Lasers for X-ray diffraction observations of solid iron at high pressures

Investigations of iron at pressures similar to those in the core of the Earth provide new constraints on planetary formation and evolution

Iron dominates the cores of rocky planets such as the Earth. Therefore, to understand how such planets formed and evolved, we need a better understanding of how iron behaves under the high temperatures and pressures found in the planet’s center. One way to do this is by hitting the iron with lasers to dynamically compress it, thereby momentarily imposing the temperature and pressure conditions found near the center of the Earth.

A research group led by Ryosuke Kodama of Osaka University’s Graduate School of Engineering, together with Michel Koening, Research Director at Ecole Polytechnique/CNRS has been using lasers to simulate the temperatures and pressures experienced by iron in the center of a planet. X-ray diffraction techniques are then used to observe changes in the molecular structure of the iron that are brought about by the increased temperature and pressure.

“There are at least two main reasons for needing to know such information about the core,”, Kodama explains. “Our results provide better constraints on the composition and formation of the cores of rocky planets, and they also help explain the generation and variability of planetary magnetic fields.”

In the past, iron and its alloys have been widely studied using static compression techniques by evaluating samples placed in large-volume pressure cells or diamond anvil cells with a probing synchrotron radiation source. The results using such methods vary widely and can sometimes be controversial.

Using the new method of laser compression of matter, the research team observed the presence of solid hexagonal close-packed (hcp structure) iron, one of a number of phases (molecular forms) of iron, at pressures of 170 GPa and temperatures of 4,150 K. At these high pressures and temperatures, previous work proposed that iron should either have a different solid structure or be liquid, but the researchers clearly identified the hcp solid structure when samples were dynamically compressed with laser-driven shocks.

This X-ray diffraction experiment confirms that laser compression is a suitable method for studying iron at the conditions found in deep planetary interiors. Such conditions are difficult to achieve with traditional static compression techniques. However, there remains some uncertainty as to whether or not phase boundaries determined at nanosecond timescales agree with those that exist under static compression. This uncertainty is going to be investigated at near future.

Please refer to the following recent publication for more information on the group’s work.
Denoeud A, Ozaki N, Benuzzi-Mounaix A, Uranishi H, Kondo Y, Kodama R, Brambrink E,
Ravasio A, Bocoum M, Boudenne JM, Harmand M. Dynamic X-ray diffraction observation of shocked solid iron up to 170 GPa. Proceedings of the National Academy of Sciences. 2016 Jul 12; 113(28): 7745-9. doi: 10.1073/pnas.1512127113

Professor Ryosuke Kodama,
Graduate School of Engineering
http://www.photon.osaka-u.ac.jp/index-e.html
kodama@eei.eng.osaka-u.ac.jp

Reference:
Denoeud A, Ozaki N, Benuzzi-Mounaix A, Uranishi H, Kondo Y, Kodama R, Brambrink E, Ravasio A, Bocoum M, Boudenne JM, Harmand M. Dynamic X-ray diffraction observation of shocked solid iron up to 170 GPa. Proceedings of the National Academy of Sciences. 2016 Jul 12; 113(28): 7745-9. doi: 10.1073/pnas.1512127113

This research project was supported by the Osaka University International Joint Research Promotion Program, which aims to further enhance research quality and promote globalization at Osaka University through advanced research with overseas collaborators. The research described in this article is part of Professor Kodama’s project under the program, for which she is jointly working with the following researchers: Dr. Michel Koening, Research Director at Ecole Polytechnique/CNRS and Osaka University Associate Professor Norimasa Ozaki, Graduate School of Eingineering, Osaka University Professor Tadashi Kondo, Graduate School of Science and a research team at Ecole Polytechnique/CNRS including A. Benuzzi-Mounaix, E. Brambrink, A. Ravasio, and M. Marmand.