Phase relations in the system Fe–Ni–Si to 200 GPa and 3900 K and implications for Earth’s core

Tetsuya Komabayashia, Giacomo Pescea, Ryosuke Sinmyob, Takaaki Kawazoec, Helene Bretona, Yuta Shimoyamad, Konstantin Glazyrine, Zuzana Konôpkováe, Mohamed Mezouarf
Earth and Planetary Science Letters 511, 12-24 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.056]
aSchool of Geo Sciences and Centre for Science at Extreme Conditions, University of Edinburgh,EH93FE,UK
bBayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
cDepartment of Earth and Planetary Systems Science, Hiroshima University, Hiroshima, Japan
dDepartment of Earth and Space Science, Osaka University, Osaka, Japan
eDeutsches Elektronen-Synchrotron(DESY), Photon Science, Notekstrasse 85, 22607 Hamburg, Germany
fEuropean SynchrotronRadiation Facility, BP220, F-38043 Grenoble Cedex, France
Copyright Elsevier

Phase relations in Fe–5 wt%Ni–4 wt%Si alloy was examined in an internally resistive heated diamond anvil cell under high pressure (P) and temperature (T) conditions to about 200 GPa and 3900 K by in-situ synchrotron X-ray diffraction. The hexagonal close-packed (hcp) structure was observed to the highest PT condition, supporting the idea that the stable iron alloy structure in Earth’s inner core is hcp. The PTlocations of the phase transition between the face-centred cubic (fcc) and hcp structures were also constrained to 106 GPa. The transition occurs at 15 GPa and 1000 K similar to for pure Fe. The Clausius–Clapeyron slope is however, 0.0480 GPa/K which is larger than reported slopes for Fe (0.0394 GPa/K), Fe–9.7 wt%Ni (0.0426 GPa/K), and Fe–4 wt%Si (0.0394 GPa/K), stabilising the fcc structure towards high pressure. Thus the simultaneous addition of Ni and Si to Fe increases the dP/dT slope of the fcc–hcp transition. This is associated with a small volume change upon transition in Fe–Ni–Si. The triple point, where the fcc, hcp, and liquid phases coexist in Fe–5 wt%Ni–4 wt%Si is placed at 145 GPa and 3750 K. The resulting melting temperature of the hcp phase at the inner core-outer core boundary lies at 550 K lower than in pure Fe.

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