Elastic isotropy of ɛ‐Fe under Earth's core conditions

Abstract

Our first‐principles calculations show that both the compressional and shear waves of ɛ‐Fe become elastically isotropic under the Earth's 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 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 ɛ‐Fe crystal along the Earth's rotation axis, account for the seismic P‐wave travel time anomalies. Either the inner core is not ɛ‐Fe, and/or the seismologically observed anisotropy is caused by inhomogeneity, i.e., multiple phases.