Credit: (a) from Ref. [5]; (b) and (c) Alan Stonebraker

Figure 1: (a) $Ca3Mn2O7$ structure in its distorted ferroelectric state. $Mn$ ions (green) are surrounded by oxygen octahedra (green with red $O$ ions) with $Ca$ ions (blue) interspersed. (b) Octahedral rotation ($R$) in $Ca3Mn2O7$, showing the oxygen (blue) and manganese (red) sites. The undistorted structure is shown in grayscale. (c) Octahedral tilt ($T$) of the same. The combination of the tilt and rotation combine to produce ferroelectric order.