  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 0.1nm 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].
