Home Physical Sciences The high-pressure anisotropic thermoelastic properties of a potential inner core carbon-bearing phase, Fe7C3, by single-crystal X-ray diffraction
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The high-pressure anisotropic thermoelastic properties of a potential inner core carbon-bearing phase, Fe7C3, by single-crystal X-ray diffraction

  • Xiaojing Lai , Feng Zhu , Jiachao Liu , Dongzhou Zhang , Yi Hu , Gregory J. Finkelstein , Przemyslaw Dera and Bin Chen
Published/Copyright: September 28, 2018
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Abstract

Carbon has been suggested as one of the light elements existing in the Earth’s core. Under core conditions, iron carbide Fe7C3 is likely the first phase to solidify from a Fe-C melt and has thus been considered a potential component of the inner core. The crystal structure of Fe7C3, however, is still under debate, and its thermoelastic properties are not well constrained at high pressures. In this study, we performed synchrotron-based single-crystal X-ray diffraction experiment using an externally heated diamond-anvil cell to determine the crystal structure and thermoelastic properties of Fe7C3 up to 80 GPa and 800 K. Our diffraction data indicate that Fe7C3 adopts an orthorhombic structure under experimentally investigated conditions. The pressure-volume-temperature data for Fe7C3 were fitted by the high-temperature Birch-Murnaghan equation of state, yielding ambient-pressure unit-cell volume V0 = 745.2(2) Å3, bulk modulus K0 = 167(4) GPa, its first pressure derivative K0 = 5.0(2), dK/dT = -0.02(1) GPa/K, and thermal expansion relation αT = 4.7(9) × 10–5 + 3(5) × 10–8 × (T – 300) K–1. We also observed anisotropic elastic responses to changes in pressure and temperature along the different crystallographic directions. Fe7C3 has strong anisotropic compressibilities with the linear moduli Ma > Mc > Mb from zero pressure to core pressures at 300 K, rendering the b axis the most compressible upon compression. The thermal expansion of c3 is approximately four times larger than that of a3 and b3 at 600 and 700 K, implying that the high temperature may significantly influence the elastic anisotropy of Fe7C3. Therefore, the effect of high temperature needs to be considered when using Fe7C3 to explain the anisotropy of the Earth’s inner core.

Acknowledgments

This work was performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source, Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation-Earth Sciences (EAR-1128799) and Department of Energy-GeoSciences (DE-FG02-94ER14466). The use of gas loading system was supported by GeoSoilEnviroCARS and by the Consortium for Materials Properties Research in Earth Sciences (COMPRES) under National Science Foundation Cooperative Agreement EAR -1606856. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Development of the ATREX software, which was used for experimental data analysis was supported by National Science Foundation (NSF) EAR GeoInformatics grant 1440005. Development of the X-ray Atlas instrument was funded by NSF EAR Infrastructure and Facilities grant 1541516. This work was supported by NSF grant EAR-1555388 to B.C. and in part supported by the Bullard award from the University of Hawai’i at Mānoa to X.L. We thank the technical support from Sergey Tkachev. We are grateful to Danielle Gray for the valuable discussion. We thank the Associate Editor R. Sinmyo, the two anonymous reviewers and the Technical Editor for their constructive comments. School of Ocean and Earth Science and Technology (SOEST) contribution 10424. Hawaii Institute of Geophysics and Planetology (HIGP) contribution 2354.

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Received: 2018-02-27
Accepted: 2018-06-11
Published Online: 2018-09-28
Published in Print: 2018-10-25

© 2018 Walter de Gruyter GmbH, Berlin/Boston

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