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, cal culations have been performed on all the candidate phases proposed for inne r core conditions, namely, body-centred cubic (bcc), body-centred tetragona l (bct), hexagonal close-packed (hcp), double-hexagonal close-packed (dhcp) and an orthorhombically distorted hcp polymorph. Our simulations are ab in itio free energy electronic structure calculations, based upon density func tional theory, within the generalised gradient approximation; we use Vander bilt ultrasoft non-normconserving pseudopotentials to describe the core int eractions, and the frozen phonon technique to obtain the vibrational charac teristics of the candidate structures. Our results show that under conditio ns of hydrostatic stress, the orthorhombic, bce and bet structures are mech anically unstable. The relative free energies of the remaining phases indic ate that dhcp and fee Fe are thermodynamically less stable than hcp Fe, the refore, we predict that the stable phase of iron at core conditions is hcp- Fe. (C) 2000 Elsevier Science B.V. All rights reserved.