Ferroelectricity in Single-Layer Ruddlesden−Popper Oxyfluorides Enabled by Low-Temperature Fluorination of Amorphous Precursors

Abstract

The development of ferroelectric materials relies on cation engineering of pure oxide systems. However, the discovery of new polar frameworks in pure oxides has become increasingly difficult because of their chemical and structural limitations. We report an approach to realize ferroelectricity in the n = 1 Ruddlesden−Popper (RP) family (A2BX4), a structure that is typically nonpolar in the oxide form because of its high crystalline symmetry. To stabilize the polar order in this rigid framework, we utilized anion engineering, specifically the partial substitution of O2− with F−. Although traditional topochemical fluorination requires a stable crystalline parent oxide, the parent n = 1 oxide Pb2MO4 (M = Fe, Co, Ni) is thermodynamically unstable and cannot be synthesized as a crystalline template. A low-temperature fluorination method using amorphous oxide film precursors was used to overcome this challenge. c-Axis-oriented epitaxial thin films of metastable Pb2M(O,F)4 were obtained, because a stable parent crystal was not required. Scanning transmission electron microscopy (STEM) and electrical measurements (P−E) hysteresis and positive-up-negative-down (PUND) measurements confirmed that these layered oxyfluorides exhibited clear and switchable ferroelectricity. This behavior probably originates from the strong O/F anisotropy, which forces significant off-center cation displacements within the single perovskite layers. Thus, amorphous-based low-temperature fluorination provides a universal route for exploring novel functional materials across various mixed-anion systems.