The structure of iron under the conditions of the Earth's inner core

The inferred density of the solid inner core indicates that it is predomina ntly made of iron. In order to indicates that it is predominantly made of i ron. In order to interpret the observed seismic anisotropy and understand t he high pressure and temperature behaviour of the core, it is essential to establish the crystal structure of iron under core conditions. On the basis of extrapolated experimental data, a number of candidate structures for th e high PIT iron phase have been proposed, namely, body-centred cubic (bcc), body-centred tetragonal (bct), hexagonal close-packed (hcp), double-hexago nal close-packed (dhcp) and an orthorhombically distorted hcp polymorph (Ma tsui, 1993; Stixrude and Cohen, 1995; Boehler, 1993; Saxena et al., 1996; A ndrault et al., 1997). Here we present the results of the first fully ab in itio free energy calculations for all of these polymorphs of iron at core p ressures and temperatures. Our results show that hcp-Fe is the most stable polymorph of iron under the conditions of the Earth's inner core.