Structural and energetic properties of domains in PbTiO ₃/ SrTiO ₃ superlattices from first principles

We report first-principles calculations, within the density functional theory, on the structural and energetic properties of 180 ^- stripe domains in (PbTiO ₃) _n/(SrTiO ₃) _n superlattices. For the explored periodicities (n=3 and 6), we find that the polydomain structures compete in energy with the monodomain phases. Our results suggest the progressive transition, as a function of n, from a strong to a weak electrostatic coupling regime between the SrTiO ₃ and PbTiO ₃ layers. Structurally, they display continuous rotation of polarization connecting 180 domains. A large offset between [100] atomic rows across the domain wall and huge strain gradients are observed. The domain wall energy is very isotropic, depending very weakly on the stripe orientation.