Epitaxially strained [001]-(PbTiO₃)₁(PbZrO₃)₁ superlattice and PbTiO₃ from first principles

The effect of layer-by-layer heterostructuring and epitaxial strain on lattice instabilities and related ferroelectric properties is investigated from first principles for the [001]-(PbTiO₃)₁(PbZrO₃)₁ superlattice and pure PbTiO₃ on a cubic substrate. The results for the superlattice show an enhancement of the stability of the monoclinic r phase with respect to pure PbTiO₃. Analysis of the lattice instabilities of the relaxed centrosymmetric reference structure computed within density functional perturbation theory suggests that this results from the presence of two unstable zone-center modes, one confined in the PbTiO₃ layer and one in the PbZrO₃ layer, which produce in-plane and normal components of the polarization, respectively. The zero-temperature dielectric response is computed and shown to be enhanced not only near the phase boundaries, but throughout the r phase. Analysis of the analogous calculation for pure PbTiO₃ is consistent with this interpretation, and suggests useful approaches to engineering the dielectric properties of artificially structured perovskite oxides.