Ab initio simulations of iron-nickel alloys at Earth's core conditions

We report ab initio density functional theory calculations on iron-nickel (Fe-Ni) alloys at conditions representative of the Earth's inner core. We test different concentrations of Ni, up to similar to 39 wt% using ab initio lattice dynamics, and investigate the thermodynamic and vibrational stability of the three candidate crystal structures (bcc, hcp and fcc). First of all, at inner core pressures, we find that pure Fe transforms from the hcp to the fcc phase at around 6000 K. Secondly, in agreement with low pressure experiments on Fe-Ni alloys, we find the fcc structure is stabilised by the incorporation of Ni under core pressures and temperatures. Our results show that the fcc structure may, therefore, be stable under core conditions depending on the temperature in the inner core and the Ni content. Lastly, we find that within the quasi-harmonic approximation, there is no stability field for Fe-Ni alloys in the bcc structure under core conditions. (C) 2012 Elsevier B.V. All rights reserved.