Bi 2 CoO 2 F 4 ─A Polar, Ferrimagnetic Aurivillius Oxide-Fluoride

Aurivillius oxides have been a research focus due to their ferroelectric properties, but by replacing oxide ions by fluoride, divalent magnetic cations can be introduced, giving Bi 2 M O 2 F 4 ( M = Fe, Co, and Ni). Our combined experimental and computational study on Bi 2 CoO 2 F 4 indicates a low-temperature polar structure of P 2 1 ab symmetry (analogous to ferroelectric Bi 2 WO 6 ) and a ferrimagnetic ground state. These results highlight the potential of Aurivillius oxide-fluorides for multiferroic properties. Our research has also revealed some challenges associated with the reduced tendency for polar displacements in the more ionic fluoride-based systems.