L.Vocadlo $^(a)$, D.Alfe $^(a,b)$, M.J.Gillan $^(b)$, G.D.Price $^(a)$ and I.G.Wood $^(a)$
$^(a)$ Department of Geological Sciences, University College London, Gower Street, London, WC1E 6BT $^(b)$ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT
Amidst the continuing controversy surrounding the stable phase of iron in the Earth s core, we report the results of the first first-principles, finite-temperature, molecular dynamics free-energy calculations on the body-centred-cubic phase (bcc) of iron under the conditions found in the Earth s core. We find that this phase becomes mechanically stable at high pressures and temperatures, with a transition to a low temperature omega phase occurring at ~3000-4000K, illustrated by the structure factors shown below: http://bowfell.geol.ucl.ac.uk/~lidunka/sf-hex2.jpg We have calculated the free energy of bcc-Fe along the melting curve and find that, in all cases, the hexagonal-close-packed (hcp) phase is thermodynamically more favourable and therefore likely to be the stable phase under core conditions. However, the free energy difference between bcc and hcp is very small, and we cannot rule out the possibility that the presence of light elements could stabilise bcc-Fe with respect to hcp-Fe in the Earth's inner core.