Structural evidence for enhanced polarization in a commensurate short-period $BaTiO3∕SrTiO3$ superlattice

A short-period $(BaTiO3)6∕(SrTiO3)5$ superlattice was characterized by x-ray diffraction and transmission electron microscopy. The superlattice is epitaxially oriented with the $c$ axes of $BaTiO3$ and $SrTiO3$ normal to the (001) surface of the $SrTiO3$ substrate. Despite the large in-plane lattice mismatch between $BaTiO3$ and $SrTiO3$ $(∼2.2%)$⁠, the superlattice interfaces were found to be nearly commensurate. The crystallographic $c∕a$ ratio of the superlattice was measured and the results agree quantitatively with first-principles calculations and phase-field modeling. The agreement supports the validity of the enhanced spontaneous polarization predicted for short-period $BaTiO3∕SrTiO3$ superlattices.