Topological defects at octahedral tilting plethora in bi-layered perovskites

Octahedral distortions have close links to the rich variety of functional properties exhibited by perovskite crystals. Sang-Wook Cheong of Rutgers University in the USA and co-workers report the discovery of a new symmetry state in the double-layered perovskite (Ca,Sr)3Ti2O7 utilizing a combination of transmission electron microscopy, synchrotron x-ray diffraction and dielectric characterizations. This intermediate tetragonal structure appears in the vicinity of the phase transition boundary at which the increasing Ca doping drives the parent tetragonal Sr3Ti2O7 unit cell to undergo an orthorhombic distortion. Associated with the oxygen octahedral tilts along the tetragonal directions in this new intermediate state, there are intriguing topological defects present, which may suggest an avenue towards the realization of unconventional functionalities ultimately.