Oxygen octahedra

Zhang, J.X, Q He, M Trassin, W Luo, D Yi, M.D Rossell, P Yu, L You, C.H Wang, C.Y Kuo, J.T Heron, Z Hu, R.J Zeches, H.J Lin, A Tanaka, C.T Chen, L.H Tjeng, Y.-H Chu, and Ramamoorthy Ramesh. "Microscopic origin of the giant ferroelectric polarization in tetragonal-like BiFeO3." Physical Review Letters 107 (2011).