Phase Transitions and Thermal Equation of State of Fe-9wt.%Si Applied to the Moon and Mercury

Accurate knowledge of the phase transitions and thermoelastic properties of candidate iron alloys, such as Fe-Si alloys, is essential for understanding the nature and dynamics of planetary cores. The phase diagrams of some Fe-Si alloys between 1 atm and 16 GPa have been back-extrapolated from higher pressures, but the resulting phase diagram of Fe_83.6Si_16.4 (9 wt.% Si) is inconsistent with temperature-induced changes in its electrical resistivity between 6 and 8 GPa. This study reports in situ synchrotron X-ray diffraction (XRD) measurements on pre-melted and powder Fe_83.6Si_16.4 samples from ambient conditions to 60 GPa and 900 K using an externally heated diamond-anvil cell. Upon compression at 300 K, the bcc phase persisted up to ∼38 GPa. The hcp phase appeared near 8 GPa in the pre-melted sample, and near 17 GPa in the powder sample. The appearance of the hcp phase in the pre-melted sample reconciles the reported changes in electrical resistivity of a similar sample, thus resolving the low-pressure region of the phase diagram. The resulting high-temperature Birch-Murnaghan equation of state (EoS) and thermal EoS based on the Mie-Gruneisen-Debye model of the bcc and hcp structures are consistent with, and complement the literature data at higher pressures. The calculated densities based on the thermal EoS of Fe-9wt.%Si indicate that both bcc and hcp phases agree with the reported core density estimates for the Moon and Mercury.