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

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 P-T condition, supporting the idea that the stable iron alloy structure in Earth's inner core is hcp. The P-T locations 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 GPO( 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. (C) 2019 The Authors. Published by Elsevier B.V.