  Short period ferroelectric/ferroelectric BaTiO3 (BTO)/PbTiO3 (PTO)
superlattices are studied using density functional theory. Contrary to the
trends in paraelectric/ferroelectric superlattices the polarization remains
nearly constant for PTO concentrations below 50%. In addition, a significant
decrease in the c/a ratio below the PTO values were observed. Using a
superlattice effective Hamiltonian we predict an enhancement in the d33
piezoelectric coefficient peaking at ~75% PTO concentration due to the
different polarization-strain coupling in PTO and BTO layers. Further analysis
reveals that these trends are bulk properties which are a consequence of the
reduced $P$ brought about by the polarization saturation in the BTO layers.
