Orientation of organic cations in hybrid inorganic-organic perovskite CH₃NH₃PbI₃ from subatomic resolution single crystal neutron diffraction structural studies

We report the crystal growth of well-faceted single crystals of methylammonium lead iodide, CH₃NH₃PbI₃, and detailed single crystal neutron diffraction structural studies aimed at elucidating the orientation of the methylammonium (CH₃NH₃⁺) cation in the tetragonal and cubic phases of the hybrid inorganic-organic perovskite. Room temperature experiments reveal a tetragonal structure where the protonated amine substituent (-NH₃⁺) of the cation is disordered in four positions, each preferentially located near the neighboring iodine of the [PbI₆] octahedra, while the methyl substituent (-CH₃) is disordered in eight positions located near the body position of the unit cell. High temperature experiments show a cubic structure where the cation aligns along the [011] (edge), the [111] (diagonal), and the [100] (face) directions of the unit cell. The resulting site occupancy ratio suggests the CH₃NH₃⁺ cation resides primarily along the [011] direction, in agreement with reported DFT calculations. One important feature that was observed for both tetragonal and cubic structures measured at 295 and 350 K, respectively, is the middle point of the C-N bond being located off-center from the high symmetry sites in the crystal structure, induced by the formation of hydrogen bond-like interactions between the -NH₃⁺ substituent of the organic cation and the iodine atoms of [PbI₆] octahedra.