Figure 1

(a)Eight preferred orientations of the organic MA⁺-ion (i.e., C-N bond axis) within the perovskite cavity. Herein, A, B, C, and D represent the projection of the MA⁺-ion orientations on the a-b plane, as measured by the azimuthal angle θ. The orientation of the MA⁺-ion with respect to the a-b in-plane is represented by the tilting angle ϕ. According to Quarti et al.²⁴ the two optimum ϕ angles are ±30°. However, we have found that the optimum ϕ angle depends sensitively on the hydrogen-bonding interaction mode (See the text for details). (b) Crystal structure of the high-temperature cubic phase viewed from the c-axis (left-hand side). The corresponding Pb-I inorganic cage characterized by the C₄-rotation axis and the mirror plane perpendicular to the C₄ axis (right-hand side). (c) Crystal structure of the tetragonal I4/mcm phase viewed from the c-axis (left-hand side). The corresponding Pb-I inorganic cage characterized by the improper S₄-rotation axis (right-hand side).