(a) The crystal structure of a cubic ABX₃ perovskite, where the green, gray, and brown balls represent the A cations, B cations, and the X anions, respectively; (b) the crystal structure of a cubic perovskite viewed from its [111] direction; (c) the calculated electronic band structure of bulk CsPbBr₃, together with its total and site-decomposed DOS; (d) the effective crystal structure of a cubic ABX₃ perovskite effectively seen by its low energy charge carriers; (e) the top view of the crystal structure of a BX₆ bilayer sliced out along the (111) plane of a cubic perovskite, which constructs the hexagonal honeycomb lattice with two equivalent BX₆ sublattices; and (f) the side view of the crystal structure of a hexagonal BX₆ bilayer, which indeed presents a buckled single quasi-atom (BX₆) layer.