  Ni is the second most abundant element in the Earth's core. Yet, its effects
on the inner core's structure and formation process are usually disregarded
because of its electronic and size similarity with Fe. Using ab initio
molecular dynamics simulations, we find that the bcc phase can spontaneously
crystallize in liquid Ni at temperatures above Fe's melting point at inner core
pressures. The melting temperature of Ni is shown to be 700-800 K higher than
that of Fe at 323-360 GPa. hcp, bcc, and liquid phase relation differ for Fe
and Ni. Ni can be a bcc stabilizer for Fe at high temperatures and inner core
pressures. A small amount of Ni can accelerate Fe's crystallization at core
pressures. These results suggest Ni may substantially impact the structure and
formation process of the solid inner core.
