Orthorhombic strontium titanate in BaTiO3-SrTiO3 superlattices

It has been suggested by several authors that SrTiO3 layers in SrTiO3-BaTiO3 superlattices should be tetragonal and ferroelectric at ambient temperatures, like the BaTiO3 layers, rather than cubic, as in bulk SrTiO3, and that free-energy minimization requires continuity of the polarization direction. A recent ab initio calculation constrained solutions to this structure. Surprisingly, our x-ray study shows that the SrTiO3 layers are orthorhombic with 0.03% in-plane strain, with the BaTiO3 c-axis matching the SrTiO3 a- and b-axis better than the c-axis; strain energy overcomes the cost in electrostatic energy.