Dimensional Reduction of Perovskite-Derived Structures in Formamidinium-Rich Lead Iodide Compositions

Formamidinium lead iodide FAPbI3 (FA+ = CH(NH2)2+) with a cubic perovskite structure is among the most-studied or-ganic-inorganic hybrid perovskites. Despite several reports only on thin films of FA-rich lead iodide perovskites, such as FA2PbI4, their structures remain unknown due to challenges in characterizing polycrystalline films. Here, we report two new phases of FA-rich lead iodide perovskite-derived structures, FA2PbI4 and FA3PbI5, in the bulk form. These materials feature two- and one-dimensional octahedral networks of perovskite, respectively, allowing for the bandgap tuning among FA-Pb-I compositions. This study presents a novel approach to controlling the structural dimensionality of perov-skites and their optical properties.