of the structural transformations.

Generation of pressures to ~60 GPa in Kawai-type apparatus and stability of MnGeO3 perovskite at high pressure and high temperature

Abstract To obtain higher pressures in a Kawai-type apparatus, we explored and tested high-pressure cell assemblies suitable for experiments using sintered diamond (SD) anvils. As a result, we succeeded in generating pressures exceeding 55 GPa at temperatures of ~1000 °C in a Kawai-type apparatus (SPEED Mk-II) at SPring-8. Using the optimized cell assembly, we examined the stability field of MnGeO3 perovskite, an analog of MgSiO3 perovskite, which was recently found to transform to a new high-pressure form under the P-T conditions near the core-mantle boundary. From our in situ X-ray observations, MnGeO3 perovskite was found to be stable at pressures up to 56.57 GPa at temperatures of 800.1050 °C.

High-Pressure Angle-Dispersive Powder Diffraction Using an Energy-Dispersive Setup and White Synchrotron Radiation

A new powder diffraction technique has been applied to collect high-pressure angle-dispersive data using a solid-state detector (SSD) and white synchrotron radiation, with the multi-anvil apparatus SPEED-1500 at SPring-8. By scanning a well-calibrated SSD over a given 2θ range at a predetermined step size, a series of one-dimensional (1D) energy dispersive data (intensity, Int, versus energy, E) are obtained as a function of 2θ. The entire intensity dataset Int (2θ, E) consists of 4048 energy bins, covering photon energies (E) up to ~ 160 keV at 600 2θ steps, forming a large two-dimensional (2D) array. These intensity data are regrouped according to photon-energy bins, which are defined by individual channels in the multi-channel analyzer, yielding a large number of intensity-versus-2θ (angle-dispersive) datasets, Int(E = const., 2θ), each of which corresponds to a given photon energy or wavelength. Experimental data obtained on a mixture of MgO and Au to 20 GPa are used to demonstrate the feasibility of this technique. The entire dataset, selected subsets or composite scans can be used for Rietveld refinement. Data subsets are selected to simulate coarse step scans. Our analysis indicates that data within certain energy bins (up to ~ 10%) may be binned together to improve counting statistics and to permit 2θ scans at 0.1–0.2° steps, without losing angular resolution. This will allow much faster data collection within about 10 min. Our test results indicate that at photon energy of ~80 keV, the angular resolution Δ θ/θ (in terms of refined FWHM) is about 0.006 within a 2θ range from 3 to 10°. This new technique is useful for high-pressure and general-purpose powder diffraction studies that have limited X-ray access to the sample using synchrotron radiation. Several advantages are discussed.

Equation of state of (Mg0.8,Fe0.2)2SiO4 ringwoodite from synchrotron X-ray diffraction up to 20 GPa and 1700 K

We present a temperature-pressure-volume ( T-P-V ) equation-of-state (EOS) of (Mg 0.8 ,Fe 0.2 ) 2 SiO 4 ringwoodite based on in situ high- T and high- P synchrotron X-ray diffraction experiments up to 1700 K and 20 GPa with a multi-anvil apparatus at SPring-8. The third-order Birch-Murnaghan equation was applied to the data between 300 and 900 K, while a constant ( ∂P/∂T) V fitting at temperatures higher than 900 K. By fixing previously measured volume thermal expansivities at 0 GPa and the isothermal bulk modulus at 300 K and 0 GPa of K 0,300k = 189.7 GPa, we derived the T-P-V EOS parameters of (Mg 0.8 ,Fe 0.2 ) 2 SiO 4 ringwoodite using least squares to be ( ∂K 0 /∂ P ) T = 4.57(7) and ( ∂K T /∂T) P = −0.0283(13) GPa/K between 300 and 900 K, and ( ∂P/∂T) V = 0.00535(11) GPa/K at temperatures above 900 K. These values compare very well with previously measured EOS data for Mg 2 SiO 4 ringwoodite of ( ∂K 0 /∂P) T = 4.6(2), ( ∂K T /∂T) P = −0.029(1) GPa/K, and ( ∂P/∂T) V = 0.0052 − 0.0055 GPa/K at high temperatures. At P = 20 GPa and T = 1800 K, as representative conditions in the lower part of the mantle transition zone, the relative V and K T values of (Mg 0.8 ,Fe 0.2 ) 2 SiO 4 ringwoodite with respect to the values at 300 K and 0 GPa are found to be V/V 0 = 0.9424, K T / K 0,300K = 1.263, based on the present EOS. These results for (Mg 0.8 ,Fe 0.2 ) 2 SiO 4 ringwoodite, combined with the corresponding data for Mg 2 SiO 4 ringwoodite, describe that the effects of Fe substitution for Mg on the T-P-V EOS of ringwoodite with (Mg 0.9 ,Fe 0.1 ) 2 SiO 4 , thought to be the composition in the mantle transition zone, are virtually negligible for V/V 0 , and less than 1% for K T /K 0,300k .