Tomoko Sato

Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature

To understand the stability of the liquid phase of ionic liquids under high pressure, we investigated the phase behavior of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim][BF4]) homologues with different alkyl chain lengths for 2 ≤ n ≤ 8 up to ∼7 GPa at room temperature. The ionic liquids exhibited complicated phase behavior, which was likely due to the conformational flexibility in the alkyl chain. The present results reveal that [Cnmim][BF4] falls into superpressed state around 2-3 GPa range upon compression with an implication of multiple phase or structural transitions to ∼7 GPa. Remarkably, a characteristic nanostructural organization in ionic liquids largely diminishes at the superpressed state. The behaviors of imidazolium-based ionic liquids can be classified into, at least, three patterns: (1) pressure-induced crystallization, (2) superpressurization upon compression, and (3) decompression-induced crystallization from the superpressurized glass. Interestingly, the high-pressure phase behavior was relevant to the glass transition behavior at low temperatures and ambient pressure. As n increases, the glass transition pressure (pg) decreases (from 2.8 GPa to ∼2 GPa), and the glass transition temperature increases. The results indicate that the p-T range of the liquid phase is regulated by the alkyl chain length of [Cnmim][BF4] homologues.

Effect of nonhydrostaticity on the pressure induced phase transformation of rhombohedral boron nitride

An in situ x-ray diffraction study of room-temperature compression of rhombohedral boron nitride (rBN) was performed up to 10 GPa. Although no phase transformation of rBN was observed in a liquid pressure transmitting medium, the structure of rBN changed to become disordered within the layered stacking sequence at less than 1 GPa with the solid-state pressure transmitting medium. Further transformation to the wurtzite structure (wBN) was observed at 6 GPa and was unquenchable upon the release of pressure at room temperature. The orientation relationship of the phase transformation of rBN to wBN was compared with that of hexagonal BN to wBN.

Differential strain and residual anisotropy in silica glass

To understand the behavior of SiO2 glass under high pressure and differential stress, we conducted radial x-ray diffraction measurements on SiO2 glass up to 60 GPa, in which x-rays irradiate the sample from a direction perpendicular to the compression axis of a uniaxial apparatus. The differential strain of SiO2 glass, determined from the azimuth angle dependence of the position of the first sharp diffraction peak, was very large especially at pressures below 20 GPa and decreased with increasing pressure. After decompression, a large differential strain, equivalent to about 2 GPa in differential stress, remained in the glass at ambient conditions. We attribute this residual anisotropy to the anisotropic permanent densification, which is caused by the anisotropic change in intermediate-range structure, i.e., the anisotropic reconstruction of the network structure consisting of SiO4 tetrahedra.

Ultrafast observation of lattice dynamics in laser-irradiated gold foils

We have observed the lattice expansion before the onset of compression in an optical-laser-driven target, using diffraction of femtosecond X-ray beams generated by the SPring-8 Angstrom Compact Free-electron Laser. The change in diffraction angle provides a direct measure of the lattice spacing, allowing the density to be calculated with a precision of ±1%. From the known equation of state relations, this allows an estimation of the temperature responsible for the expansion as <1000 K. The subsequent ablation-driven compression was observed with a clear rise in density at later times. This demonstrates the feasibility of studying the dynamics of preheating and shock formation with unprecedented detail.

Dynamic fracture of tantalum under extreme tensile stress

The dynamic fracture of tantalum is observed at the atomic scale using an x-ray monitoring technique at the SACLA XFEL facility. The understanding of fracture phenomena of a material at extremely high strain rates is a key issue for a wide variety of scientific research ranging from applied science and technological developments to fundamental science such as laser-matter interaction and geology. Despite its interest, its study relies on a fine multiscale description, in between the atomic scale and macroscopic processes, so far only achievable by large-scale atomic simulations. Direct ultrafast real-time monitoring of dynamic fracture (spallation) at the atomic lattice scale with picosecond time resolution was beyond the reach of experimental techniques. We show that the coupling between a high-power optical laser pump pulse and a femtosecond x-ray probe pulse generated by an x-ray free electron laser allows detection of the lattice dynamics in a tantalum foil at an ultrahigh strain rate of ε. ~2 × 108 to 3.5 × 108 s−1. A maximal density drop of 8 to 10%, associated with the onset of spallation at a spall strength of ~17 GPa, was directly measured using x-ray diffraction. The experimental results of density evolution agree well with large-scale atomistic simulations of shock wave propagation and fracture of the sample. Our experimental technique opens a new pathway to the investigation of ultrahigh strain-rate phenomena in materials at the atomic scale, including high-speed crack dynamics and stress-induced solid-solid phase transitions.

Crystal structures and stabilities of cristobalite-helium phases at high pressures

Abstract First-principles calculations were used to study the structural and energetic properties of cristobalite- He I and II at high pressures, both of which were recently found in high-pressure powder X‑ray diffraction experiments of a-cristobalite with helium pressure-medium at room temperature. These calculations have revealed that both cristobalite-He I and II contain one helium atom per SiO2 with the formula SiO2He. It has also been revealed that cristobalite-He I is energetically favored above 6.4 GPa, cristobalite-He II is the stable phase at pressures between 1.7 and 6.4 GPa, and the mixture of cristobalite II and crystalline He is more stable than either cristobalite-He I or II below 1.7 GPa, in general agreement with the observation. Cristobalite-He I and II have been predicted to be monoclinic with space group P21/c, and rhombohedral with space group R3̅c, respectively. The unit-cell parameters of both cristobalite-He I and II were re-determined from the previously measured high-pressure X-ray diffraction data on the basis of these predicted cells. There is an excellent agreement between the observed (re-determined) and calculated pressure dependence of the cell parameters for the both phases. The calculated X-ray diffraction patterns for both cristobalite-He I and II are also consistent with the observed data. Cristobalite-He I and II have been predicted to have molar volumes 21% larger at 10 GPa and 23% larger at 4 GPa than cristobalite II due to the penetration of helium atoms into large voids of the structure.

Indirect monitoring shot-to-shot shock waves strength reproducibility during pump–probe experiments

We present an indirect method of estimating the strength of a shock wave, allowing on line monitoring of its reproducibility in each laser shot. This method is based on a shot-to-shot measurement of the X-ray emission from the ablated plasma by a high resolution, spatially resolved focusing spectrometer. An optical pump laser with energy of 1.0 J and pulse duration of similar to 660 ps was used to irradiate solid targets or foils with various thicknesses containing Oxygen, Aluminum, Iron, and Tantalum. The high sensitivity and resolving power of the X-ray spectrometer allowed spectra to be obtained on each laser shot and to control fluctuations of the spectral intensity emitted by different plasmas with an accuracy of similar to 2%, implying an accuracy in the derived electron plasma temperature of 5%-10% in pump-probe high energy density science experiments. At nano-and sub-nanosecond duration of laser pulse with relatively low laser intensities and ratio Z/A similar to 0.5, the electron temperature follows T-e similar to I-las(2/3). Thus, measurements of the electron plasma temperature allow indirect estimation of the laser flux on the target and control its shot-to-shot fluctuation. Knowing the laser flux intensity and its fluctuation gives us the possibility of monitoring shot-to-shot reproducibility of shock wave strength generation with high accuracy. Published by AIP Publishing.

Shock Compression and Melting of an Fe‐Ni‐Si Alloy: Implications for the Temperature Profile of the Earth's Core and the Heat Flux Across the Core‐Mantle Boundary

Understanding the melting behavior and the thermal equation of state of Fe-Ni alloyed with candidate light elements at conditions of the Earth’s core is critical for our knowledge of the region’s thermal structure and chemical composition and the heat flow across the liquid outer core into the lowermost mantle. Here we studied the shock equation of state and melting curve of an Fe-8 wt% Ni-10 wt% Si alloy up to ~250 GPa by hypervelocity impacts with direct velocity and reliable temperature measurements. Our results show that the addition of 10 wt% Si to Fe-8 wt% Ni alloy slightly depresses the melting temperature of iron by ~200–300 (±200) K at the core-mantle boundary (~136 GPa) and by ~600–800 (±500) K at the inner core-outer core boundary (~330 GPa), respectively. Our results indicate that Si has a relatively mild effect on the melting temperature of iron compared with S and O. Our thermodynamic modeling shows that Fe-5 wt% Ni alloyed with 6 wt% Si and 2 wt% S (which has a density-velocity profile that matches the outer core’s seismic profile well) exhibits an adiabatic profile with temperatures of ~3900 K and ~5300 K at the top and bottom of the outer core, respectively. If Si is a major light element in the core, a geotherm modeled for the outer core indicates a thermal gradient of ~5.8–6.8 (±1.6) K/km in the D′′ region and a high heat flow of ~13–19 TW across the core-mantle boundary.

Muonium in Stishovite: Implications for the Possible Existence of Neutral Atomic Hydrogen in the Earth's Deep Mantle

Hydrogen in the Earths deep interior has been thought to exist as a hydroxyl group in high-pressure minerals. We present Muon Spin Rotation experiments on SiO2 stishovite, which is an archetypal high-pressure mineral. Positive muon (which can be considered as a light isotope of proton) implanted in stishovite was found to capture electron to form muonium (corresponding to neutral hydrogen). The hyperfine-coupling parameter and the relaxation rate of spin polarization of muonium in stishovite were measured to be very large, suggesting that muonium is squeezed in small and anisotropic interstitial voids without binding to silicon or oxygen. These results imply that hydrogen may also exist in the form of neutral atomic hydrogen in the deep mantle.

Macroscopic and microscopic strain of SiO2 glass under uniaxial compression

Although SiO2 glass is brittle due to its covalency and the lack of dislocation movement seen in crystals, it can deform without fracturing when compressed to high pressures. The phenomenon may be attributable to the well-known permanent densification by the reconstruction of the network structure consisting of SiO4 tetrahedra. To explore so-called plastic deformation without permanent densification, we measured the change in size (macroscopic strain) of uniaxially-compressed disk-shaped SiO2 glass by an optical microscope [1]. Also, to understand the anisotropy in structure (microscopic strain), we measured the azimuth-angle dependence of the position of the first sharp diffraction peak (FSDP) of uniaxially-compressed SiO2 glass with a radial X-ray diffraction technique [2]. In the microscope observation, the glass was found to deform largely without fracturing up to at least 20 GPa from 6-8 GPa, where uniaxial conditions were achieved. In the X-ray diffraction observation, a large anisotropy was found in the FSDP which corresponds to the intermediate-range network structure of the glass. The recovered glass was examined by the radial X-ray diffraction up to a high-Q range and was found to remain largely anisotropic (equivalent to about 2 GPa in differential stress) in the intermediate-range network structure and not to remain anisotropic in the short-range SiO4 tetrahedral structure. It seems intuitive that the residual anisotropy is due to the anisotropic reconstruction of the network structure during permanent densification. However, the macroscopic strain measured in the microscope observation was an order of magnitude larger than the microscopic strain in the X-ray diffraction observation, and therefore it cannot be explained solely by the anisotropic permanent densification. The permanent densification may also enhance the reconstruction of the network structure and therefore plastic deformation.