Inner core composition paradox revealed by sound velocities of Fe and Fe-Si alloy.

Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (V) and high Poisson's ratio (σ) in the solid inner core. To date, neither V nor σ of Fe and Fe-Si alloy have been measured under core conditions. Here, we present V and σ derived from direct measurements of the compressional wave velocity, bulk sound velocity, and density of Fe and Fe-8.6 wt%Si up to ~230 GPa and ~5400 K. The new data show that neither the effect of temperature nor incorporation of Si would be sufficient to explain the observed low V and high σ of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease V and increase σ. However, the physical property-based Fe-Si-C core models seemingly conflict with the partitioning behavior of Si and C between liquid and solid Fe.