Orthorhombic strontium titanate in BaTiO₃–SrTiO₃superlattices

Abstract

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.