­Phase transitions of Fe-9wt%Si at high pressure-temperature conditions

The effects of pressure and temperature on the phase transitions of Fe-Si alloys under deep Mercury conditions are not well defined. Yet, temperature-induced changes in the electrical resistivity (ρ) of Fe-Si alloys have been known to indicate changes in scattering mechanisms and the solid-liquid phase transition. The nature of the ρ changes while in the solid state for Fe-Si alloys remains controversial and requires further studies. Recent ρ measurements of Fe-8.5wt%Si indicate variations below the magnetic transition, 6-8 GPa and 500-1000 K, although no phase transition is reported in the literature for this composition at the observed pressure-temperature (P-T) conditions. In fact, no phase transition is observed at these P-T conditions in Fe, while the magnetic transition, indicating the structural change from bcc-Fe to fcc-Fe, is the nearest well-defined transition. We performed in situ synchrotron X-Ray Diffraction measurements for an Fe-9wt%Si sample in an externally heated diamond-anvil cell from ambient conditions to 25 GPa and 1000 K, in order to identify and outline the possible phase transition. Results suggest the presence of a phase transition correlating with the ρ variations below the magnetic transition. The phase transition from hcp+B2 to hcp at low T and high P is constrained for the first time. The results are also compared to the reported Fe-9wt%Si phase diagram above 20 GPa. This analysis sheds light on the stable phases on the core side of Mercury's core-mantle boundary (5-7 GPa and 1800-2100 K) and deep core (up to 36 GPa) which is expected to contain approximately 10.5wt% Si.