Multiferroism in Iron-based Oxyfluoride Perovskites*

Hybrid improper ferroelectricity is generated by the combination of layered cation ordering at the A-site and octahedral tilts, and unlike conventional ferroelectricity, it does not conflict with magnetism. Therefore, it provides a route to achieve multiferroic materials. In this work, we use anion engineering to design perovskite ferroelectrics with iron in +3 oxidation state to take advantage of its high magnetic-ordering temperature. To maintain charge neutrality in AA’Fe₂O₆ perovskites, previous Fe +3 perovskite ferroelectrics have been restricted to using A and A’ both having +3 oxidation state, which limits the polarization. Using first-principles density functional theory calculations, we show polarization as high as 19.3 μC/cm² in AA’Fe₂O₅F perovskites with A and A’ cations being +2 and +3, respectively. We also show strong superexchange interactions in these perovskites, leading to a new family of potential room-temperature multiferroics. Design rules to maximize the stable polarization as a function of the combination of A and A’ cations will also be discussed.