Ab initio free energy calculations on the polymorphs of iron at core conditions

In order to predict the stable polymorph of iron under core conditions, calculations have been performed on all the Ž . Ž . candidate phases proposed for inner core conditions, namely, body-centred cubic bcc , body-centred tetragonal bct , Ž . Ž . hexagonal close-packed hcp , double-hexagonal close-packed dhcp and an orthorhombically distorted hcp polymorph. Our simulations are ab initio free energy electronic structure calculations, based upon density functional theory, within the generalised gradient approximation; we use Vanderbilt ultrasoft non-normconserving pseudopotentials to describe the core interactions, and the frozen phonon technique to obtain the vibrational characteristics of the candidate structures. Our results show that under conditions of hydrostatic stress, the orthorhombic, bcc and bct structures are mechanically unstable. The relative free energies of the remaining phases indicate that dhcp and fcc Fe are thermodynamically less stable than hcp Fe, therefore, we predict that the stable phase of iron at core conditions is hcp-Fe. q 2000 Elsevier Science B.V. All rights reserved.