Atomic Structure and Dynamics of Organic–Inorganic Hybrid Perovskite Formamidinium Lead Iodide

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The atomic dynamic behaviors of formamidinium lead iodide [HC(NH 2 ) 2 PbI 3 ] are critical for understanding and improving photovoltaic performances. However, they remain unclear. Here, we investigate the structural phase transitions and the reorientation dynamics of the formamidinium cation [HC(NH 2 ) 2 +, FA + ] of FAPbI 3 using neutron scattering techniques. Two structural phase transitions occur with decreasing temperature, from cubic to tetragonal phase at 285 K and then to another tetragonal at 140 K, accompanied by gradually frozen reorientation of FA cations. The nearly isotropic reorientation in the cubic phase is suppressed to reorientation motions involving a two-fold ( C 2 ) rotation along the N···N axis and a four-fold ( C 4 ) rotation along the C–H axis in the tetragonal phase, and eventually to local disordered motion as a partial C 4 along the C–H axis in another tetragonal phase, thereby indicating an intimate interplay between lattice and orientation degrees of freedom in the hybrid perovskite materials. The present complete atomic structure and dynamics provide a solid standing point to understand and then improve photovoltaic properties of organic–inorganic hybrid perovskites in the future.