New Data on the Stability of bcc Iron in the Earth Inner Core

International audienceA comparison between recent experimental [1] and theoretical [2] data on melting of Xe at high pressure has demonstrated that the close-packed structure to bcc structure transition might be misinterpreted as a melting transition. This suggested that the so-called "low" iron melting curve [3], measured using the same experimental technique as in the case of Xe, might also represent a solid-solid temperature induced transition. We studied this possibility using molecular dynamics employing first principles, pairwise and EAM models of interatomic interactions, as well as the first principles full potential linear muffin tin orbitals method. The results demonstrate that the bcc high-TP phase is at least dynamically stable. The stabilization occurs due to effective mixing of first and second neighbours in the bcc crystal. The iron atoms become effectively 14-coordinated, which removes the dynamic instability of bcc iron at low temperature and high pressure. This identification of a hcp-bcc transition brings all the data on iron, both theoretical [4-6] and experimental [3,7,8], into total agreement. It is thus possible that the Earth inner core is not hcp, but rather bcc iron. [1] R. Boehler et al., Phys. Rev. Lett. 86, 5731(2001). [2] A. B. Belonoshko, R. Ahuja and B. Johansson, Phys. Rev. Lett. 87,(2001). [3] R. Boehler, Nature 363, 534(1993). [4] D. Alfe, M. J. Gillan and G. D. Price, Nature 401, 462(1999). [5] A. Laio et al., Science 287, 1027(2000). [6] A. B. Belonoshko, R. Ahuja and B. Johansson, Phys. Rev. Lett. 84, 3638(2000). [7] J. M. Brown and R. G. McQueen, J. Geophys. Res. 91, 7485(1986]. [8] Q. R. Williams et al., Science 236, 181(1987)