Order-disorder transition in B2 phase Fe-Si alloy and implications on transport property at the Earth's core conditions

The Earth's core primarily consists of iron and nickel, with minor amounts of light elements such as silicon, sulfur, oxygen, and carbon. A long-standing debate has centered on whether body-centered cubic (bcc) iron exists in the core or if light elements promote the formation of the B2 phase in iron alloys. Through first-principles calculations, we found that Fe-Si alloys in the B2 phase undergo an order-to-disorder phase transition under Earth's core conditions. Enthalpy comparisons reveal that the B2 phase is more stable than hexagonal close-packed (hcp) and face-centered cubic (fcc) phases at high pressures. We constructed a detailed binary phase diagram for Fe-Si alloys at high temperatures and pressures, showing that doping silicon into iron alloys results in a B2+hcp mixture above 10 wt\% Si, with a complete B2 phase forming above 25 wt\% Si. Furthermore, calculations of electrical transport properties using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) show that B2 phase Fe-Si alloys exhibit higher electrical resistivity than the hcp and fcc phases at lower temperatures, though the difference in thermal conductivity is negligible. These findings provide important insights into the behavior of materials under extreme conditions, with potential implications for understanding Earth's core dynamics.