  Using first-principles density functional calculations, we study the
interplay of ferroelectricity and polar discontinuities in a range of 1-1 oxide
superlattices, built out of ferroelectric and paraelectric components. Studies
have been carried out for a varied choice of chemical composition of the
components. We find that, when polar interfaces are present, the polar
discontinuities induce off- centric movements in the ferroelectric layers, even
though the ferroelectric is only one unit cell thick. The distortions yield
non-switchable polarizations, with magnitudes comparable to those of the
corresponding bulk ferroelectrics. In contrast, in superlattices with no polar
discontinuity at the interfaces, the off-centric movements in the ferroelectric
layer are usually suppressed. The details of the behavior and functional
properties are, however, found to be sensitive to epitaxial strain, rotational
instabilities and second-order Jahn-Teller activity, and are therefore strongly
in uenced by the chemical composition of the paraelectric layer.
