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.