Giant Polarizability and Origin of Ferroelectricity in Layered Materials with a Litharge-Type Structural Unit

We discover that a large family of [Pb2F2]- and [Bi2O2]-based mixed-anion materials with a litharge-type structural unit are highly polarizable layered semiconductors in proximity to strain-induced ferroelectricity. First-principles calculations demonstrate that in this family of materials, compounds as diverse as PbFBr, BiOCl, BiCuOSe, Bi2OS2, and Bi5O4S3Cl exhibit static dielectric constants an order of magnitude higher than typical semiconductors. Additionally, they undergo a ferroelectric transition when subjected to a few percent of tensile strain. The ferroelectric transitions of these materials are found to have a universal origin in the strong cross-bandgap hybridization of the cation p orbitals, enabled by the cation 6s2 lone-pair electrons and the litharge-type structure of the [Pb2F2] and [Bi2O2] layers, as demonstrated by the strain-induced ferroelectric transition in the archetypal litharge α-PbO. These results establish materials with a litharge-type structural unit as a large and versatile family of highly polarizable layered semiconductors in proximity to ferroelectricity.