Maximum of dielectric permittivity caused by structural transition in ferroelectric BaTiO3-SrTiO3 superlattices

We invoke the onset of dislocations along the BaTiO3-SrTiO3 interface as reported by Wunderlich et al. to explain the non-monotonic behaviour of the dielectric permittivity as a function of superlattice periodicity and the less than four-fold in-plane symmetry at the dielectric maximum. At a periodicity of about 10/10, depending on composition and growth mechanism, several groups report a maximum of dielectric permittivity. In addition to that we observe in-plane symmetry less than tetragonal for 10/10 superlattices by HR-XRD, in contrast to initial low-resolution data from Tabata et al. thus challenging the assumption of unrelaxed strain all the way through the superlattice. The aim of this article is to link both effects to the increasing volume fraction of conducting layers close to the interface in series with the superlattice layers.