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.