Unveiling the effect of Ni on the formation and structure of Earth's inner core*

Ni is the second most abundant element in the Earth's core. However, its effects on the inner core's structure and formation process are often overlooked due to its similar atomic number to Fe. Using ab initio simulations, we have found that the bcc phase can spontaneously crystallize in liquid Ni at temperatures above Fe's melting point under inner core pressures. The phase relations among hcp, bcc, and liquid differ between Fe and Ni. Ni can act as a bcc stabilizer for Fe at the inner core's temperatures and pressures. A small amount of Ni can significantly accelerate the crystallization of Fe under core conditions. These results suggest that Ni may have a substantial impact on the structure and formation process of the solid inner core.