Crystal structures of perovskite. Since the formamidinium lead iodide perovskite crystal of cubic structure system is widely used, this paper explores the properties of perovskite cubic crystal. The divalent cations sit at the corners of the cube surrounded by halogen atoms in an octahedral environment. The FA cation sits in the center of the cube. (a) Cubic structure, (b) tetragonal structure, (c) orthogonal structure, and (d) comparison of crystal structure.