Cooperative diffusion in body-centered cubic iron in Earth and super-Earths’ inner core conditions

Abstract

The physical chemistry of iron at the inner-core conditions is key to understanding the evolution and habitability of Earth and super-Earth planets. Based on full first-principles simulations, we report cooperative diffusion along the longitudinally fast ⟨111⟩ directions of body-centered cubic (bcc) iron in temperature ranges of up to 2000-4000 K below melting and pressures of ∼300-4000 GPa. The diffusion is due to the low energy barrier in the corresponding direction and is accompanied by mechanical and dynamical stability, as well as strong elastic anisotropy of bcc iron. These findings provide a possible explanation for seismological signatures of the Earth’s inner core, particularly the positive correlation between P wave velocity and attenuation. The diffusion can also change the detailed mechanism of core convection by increasing the diffusivity and electrical conductivity and lowering the viscosity. The findings need to be considered in future geophysical and planetary models and should motivate future studies of materials under extreme conditions.

Ab Initio Melting Temperatures of Bcc and Hcp Iron Under the Earth’s Inner Core Condition

“…Therefore, the hcp phase should be the stable phase for pure Fe at inner-core conditions. While there are increasing evidence of the bcc phase in the inner core (Belonoshko, Simak, et al, 2022;Ghosh et al, 2023), our ab initio calculations, regardless of the PAW potential employed, suggest that the stability of the bcc phase should not be due to Fe itself. The free energy difference between bcc and hcp is tiny, ∼10 meV/atom at 323 GPa.…”

Collective motion in hcp-Fe at Earth’s inner core conditions

“…However, it is still highly controversial whether C and/or H light elements exist in Earth’s inner core ( 24 ). Bcc-Fe has also been proposed as a possible explanation for the origin of the low shear wave velocity in the inner core ( 41 – 43 ), but in situ x-ray diffraction (XRD) experiments did not observe its existence at the relevant P–T conditions ( 19 , 20 , 44 ). Possible presence of melt pockets in the inner core has been suggested as an explanation for the high Poisson’s ratio ( 45 ), although the inner core growth models indicate that interstitial liquid would have been squeezed out during its complete solidification and compaction below the solidus ( 46 ).…”