Interplay of epitaxial strain and rotations in PbTiO3/PbZrO3 superlattices from first principles

We present first-principles calculations of the structural phase behavior of the [1:1] PbTiO3/PbZrO3 superlattice and the PbTiO3 and PbZrO3 parent compounds as a function of in-plane epitaxial strain. A symmetry analysis is used to identify the phases and clarify how they arise from an interplay between different structural distortions, including out-of-plane and in-plane polar modes, rotation of oxygen octahedra around out-of-plane or in-plane axes, and an antipolar mode. Symmetry-allowed intermode couplings are identified and used to elucidate the nature of the observed phase transitions. For the minimum-period [1:1] PbTiO3/PbZrO3 superlattice, we identify a sequence of three transitions that occur as the in-plane lattice constant is increased. All four of the phases involve substantial oxygen octahedral rotations, and an antipolar distortion is important in the high-tensile-strain phase. Inclusion of these distortions is found to be crucial for an accurate determination of the phase boundaries.