  Oxygen vacancies in two-dimensional metal-oxide structures garner much
attention due to unique conductive, magnetic and even superconductive
functionalities they induce. Ferroelectric domain walls have been a prominent
recent example because they serve as a hub for topological defects that enable
unusual symmetries and are relevant for low-energy switching technologies.
However, owing to the light weight of oxygen atoms and localized effects of
their vacancies, the atomic-scale electrical and mechanical influence of oxygen
vacancies has remained elusive. Here, stable individual oxygen vacancies were
found and engineered in situ at domain walls of seminal titanate perovskite
ferroics. The atomic-scale strain, electric-field, charge and dipole-moment
distribution around these vacancies were characterized by combining advanced
transmission electron microscopy and first-principle methodologies. 3-5 %
tensile strain was observed at the immediate surrounding unit cells of the
vacancies. The dipole-moment distribution around the vacancy was found to be an
alternating head-to-head $-$ tail-to-tail $-$ head-to-head structure, giving
rise to a quasi-linear quadrupole topology. Reduction of the nearby Ti ion as
well as enhanced charging and electric-field concentration near the vacancy
confirmed the quadrupole structure and illustrated its local effects on the
electrical and structural properties. Significant intra-band states were found
in the unit cell of the vacancies, proposing a meaningful domain-wall
conductivity. Oxygen-vacancy engineering and controllable quadrupoles that
enable pre-determining both atomic-scale and global functional properties offer
a promising platform of electro-mechanical topological solitons and device
miniaturization in metal oxides.
