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 BA2PbCl4 (BA = C6H5CH2NH3+)] 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 Pb2+ ions that is essential to the ferroelectricity in 2D BA2PbCl4. The nonpolar ground state of the closely related compounds BA2PbBr4 and BA2PbI4 can also be explained in terms of the weakened cooperative coupling. We further predict that 2D BA2PbF4 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.