Stabilization of highly polar BiFeO₃-like structure: A new interface design route for enhanced ferroelectricity in artificial perovskite superlattices

In ABO₃ perovskites, oxygen octahedron rotations are common structural distortions that can promote large ferroelectricity in BiFeO₃ with an R3c structure [1] but suppress ferroelectricity in CaTiO₃ with a Pbnm symmetry [2]. For many CaTiO₃-like perovskites, the BiFeO₃ structure is a metastable phase. Here, we report the stabilization of the highly polar BiFeO₃-like phase of CaTiO₃ in a BaTiO₃/CaTiO₃ superlattice grown on a SrTiO₃ substrate. The stabilization is realized by a reconstruction of oxygen octahedron rotations at the interface from the pattern of nonpolar bulk CaTiO₃ to a different pattern that is characteristic of a BiFeO₃ phase. The reconstruction is interpreted through a combination of amplitude-contrast sub-0.1-nm high-resolution transmission electron microscopy and first-principles theories of the structure, energetics, and polarization of the superlattice and its constituents. We further predict a number of new artificial ferroelectric materials demonstrating that nonpolar perovskites can be turned into ferroelectrics via this interface mechanism. Therefore, a large number of perovskites with the CaTiO₃ structure type, which include many magnetic representatives, are now good candidates as novel highly polar multiferroic materials [3].