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Sci Adv. 2018 Apr 25;4(4):eaao5864. doi: 10.1126/sciadv.aao5864. eCollection 2018 Apr.

Crystal structure and equation of state of Fe-Si alloys at super-Earth core conditions.

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Department of Geosciences, Princeton University, Princeton, NJ 08544, USA.
Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA.
Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94550, USA.
Division of Engineering and Applied Sciences, California Institute of Technology, 1200 E California Boulevard, Pasadena, CA 91125, USA.
Laboratory for Laser Energetics, Department of Physics and Astronomy, and Department of Mechanical Engineering, University of Rochester, Rochester, NY 14623-1299, USA.


The high-pressure behavior of Fe alloys governs the interior structure and dynamics of super-Earths, rocky extrasolar planets that could be as much as 10 times more massive than Earth. In experiments reaching up to 1300 GPa, we combine laser-driven dynamic ramp compression with in situ x-ray diffraction to study the effect of composition on the crystal structure and density of Fe-Si alloys, a potential constituent of super-Earth cores. We find that Fe-Si alloy with 7 weight % (wt %) Si adopts the hexagonal close-packed structure over the measured pressure range, whereas Fe-15wt%Si is observed in a body-centered cubic structure. This study represents the first experimental determination of the density and crystal structure of Fe-Si alloys at pressures corresponding to the center of a ~3-Earth mass terrestrial planet. Our results allow for direct determination of the effects of light elements on core radius, density, and pressures for these planets.

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