Fig. 1: The n = 2 Ruddlesden–Popper structure.
From: Suppressing the ferroelectric switching barrier in hybrid improper ferroelectrics
a The high symmetry body-centered-tetragonal phase (I4/mmm) of A3B2O7 RP-phase perovskites. b Compounds with tolerance factor <1 develop octahedral rotation/tilt distortions, which are usually associated with normal modes at the X point of the Brillouin zone. (The figure shows the \({X}_{2}^{+}\) mode.) These distortions double the original unit cell and symmetry becomes orthorhombic. c Orientations of the crystal axes in the orthorhombic cell are different from those in the high-symmetry tetragonal cell. Throughout this paper, we use the axes of a pseudo-tetragonal cell (shown in black) that can be defined within the orthorhombic cell (shown in light blue).
