Phase Stability of Fe–Si Alloy at Super‐Earth Core Conditions

Abstract

Silicon is proposed to be a major impurity in the Fe‐rich cores of the Earth and large rocky exoplanets, where it may influence the density, phase equilibrium, and thermal evolution of planetary interiors. Here, we constrain the composition‐dependent phase boundary between hcp and B2 stability in Fe–Si alloys at terapascal (TPa) pressures (1 TPa = 1,000 GPa = 10 million atmospheres), using Fe–11 wt.%Si as an intermediate composition. By employing laser‐driven ramp compression combined with nanosecond in situ X‐ray diffraction, we determined that Fe–11 wt.%Si retains a hexagonal‐closed‐packed (hcp) structure up to pressures of 1.15 TPa. This data set, compared to previous studies, shows that at these extreme pressure states there is a transition from hcp  B2 stability for Si wt.% impurities in the 11%–15% range. These results provide new constraints on Fe–Si phase boundaries and improve the understanding of equations of state and compositions relevant to the core of super‐Earth planets. We used our experimental results to calculate the mass–radius relationships for planets composed of a Fe–Si core and a mantle. The results suggest that some exoplanets previously modeled with a pure Fe core may have larger cores when silicon impurities are considered.