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

Creators: Wicks, June K.; Smith, Raymond F.; Fratanduono, Dayne E.; Coppari, Federica; Kraus, Richard G.; Newman, Matthew G.; Rygg, J. Ryan; Eggert, Jon H.; Duffy, Thomas S.

Abstract

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.

Additional Information

© 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited. Received for publication August 5, 2017. Accepted for publication March 2, 2018. We thank the Omega Laser operations staff and the Target Engineering Team at Lawrence Livermore National Laboratory (LLNL) for assistance in these experiments. We acknowledge M. Millot (LLNL) for the helpful comments on the use of the Hyades hydrocode and C. Unterborn (Arizona State University) for providing an early version of the Planet Builder module within the BurnMan software. The research was supported by National Nuclear Security Administration through the National Laser Users' Facility Program (contract nos. DE-NA0002154 and DE-NA0002720) and the Laboratory Directed Research and Development Program at LLNL (project no. 15-ERD-012). This work was performed under the auspices of the U.S. Department of Energy by LLNL (contract no. DE-AC52-07NA27344). Author contributions: J.K.W. and R.F.S. were responsible for the design and execution of the experiments. D.E.F. developed the backward characteristics analysis for pressure determination. J.H.E. and J.R.R. developed the analysis tools for PXRDIP image processing. J.K.W., R.F.S., F.C., R.G.K., M.G.N., and T.S.D. carried out the experiments. All authors were involved in discussions related to experimental design and data analysis. The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

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