Composition of the Earth’s inner core from high pressure sound velocity measurements of Fe-Ni-Si alloys

Earth’s inner core likely consists of Fe-Ni alloy(s) plus a minor fraction of light element(s) to match the density and sound wave velocities of seismological models such as the preliminary reference Earth model (PREM). Among possible alloying light elements (e.g., Si, O, H, S, C), silicon is a popular candidate based on its cosmochemical abundance and potential involvement in chemical reactions at the core-mantle boundary. Previous work has shown that the solubility of Si in hcp-(Fe,Ni) alloy increases the stability field of the bcc-phase at high pressure. Comparison of sound velocity and density data of Fe-Ni-Si alloys with geophysical observations and theoretical predictions provide important constraints on the structure and dynamics of Earth’s inner core. However, knowledge of the high-pressure and high-temperature behaviour and properties of Fe-Ni alloys that contain light elements is limited. We therefore investigated bcc-Fe_0.78Ni_0.07Si_0.15 alloy to compare its sound velocity and density with ab initio calculations and PREM in order to clarify the role of Si as a light element in Earth’s inner core.

Compressional velocities and densities of bcc-Fe_0.78Ni_0.07Si_0.15 alloy have been measured using inelastic X-ray scattering (IXS) and powder X-ray diffraction at the SPring-8 synchrotron facility (BL35XU beamline). High pressure was generated using a BX90-type diamond anvil cell. The metal alloy sample was loaded together with Ne (pressure medium) in a Re sample chamber and was mechanically compressed to 75 GPa through steps of 10 GPa at room temperature. IXS data were acquired at each pressure point in the range of momentum transfer of 4.24 to 7.63 nm^-1. To determine density, we collected X-ray diffraction patterns of the sample before acquisition of each IXS spectrum using a flat panel detector installed in the optical system. All IXS spectra were fitted using Lorentzian functions. The dispersion relationship between energy (E) and momentum transfer (Q) was obtained by fitting all data with the following equation:

E (meV) = 4.192 x 10^-4 vp (m/s) x Q_max (nm^-1) x sin (π/2 x Q (nm^-1)/ Q_max (nm^-1),

where vp is the sound velocity of the sample.

Preliminary results for bcc-Fe_0.78Ni_0.07Si_0.15 show that the energy of the longitudinal acoustic phonon increases with increasing pressure. Additionally, we found that vp follows Birch’s Law, i.e., there is a linear relationship between density and sound velocity. Based on the comparison of our results and those for hcp-Fe and Fe-Si alloys reported previously with PREM, we propose that bcc-Fe_0.78Ni_0.07Si_0.15 alloy is a viable candidate as a component of Earth’s inner core.