Elastic isotropy of epsilon-Fe under Earth's core conditions

Our first-principles calculations show that both the compressional and shear waves of epsilon-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 epsilon-Fe crystal along the Earth's rotation axis, account for the seismic P-wave travel time anomalies. Either the inner core is not epsilon-Fe, and/or the seismologically observed anisotropy is caused by inhomogeneity, i.e., multiple phases. Citation: Sha, X., and R. E. Cohen (2010), Elastic isotropy of epsilon-Fe under Earth's core conditions, Geophys. Res. Lett., 37, L10302, doi:10.1029/2009GL042224.