Hybrid functional study on the ferroelectricity of domain walls with O-vacancies in PbTiO3

Nonlinear defect interactions between the 180 domain wall and oxygen-vacancies (O-vacancies) in PbTiO3, and the characteristic ferroelectricity due to the interactions are investigated using first-principles calculations based on the hybrid Hartree-Fock (HF) density functionals, which correctly reproduce the band gap and provide the accurate defect electronic structures. We show that an oxygen vacancy is likely to form at 180 domain walls than inside the bulk and the vacancy behaves as a double shallow donor that contributes to partial conductivity preferentially than that inside the bulk. The defect interactions between 180 domain walls and oxygen vacancies have a significant influence on the polarization distribution in PbTiO3, and the effect differs depending on the location of O-vacancies with respect to the domain wall. An oxygen vacancy that is located in the polar [001] direction relative to the Ti atom suppresses ferroelectricity around the vacancy in front of a domain wall, and enhances ferroelectricity only an the center of domain wall, which leads to a shift of the domain wall towards the vacancy and pinning of the domain wall. On the other hand, an O-vacancy that is located in the non-polar [100] and [010] directions relative to the Ti atom induces polarization perpendicular to the [010] axis and outward from the vacancy. Furthermore, the magnitude of polarization change around the O-vacancy inside the 180 domain wall is larger than inside the bulk, which originates from the strong interaction between the 180 domain wall and O-vacancy. These results will provide significant fundamental insight for the design of ferroelectrics.