  The crystal structure and local spontaneous polarization of
(BaTiO3)m/(SrTiO3)n superlattices is calculated using a first-principles
density functional theory method. The in-plane lattice constant is 1% larger
than the SrTiO3 substrate to imitate the relaxed superlattice structure and the
symmetry is lowered to monoclinic space group Cm which allows polarization to
develop along the [110] and [001] directions. The polarization component in the
[110] direction is found to develop only in the SrTiO3 layers and falls to zero
in the BaTiO3 layers, whereas the polarization in the [001] direction is
approximately uniform throughout the superlattice. These findings are
consistent with recent experimental data and first-principles results for
epitaxially strained BT and ST.
