Elastic wave velocity and density of fcc-Fe and fcc-Fe–Si alloys at high pressures and temperatures

The solid cores of moderate-sized terrestrial planets are hypothesized to comprise face-centered cubic (fcc) FeSi alloys, making it essential to understand the elastic properties of these materials under extreme conditions (high temperatures and pressures) for constraining planetary core compositions. However, there are few studies on the elastic properties of fcc-Fe–Si alloys. We report comprehensive measurements of longitudinal elastic wave velocities (V_P) and densities (ρ) for fcc-Fe and fcc-Fe–5Si (5 wt% Si) alloys at pressures up to 15 GPa and temperatures to 1700 K, utilizing simultaneous in situ ultrasonic measurements, X-ray radiography, and X-ray diffraction techniques. Our findings reveal that the V_P difference between pure Fe and Fe5Si was minimal at 6–8 GPa but diverged significantly at 12–14 GPa, demonstrating that Si incorporation increases the pressure dependence of V_P. Linear regression of V_P–ρ relationships yielded distinct equations for fcc-Fe (V_P [m/s] = 1.35(16) × ρ [kg/m³] − 5.3(14) × 10³) and fcc-Fe–5Si (V_P [m/s] = 2.09(17) × ρ [kg/m³] − 10.6(13) × 10³). Critically, although the V_P of the fcc-Fe–5Si alloy closely matched Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport mission predictions for the Martian inner core, its density exceeded observational constraints by 800–1000 kg/m³. This discrepancy suggests the presence of additional light elements—potentially H—that could reduce density without substantially modifying elastic wave velocities, providing a novel constraint on Martian core composition.