(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.