Topological defects at octahedral tilting plethora in bi-layered perovskites

Oxygen octahedral distortions, including tilts/rotations, deformations and off-centring in (layered) perovskites, have the key role in their numerous functional properties. Near the polar-centrosymmetric phase boundary in bi-layered perovskite Ca3-xSrxTi2O7 with x approximate to 1, we found the presence of abundant topological eight-state vortex-antivortex pairs, associated with four oxygen octahedral tilts at domains and another four different oxygen octahedral tilts at domain walls. Our discovery opens a new revenue to unveil real-space topological defects associated with the possible vector choices in one specific lattice mode.