Polymorph Selection During the Crystallization of Iron Under the Conditions of Earth's Inner Core

Using classical molecular dynamics simulations together with a many-body embedded atom model (EAM) potential, we study the crystallization of iron under conditions typical of the Earth's inner core. We simulate this process at a pressure of 330 GPa and at two different temperatures, 5680 K and 4970 K, corresponding to temperatures 20 % and 30 % below the melting point of pure iron, respectively. We show that, for both temperatures, crystal nucleation proceeds with the formation of a critical nucleus, in which all three main metallic structures, i.e. the body-centered cubic (BCC) polymorph, the hexagonal-close packed (HCP) polymorph and the face-centered cubic (FCC) polymorph, are present. While we find that the critical nucleus is predominantly BCC, we also identify a significant HCP signature and, to a lesser extent, a FCC signature. During the crystal growth at the higher temperature, we observe that the BCC fraction in the nucleus constantly rises at the expense of both close-packed structures, leading to post-critical nuclei largely dominated by the BCC polymorph. However, we show that the crystallization mechanisms and the polymorph selection process strongly depend on temperature since crystal growth at the lower temperature yields a highly polycrystalline, phase composed of blocks of the BCC, FCC and HCP structure. Depending on the actual temperature of the Earth's inner core, our results suggest that the Earth's inner core could be either predominantly BCC or a polycrystalline mixture of BCC, HCP and FCC.