Pressure-induced phase stability and metallization of monoclinic FeO from first-principles calculations

The phase relations of FeO under high-pressure and high-temperature (high P–T) conditions exhibit considerable complexity, hindering understanding of the physicochemical properties of the Earth’s deep interior. In particular, recent experimental studies have shown that FeO can stabilize in two monoclinic phases under extreme conditions, with space groups C2/m and P21/m, respectively. Here, we perform first-principles calculations based on density functional theory with Hubbard U corrections to investigate the structural stability, magnetic behavior, and electronic properties of the two phases under compression. The calculated energy–volume and enthalpy–pressure relations indicate that the C2/m phase is more stable at low pressures, whereas a C2/m-to-P21/m phase transition occurs at pressures above ∼83 GPa. The local magnetic moment of Fe gradually decreases with increasing pressure, accompanied by enhanced electronic delocalization. Projected density-of-states analyses reveal enhanced Fe–O orbital hybridization and increased electronic delocalization under compression. Notably, the P21/m phase undergoes an insulator-to-metal transition above ∼80 GPa. In contrast to the metallization of B1–FeO reported previously, this transition is not accompanied by a pronounced spin-state change, suggesting a distinct pressure-induced electronic mechanism dominated by enhanced electronic delocalization. These results provide new insights into the structural and electronic evolution of FeO under extreme conditions and may have implications for the transport properties of Fe-bearing materials in Earth’s deep interior.