Fig. 16: Crystal structure and structural distortions in hybrid improper ferroelectric materials of Ca3Mn2O7.

a Ball-and-stick model of ferroelectric ground state structure of Ca3Mn2O7 with A21am space group symmetry. b Structural distortion of oxygen octahedral rotation (X2 mode). c Structure distortion of oxygen octahedral tilt (X3− mode). d Layer decomposition of polarizations within the plane along [110] direction. Figure 16a–c are reproduced with permission from ref. 170. Copyright [2011] {American Physical Society}. Figure d is reprinted from ref. 200, Copyright (2012), with permission from Elsevier.