Elastic isotropy of hcp-Fe under Earth core conditions

TL;DR

First-principles calculations reveal that hcp-Fe becomes elastically isotropic at Earth's inner core conditions, suggesting other mechanisms explain seismic anisotropy.

Contribution

This study provides the first detailed thermodynamic and elastic analysis of hcp-Fe at core conditions, showing isotropy at high temperatures.

Findings

hcp-Fe becomes elastically isotropic at core conditions

Anisotropic effects are strongly temperature dependent

Seismic anisotropy likely caused by inhomogeneity or different phases

Abstract

Our first-principles calculations show that both the compressional and shear waves of hcp-Fe become elastically isotropic under the high temperatures of Earth inner core conditions, with the variation in sound velocities along different angles from the c axis within 1%. We computed the thermoelasticity at high pressures and temperatures from quasiharmonic linear response linear-muffin-tin-orbital calculations in the generalized-gradient approximation. The calculated anisotropic shape and magnitude in hcp-Fe at ambient temperature agree well with previous first-principles predictions, and the anisotropic effects show strong temperature dependences. This implies that other mechanisms, rather than the preferential alignment of the hcp-Fe crystal along the Earth rotation axis, account for the seismic P-wave travel time anomalies. Either the inner core is not hcp iron, and/or the seismologically observed anisotropy is caused by inhomogeneity, i.e. multiple phases.