Solidus and Phase Relations of Fe _0.8 Ni _0.1 Si _0.1 at Earth's Core Conditions

Abstract

Determining the solidus of Fe‐Ni‐Si alloys at high pressure is important for constraining the thermal and compositional structure of Earth's core, where silicon is widely considered a major light element. However, large uncertainties remain due to methodological differences in melt‐detection criteria and limited measurements in the hexagonal close‐packed phase field. In this study, we sampled the phase diagram of from 58 to 157 GPa and temperatures up to 3500 K using synchrotron Mössbauer spectroscopy (SMS) that monitors the atomic dynamics of iron nuclei, and synchrotron X‐ray diffraction (XRD) to detect liquid diffuse scattering. SMS measurements constrained the solidus by tracking the collapse of hyperfine parameters across the solid–liquid boundary. Complemented by burst‐heating XRD with pixel‐level background analysis that enables robust identification of the appearance of liquid diffuse scattering, this dual‐technique approach provides a precise determination of the solidus. Our results demonstrate that silicon significantly lowers the solidus temperatures of Fe‐Ni alloys in both and structures, and the ‐‐ liquid quasi‐triple point occurs at GPa and K. At the highest solidus pressure of 150 GPa, we determine a low solidus temperature of K. This integrated framework enables a more precise determination of the solidus and provides broader insight into melting detection strategies relevant to Earth's deep interior.