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 Fe83.6Si16.4 (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 Fe83.6Si16.4 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. Plain Language Summary The compositions of the interiors of planetary bodies such as the Moon and Mercury can be inferred by examining how core analogue materials (e.g., Fe‐Si alloys) behave at high pressures and temperatures. However, phase transitions of Fe‐Si alloys under conditions relevant to the Moon and Mercury have only been inferred from the extrapolations of data acquired at higher pressures. Additionally, electrical resistivity measurements have suggested that Fe‐Si alloy containing 9 wt.% Si undergoes pressure‐induced changes that are not well understood. To clarify these ambiguities, we acquired XRD patterns of Fe‐9wt.%Si up to 60 GPa and 900 K using an externally heated diamond‐anvil cell. The results show that Fe‐9wt.%Si can exist in two different crystal structures, body‐centered cubic and hexagonal close‐packed, depending on the pressure‐temperature conditions. The transition between these structures occurs at different pressures for the pre‐melted and powder samples. By modeling how the volume of the sample chang