Achieving the extreme pressure and temperature conditions found in the Earth’s inner core has been one of the major experimental challenges. In this article, recent development of ultrahigh-pressure and high-temperature techniques in multimegabar pressures region using a laser heated diamond anvil cell is described. Results from recent experiments about phase relations of silica and iron up to 300 GPa and 2000 K are reviewed.
With recent technological advances in both high-pressure techniques and analytical methods with diamond-anvil cells, it is now possible to generate pressure over 3 Mbar and probe the unique and novel behavior of materials under extreme conditions of multimegabar pressures. The synchrotron radiation techniques associated with high-pressure research have been developed by optimization of X-ray source, X-ray optics, sample environment, experimental setup, and detectors. Multimegabar experiments on Earth and planetary materials with the recent X-ray techniques of micro-X-ray diffraction and spectroscopy are providing new insights into the nature of the Earth’s core and the deep giant planets.
Brillouin scattering measurements of the aggregate shear wave velocities in MgSiO3 perovskite and post-perovskite phase were conducted at high-pressure conditions relevant to the Earth’s lowermost mantle. Infrared laser annealing of samples in a diamond anvil cell enabled to obtain high quality Brillouin spectra and to extend the upper limit of pressure of Brillouin measurements. The large pressure range over which acoustic measurements of MgSiO3 perovskite and post-perovskite phase were performed has thereby allowed us to put tighter constraints on compositional models of the Earth’s lower mantle and enigmatic seismic observations in the lowermost mantle.
In this article, recent advances in X-ray spectroscopy, in particular X-ray Raman scattering (XRS), under high pressure are introduced. There are problems to overcome when these techniques are applied to high-pressure materials science. Solutions for the problems are shown. As examples of XRS study under pressure conditions, experimental results on H2O and SiO2 are reviewed. Further prospect of these techniques on materials of the earth’s interior is discussed.
Analytical transmission electron microscope (ATEM) is a powerful tool for analyses of the samples recovered from ultrahigh pressure experiments. Recently, Focused Ion Beam (FIB) system has been applied to prepare a TEM foil of sample recovered from laser heated diamond anvil cell (LHDAC). It has some advantages compared to conventional argon ion milling method. In this article, recent advances in the DAC sample preparation for TEM observation using FIB system are reviewed.
Solubility of noble gas in silicate melt under high pressures is important for understanding the origin of the Earth’s atmosphere and evolution of the Earth’s interior. In this article, previous studies of noble gas solubility based on high pressure experiments are reviewed. Then we present new experimental results on Ar solubility in silica melt up to about 12 GPa by using a laser-heated diamond-anvil cell (LHDAC). It was found that solubility of noble gas increased and then saturated with increasing pressure, which is completely different from the results of previous LHDAC experiments. The source of discrepancy and problems of solubility measurements using LHDAC are discussed.
We have been developing experiments on shock compression relevant to Earth core condition with intense laser irradiation. Multiple shock compression method is applied to create pressure and temperature (P, T) conditions of geophysical interest. We measured sound velocity of shock-compressed iron foils with two-stepped laser pulse irradiation by x-ray radiography technique. We have also developed nanosecond x-ray diffraction in order to measure the structure of shock-compressed materials.
The strongly correlated Ir oxide BaIrO3 shows a weak ferromagnetism and a gap-type order below the transition temperature Tc(= 180 K at ambient pressure). An external pressure decreases Tc by a rate of -29 K/GPa, removes the metallic conduction below Tc, and suppresses the nonlinear conduction at 4.2 K. These observations are highly incompatible with a charge-density-wave state that was believed to be the origin for the 180-K transition. An alternative model for the transition is proposed, which can be regarded as a transition from the Mott-insulator to the charge-transferred insulator transition.
It is expected that larger grains are to be produced when longer durations of shock are applied for shock synthesis. A shrinkage tube in the explosive loading system is developed to realize longer effective loadings, and was applied for diamond synthesis. We observed a remarkable increase in crystallite size of diamond obtained by the help of this new device.
The optical measurements by the use of a DAC have developed the studies of solid hydrogen at high pressures. Those studies have disclosed that solid hydrogen at high pressures exhibits a variety of structural phase transitions. The researches about the metallization of solid hydrogen, which is a longstanding issue in high pressure science, have been also progressed in recent years. In this article, we briefly review the pressure-induced structural phase transitions and the quest for metallic phase in solid hydrogen at high pressures.