Cooperative Nature of Ferroelectricity in Two-Dimensional Hybrid Organic-Inorganic Perovskites.

Two-dimensional (2D) ferroelectric (FE) hybrid organic-inorganic perovskites (HOIPs) are promising for potential applications as miniaturized flexible ferroelectric/piezoelectric devices. Recently, several 2D HOIPs [e.g., Ruddlensden-Popper type HOIP BAPbCl (BA = CHCHNH)] were reported to possess room-temperature ferroelectricity. However, the underlying microscopic mechanisms for ferroelectricity in 2D HOIPs remain elusive. Here, by performing first-principles calculations and symmetry mode analysis, we demonstrate that there exists a cooperative coupling between A-site organic molecules and B-site inorganic Pb ions that is essential to the ferroelectricity in 2D BAPbCl. The nonpolar ground state of the closely related compounds BAPbBr and BAPbI can also be explained in terms of the weakened cooperative coupling. We further predict that 2D BAPbF displays in-plane ferroelectricity with a higher Curie temperature and larger electric polarization. Our work not only reveals the unusual FE mechanism in 2D HOIPs but also provides a solid theoretical basis for the rational design of 2D multifunctional materials.