Highly Polarizable Semiconductors and Universal Origin of Ferroelectricity in Materials with a Litharge-Type Structural Unit

Abstract

We discover that a large family of [Pb${}_2$F${}_2$]- and [Bi${}_2$O${}_2$]-based mixed-anion materials with a litharge-type structural unit are highly polarizable layered semiconductors on the edge of ferroelectricity. First-principles calculations demonstrate that in this family of materials, compounds as diverse as PbFBr, BiOCl, BiCuOSe, Bi${}_2$OS${}_2$, and Bi${}_5$O${}_4$S${}_3$Cl 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 6s${}^2$ lone-pair electrons and the litharge-type structure of the [Pb${}_2$F${}_2$] and [Bi${}_2$O${}_2$] layers, as demonstrated by the strain-induced ferroelectric transition in the archetypal litharge $\alpha$-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, offering vast opportunities for multifunctional materials design.

7 pages, 3 figures

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