Structure of Iron to 1 Gbar and 40 000 K

First-principles calculations show that compression-induced electronic transitions produce a rich phase diagram featuring reentrant stability of the fcc phase with an extremum on the fcc to hcp boundary at 23 Mbar and 19 000 K, conditions similar to those expected at the center of super-Earth exoplanets. Transformations of hcp to fcc at 57 Mbar due to 4s to 3d electron transfer and from fcc to a body-centered structure at 320 Mbar due to hybridization of the 3d with the core 3p band are experimentally observable along a quasi-isentrope generated by laser-induced ramp compression.