Sound Velocity of Fe and Fe-Si Alloys at Mbar Pressures by a Multi-Technique Approach

Abstract:

The presence of light elements alloyed to iron in the Earth’s core is well established, and many studies point at silicon as the major light element in the inner core. However, attempts to constrain Si abundance on the basis of comparison of measured velocities in Fe-Si alloys at high pressure with seismic observations do not provide a unique answer. Available data obtained in the 40-100 GPa range are indeed limited in number and, more importantly, in open disagreement.

Here we present new data on pure Fe and on Fe-Si alloys in the hcp structure at pressure exceeding 100 GPa, obtained by inelastic x-ray scattering and by picosecond acoustic measurements. The comparison of measurements by these complementary techniques provides reliable relations for the pressure and density evolution of the compressional sound velocity of the investigated alloys. These results are compared to results in literature and allow us to model the effect of Si content on the velocity-density relationship in the Fe-FeSi system and to constrain the Si abundance in the Earth’s inner core.