Stability threshold of formamidinium lead iodide determined by strain amplitudes

Abstract

The mechanical stability of α-phase formamidinium (FA) lead iodide perovskite is investigated using a systematic finite deformation method based on all the possible FA cation orientations, inside an inorganic cage. Two specific strain boundaries of deformation or strain amplitudes were efficiently examined over this optimized pseudo-cubic structure, which needs nine independent elastic constants (EC) to determine the stability. The stable structure was found while applying an initial amplitude limit (0.02), which is attributed to the balanced charge distribution in between the hydrogens (H4, H5 and H2, H3) to the specific iodine (I2) atom. Surprisingly, with increasing amplitude (0.04), the structure was unstable due to their disturbed charge distribution and FA cation tilting as found from electron density difference (EDD) spectrum. From the computational perspective, the obtained EC values, Mulliken charge, and EDD results provide some crucial information’s to understand the reason behind the instability issues based on the FA cation position which is reported so far. Hence, we provide a solid justification regarding the stability threshold of a pseudo-cubic model based on the FA cation position to get an insight into this FA cation-based perovskite material.