Interplay of couplings between antiferrodistortive, ferroelectric, and strain degrees of freedom in monodomain PbTiO<inf>3</inf>/SrTiO<inf>3</inf> superlattices

We report first-principles calculations on the coupling between epitaxial strain, polarization, and oxygen octahedra rotations in monodomain (PbTiO3)n/(SrTiO3)n superlattices. We show how the interplay between (i) the epitaxial strain and (ii) the electrostatic conditions can be used to control the orientation of the main axis of the system. The electrostatic constrains at the interface facilitate the polarization rotation and, as a consequence, we predict large piezoelectric responses at epitaxial strains smaller than those required considering only strain effects. In addition, ferroelectric (FE) and antiferrodistortive (AFD) modes are strongly coupled. Usual steric arguments cannot explain this coupling and a covalent model is proposed to account for it. The energy gain due to the FE-AFD coupling decreases with the periodicity of the superlattice, becoming negligible for n?3.. We acknowledge financial support from Grants No. FIS2009- 12721-C04-02 and No. CP-FP 228989-2 OxIDes. We acknowledge the computer resources, technical expertise, and assistance provided by the Red Espan˜ola de Supercomputacio´n. Calculations were also performed at the ATC group of the University of Cantabria