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 one of the most well-known compounds among organic-inorganic hybrid perovskites. There are several reports on thin films of FA-rich lead iodide perovskite compounds, such as FA2PbI4, but the structures remain unclear due to the poor characterization of the thin film forms. Herein, we identify two new phases of FA-rich lead iodide perovskite-derived compounds, FA2PbI4 and FA3PbI5, in the bulk form. These materials feature two- and one-dimensional octahedral networks of perovskite, respec-tively, allowing for the bandgap tuning among FA-Pb-I compositions. This study presents a novel approach to controlling the structural dimensionality of perovskites and their optical properties.