fig2

Figure 2. Improper ferroelectric structure with geometric distortion. (A) Hexagonal YMnO₃ structure with centrosymmetric non-polar (left), down-polar (middle), and up-polar (right). Distortion of MnO₅ triangular bipyramids is indicated with arrows. (B) Ruddlesden-Popper Sr₃Zr₂O₇ layered structures showing polar and anti-polar characteristics. ZrO₆ octahedral rotations determine the polarity of the structure. Reproduced with permission^[52]. Copyright 2018, WILEY-VCH Verlag Gmbh & Co. KGaA, Weinheim.