Topological defects at octahedral tilting plethora in bi-layered   perovskites
  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. 
F.-T. Huang, B. Gao, J.-W. Kim, X. Luo, Y. Wang, M.-W. Chu, C.-K.   Chang, H.-S. Sheu and S.-W. Cheong
cond-mat.mtrl-sci