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2016-7-1

A group of researchers led by Associate Professor OZAKI Norimasa and Professor KODAMA Ryousuke at the Graduate School of Engineering, Osaka University, Dr. A. Denoeud at the École Polytechnique, France, and Dr. A. Benuzzi-Mounaix at the Centre national de la recherche scientifique, using the Gekko XII laser facility at the Osaka University Institute of Laser Engineering and the LULI2000 laser at the École Polytechnique, produced super high pressure conditions close to what was found in the earth’s core, verifying that iron crystal structures drastically changed at high speed under super-high pressure.

Iron is a very common metal for humans, and it's no exaggeration to say that iron has been studied the most; however, its behavior at super-high pressures is wrapped in mystery.

It was very difficult to directly see structural changes themselves under dynamic compression. However, this group was able to directly observe them with their own X-ray diffraction imaging, which applied a laser-driven shock-compressed method and an X-ray spectrometer, succeeding in clarifying structures at a super-high pressure of nearly 200 Gpa.

As it was verified that it was possible to produce and study various material structures at conditions of deep planetary interiors, a new possibility was opened in the fields of material science and planetary science. It is also expected that these results will produce new materials, new structures, and new material properties with high-power lasers at the nanosecond and picosecond timescales, or dream-like applications such as “material factories at conditions of deep planetary interiors.”

Abstract

Investigation of the iron phase diagram under high pressure and temperature is crucial for the determination of the composition of the cores of rocky planets and for better understanding the generation of planetary magnetic
fields. Here we present X-ray diffraction results from laser-driven shock-compressed single-crystal and polycrystalline iron, indicating the presence of solid hexagonal close-packed iron up to pressure of at least 170 GPa along the principal Hugoniot, corresponding to a temperature of 4,150 K. This is confirmed by the agreement between the pressure obtained
from the measurement of the iron volume in the sample and the inferred shock strength from velocimetry deductions. Results presented in this study are of the first importance regarding pure Fe phase diagram probed under dynamic compression and can be applied to study conditions that are relevant to Earth and super-Earth cores.

Figure 1. Schematic phase diagram of iron
The solid lines represent explored phase boundaries.The area of the pink circle is the ultra high pressure region in which we here directly observed the structural change. We demonstrated the fast structural change of iron from ε-phase to α-phase. Dashed lines and "?" marks represent unexplored regions, which are expected to be explored using a high-power laser in the near future. The α phase and δ phase have body-centered cubic structure, γ-phase
face-centered cubic structure, ε-phase hexagonal close-packed structure.

Figure 2.
(Upper Panel) Schematic diagram of the experimental configuration of the
flash X-ray*7 diffraction imaging.
(Bottom Panel) Photo of the inside of the ~2-m diameter vacuum chamber.
The few-mm size targets are precisely positioned in the center of the
chamber. The targets are irradiated by the high-power laser to create and observe extremely high-pressured matter.

To learn more about this research, please view the full research report entitled “Dynamic X-ray diffraction observation of shocked solid iron up to 170 GPa” at this page of the Journal of Neuroscience website.


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