Effects of strain and oxygen vacancies on the ferroelectric and antiferrodistortive distortions in PbTiO3/SrTiO3 superlattice

The effects of oxygen vacancy (VO) and epitaxial strain on the ferroelectric (FE) and antiferrodistortive (AFD) properties of the [001]-oriented PbTiO3/SrTiO3 1/1 superlattice are comprehensively studied using first-principles calculations. It is found that the oxygen vacancies form most easily when the superlattice in-plane lattice constant is between those of single-crystal PbTiO3 and SrTiO3. The polarization in the direction of the Ti-VO-Ti chain is remarkably reduced due to the VO-induced local tail-to-tail polarization patterns, and consequently, the VO can pin the polarization to a certain direction. Moreover, the octahedral rotation or tilting around the direction of the Ti-VO-Ti chain is also suppressed while the rotations along the other two orthogonal directions are enhanced. In both perfect and oxygen-deficient superlattices, the ferroelectric phase has lower energy when the octahedra rotate around the polar axis. These results suggest a promising way to mediate the FE and AFD properties in oxygen-deficient superlattices.