Predictive Quantum Mechanics-Based Force Field for Iron Oxide Systems: Mechanical, Dielectric, and Piezoelectric Response in Hematite, Magnetite, Maghemite, and Wüstite

Iron oxide systems are well-known for their diverse magnetic and electronic properties, making them pivotal in materials science, catalysis, and biomedical applications. Among these, Fe₃O₄ (magnetite) stands out as a ferrimagnetic half-metallic material with exceptional versatility. Through controlled oxidation or reduction, Fe₃O₄ can transform into other iron oxide phases, such as wüstite (Fe_1–xO), an antiferromagnetic phase, or γ-Fe₂O₃ and α-Fe₂O₃, which exhibit ferrimagnetic and antiferromagnetic insulating behaviors, respectively. These phase transitions provide a unique platform for tuning the magnetic and electrical properties of iron oxides. In this work, we present the development of a novel force field (FF′) specifically designed to model the structural, mechanical, dielectric, and piezoelectric properties of iron oxide systems. By capturing the intrinsic relationships between Fe₃O₄ and its oxidized and reduced counterparts, this force field provides a unified framework for simulating phase transitions and property tuning in iron oxides. The force field is parametrized based on the quantum-mechanical structure of Fe₃O₄ and extended to accurately describe the properties of γ-Fe₂O₃, α-Fe₂O₃, and Fe_1–xO. Our FF′ successfully reproduced quantum mechanical calculations for the elastic constants, dielectric responses, and piezoelectric coefficients across these phases. This study highlights the potential of FF′ as a robust tool for molecular dynamics simulations of iron oxide systems across diverse compositions and applications. The ability to accurately model phase-dependent magnetic and electric properties makes this force field particularly valuable for advancing the design of magnetoelectric devices, catalysts, sensors, and biomedical materials.