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Stability of the body-centred-cubic phase of iron in the Earth's inner core

Abstract

Iron is thought to be the main constituent of the Earth's core1, and considerable efforts2,3,4,5,6,7,8,9,10,11,12,13,14 have therefore been made to understand its properties at high pressure and temperature. While these efforts have expanded our knowledge of the iron phase diagram, there remain some significant inconsistencies, the most notable being the difference between the ‘low’ and ‘high’ melting curves15. Here we report the results of molecular dynamics simulations of iron based on embedded atom models fitted to the results of two implementations of density functional theory. We tested two model approximations and found that both point to the stability of the body-centred-cubic (b.c.c.) iron phase at high temperature and pressure. Our calculated melting curve is in agreement with the ‘high’ melting curve, but our calculated phase boundary between the hexagonal close packed (h.c.p.) and b.c.c. iron phases is in good agreement with the ‘low’ melting curve. We suggest that the h.c.p.–b.c.c. transition was previously misinterpreted as a melting transition, similar to the case of xenon16,17,18, and that the b.c.c. phase of iron is the stable phase in the Earth's inner core.

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Figure 1: Temperature dependence of the volume of iron.
Figure 2: Energies of the iron phases at constant volume as a function of temperature.
Figure 3: The axial ratio of h.c.p. iron as a function of temperature.
Figure 4: High-pressure iron phase diagram.

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Acknowledgements

Discussions with P. Korzhavy were helpful. The calculations were done using the resources of the Swedish National Supercomputer Centre in Linköping. The study was supported by the Swedish Research Council (VR) and the Swedish Foundation for Strategic Research (SSF).

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Correspondence to Anatoly B. Belonoshko.

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Belonoshko, A., Ahuja, R. & Johansson, B. Stability of the body-centred-cubic phase of iron in the Earth's inner core. Nature 424, 1032–1034 (2003). https://doi.org/10.1038/nature01954

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