Fig. 7.

a Schematic structures of mixed FA/PEA Sn perovskites with PEAI/FAI doping ratios of 0, 20, 40, 60, 80, and 100%. Reproduced with permission from Ref. [127]. b Three alternative views of the archetypal 3D perovskite structure viewed along the (100), (110), and (111) cleavage planes. c “Perovskitoid” face-sharing building block of the 1D structures obtained for t > 1 representing the archetypal hexagonal polytype. d Perovskite structures obtained through dimensional reduction featuring corner-sharing 2D sheets and 1D chains through the (100) and (110) cleavage planes, respectively. e Hexagonal perovskite polytypes obtained from a linear combination of the corner-sharing 3D perovskite along the (111) cleavage plane and the 1D face-sharing polytype. The “h” and “c” symbols indicate hexagonal and cubic layers, respectively, and serve in identifying the layer sequence that characterizes the polytype. Reproduced with permission from Ref. [135]