CONSTRAINTS ON THE CRYSTALLINE-STRUCTURE OF THE INNER-CORE - MECHANICAL INSTABILITY OF BCC IRON AT HIGH-PRESSURE

We examine the energetics of close-packed phases of iron (bcc, fee, hcp) throughout the pressure regime of the earth with elaborate first principles electronic structure calculations. The calculations reproduce the relevant experimental observations including the equation of state to over 300 GPa and the pressure of the bcc to hcp phase transition. The bcc structure is found to be energetically unfavorable and mechanically unstable with respect to a tetragonal strain at high pressure (P>150 GPa). This phase is thus highly unlikely to exist in the earth's inner core as has frequently been proposed.