fig1

Figure 1. Detail of the tetragonal room-temperature structure of MAPbI₃ with its large iodine anisotropic temperature factors (ADPs) (their flat shape indicates a transverse displacement perpendicular to the Pb-I-Pb bonds and in the direction of the MA molecule), the coordination polyhedra of the MA molecule [MAI₁₂], a cuboctahedron (light yellow), and one [PbI₆] octahedra (light gray)^[37]. The anisotropic shape and orientation of the halide ADPs are partly due to the strongly anharmonic metal-halide bond (see EXAFS section) and not only to the hydrogen bond between the MA and the halide atoms (formed between the jump rotations). In this representation of a single MA molecule, neither the 4-fold rotational axis nor the 2-fold rotational axis of the I4/mcm space group symmetry seems to exist for the molecule. However, since the MA molecule jump rotates^[8] and there are, on average, eight MA molecule orientations in the cage, an average structure with I4/mcm symmetry is observed in diffraction.