Sound Velocity of Hcp-Fe at High Pressure: Experimental Constraints, Extrapolations and Comparison with Seismic Models

Sound velocity determination of iron under extreme thermodynamic conditions is essential for a proper interpretation of seismic observations of the Earth's core, but static high-pressure measurements proven to be an experimental challenge. Here we review techniques and methodologies used to measure sound velocities in metals at Mbar pressures, with specific focus on the compressional sound velocity of hexagonal close-packed iron. A critical comparison of literature results, coherently analyzed using the same metrology (pressure scale, equation of state,), allowed us to propose reference relations for the pressure and density evolution of the compressional velocity of hexagonal close-packed iron at ambient temperature. This provides a key base line upon which build on complexity, including high-temperature effects, pre-melting effects, effects of nickel and/or light element inclusion, necessary for an accurate comparison with seismic models, and ultimately to constrain Earth's inner core composition.