Primary slip system of ε-iron and anisotropy of the Earth's inner core

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Models which attribute the seismic anisotropy of the Earth's inner core to lattice preferred orientation due to plastic deformation, rely on the assumption that the slip systems are the same for hexagonal ε-iron and for titanium, taken as an analogue because of a similar c/ a ratio. However, metals with similar c/ a ratio may predominantly slip on different planes. In the absence of experimental deformation data on ε-iron, we have estimated its primary slip system using a theoretical criterion which has been shown to give correct results in all the cases where it has been tested. The criterion depends on the values of the stacking fault energies on basal and prism planes, which we have calculated from first principles. We find it probable that ε-iron predominantly slips on the basal plane, rather than on the prism planes as titanium. The consequences on the seismic anisotropy of the preferred orientation caused by plastic deformation are analyzed. It is suggested that the observed seismic anisotropy is unlikely to be produced by low-order convection in the inner core.