Interfacial enhancement of ferroelectricity in CaTiO3/BaTiO3 superlattices

We carry out first-principles calculations for CaTiO3/BaTiO3superlattices with epitaxial strain corresponding to growth on a SrTiO3 substrate, and consider octahedral rotations as well as ferroelectric distortions. The calculations are done as a function of electric displacement field, and both a macroscopic and a local electrostatic analysis are carried out. We find that strong octahedral rotations occur for TiO6 octahedra sandwiched between CaO layers on both sides, but are strongly suppressed if either neighboring layer is a BaO layer. Due to the resulting enhancement of the ferroelectric instability in these octahedra, we find that overall the ferroelectric instability of the superlattice is enhanced by the interface. Thus, short-period superlattices in this system have a larger ferroelectric polarization than longer-period ones of the same average composition, contrary to the expected trend.