Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (VS) and high Poisson’s ratio (σ) in the solid inner core. To date, neither VS nor σ of Fe and Fe-Si alloy have been measured under core conditions. Here, we present VS 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 VS and high σ of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease VS 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.

Using dynamic compression technique, the equation of state for Fe‐8.6 wt% Si was measured up to 240 GPa and 4,670 K. A least squares fit to the experimental data yields the Hugoniot parameters C0 = 4.603±0.101 km/s and λ = 1.505±0.037 with initial density ρ0=7.386±0.021 g/cm3. Based on the Hugoniot data, the calculated isothermal equation of state is consistent with static compression data when the lattice Grüneisen parameter γl =1.65(7.578/ρ) and electronic Grüneisen parameter γe=1.83. The calculated pressure‐density data at 300 K were fitted to a third‐order Birch‐Murnaghan equation of state with zero pressure the parameters K0=192.1±6.3 GPa, K0'=4.71±0.27 with fixed ρ0ε =7.578±0.050 g/cm3. Under the conditions of Earth's core, the densities of Fe‐8.6±2.0 wt% Si and Fe‐3.8±2.9 wt% Si agree with preliminary reference Earth mode (PREM) data of the outer and the inner core, respectively. These are the upper limits for Si in the core assuming Si is the only light element. Simultaneously considering the geophysical and geochemical constraints for a Si‐S‐bearing core, the outer core may contain 3.8±2.9 wt% Si and 5.6±3.0 wt% S.

Using the dynamic compression technique, the sound velocities of Fe‐11.8 wt % S were measured up to 211.4 (4.5) GPa and 6,150 K. Discontinuities both in shock velocity and sound velocity indicate that Fe‐11.8 wt % S completely melts at a pressure of 111.3 (2.3) GPa. By the energy conservation law, the calculated liquidus temperature is about 2,500 (300) K. Extrapolated to the inner‐core boundary based on the Lindeman law, the liquidus temperature of Fe‐11.8 wt % S is 4,300 (300) K. We developed a thermodynamic model fit to the experimental data, which allows calculation of the densities and sound velocities of liquid Fe‐S under core conditions. For liquid Fe‐11.8 wt % S and Fe‐10 wt % S, good agreement was achieved between the extrapolations using our model and experimental measurements at very low pressure. Under the conditions of the outer core, the densities and bulk sound velocities of Fe‐10 wt % S provide a good fit to observed seismic profiles of Earth's core. Our results imply that an upper limit of 10 wt % S content in Earth's core satisfies the geophysical constraints. Simultaneously considering other geochemical constraints, the outer core may contain about 6 wt % sulfur and 4 wt % silicon.

We report Hugoniot measurements on a mixture of cubic boron nitride (cBN) and hexagonal boron nitride (hBN, ∼10% in weight) to investigate the shock compression behavior of BN at Hugoniot stresses up to 110 GPa. We observed a discontinuity at ∼77 GPa along the Hugoniot and interpreted it as the manifestation of the shock-induced phase transition of hBN to cBN. The experimental stress at 77–110 GPa shows significant deviation from the hydrodynamic Hugoniot of cBN calculated using the Mie-Grüneisen model coupled with the reported 300 K-isotherms of cBN. Our investigation reveals that material strength in cBN increases with the experimental stress at least up to 110 GPa. The material strength might be preserved at higher stress if we consider the previously reported high stress data.

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