Pressure-Induced Crystal Structure and Spin-State Transitions in Magnetite (Fe3O4)

High pressure is an important dimension for the emergent phenomena in transition metal oxides, including high-temperature superconductivity, colossal magnetoresistance, and magnetoelectric coupling. In these multiply correlated systems, the interplay between lattice, charge, orbital, and spin is extremely susceptible to external pressure. Magnetite (Fe3O4) is one of the oldest known magnetic materials and magnetic minerals, yet its high pressure behaviors are still not clear. In particular, the crystal structure of the high-pressure phase has remained contentious. Here, we investigate the pressure-induced phase transitions in Fe3O4 from first-principles density-functional theory. It is revealed that the net magnetic moment, arising from two ferrimagnetically coupled sublattices in Fe3O4, shows an abrupt drop when entering into the high-pressure phase but recovers finite value when the pressure is beyond 65.1 GPa. The origin lies in the redistribution of Fe 3d orbital occupation with the change of crystal field, where successive structural transitions from ambient pressure phase Fd (3) over barm to high pressure phase Pbcm (at 29.7 GPa) and further to Bbmm (at 65.1 GPa) are established accurately. These findings not only explain the experimental observations on the structural and magnetic properties of the highly compressed Fe3O4 but also suggest the existence of highly magnetized magnetite in the Earth's lower mantle.