  Oxygen octahedral distortions including tilts/rotations, deformations and
off-centering in (layered) perovskites play the key role in their numerous
functional properties. At the polar-centrosymmetric phase boundary in
bi-layered perovskite Ca3-xSrxTi2O7 with x~1, we found the presence of abundant
topological Z8 vortex-antivortex pairs, associated with four oxygen octahedral
tilts at domains and another four different oxygen octahedral tilts at domain
boundaries. Our discovery opens a new revenue to unveil topological defects
associated with various types of oxygen octahedral distortions.
