First-principles study of symmetry lowering and polarization in BaTiO₃∕SrTiO₃ superlattices with in-plane expansion

The crystal structure and local spontaneous polarization of (BaTiO₃)_m∕(SrTiO₃)_n superlattices are calculated using first-principles density-functional theory. The in-plane lattice constant is constrained to be 1% larger than the SrTiO₃ (ST) substrate to take into account the in-plane expansion observed in recent experiments. The symmetry is lowered to monoclinic space group Cm allowing for polarization 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 (BT) 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.