Reentrant Structural and Optical Properties and Large Positive Thermal Expansion in Perovskite Formamidinium Lead Iodide

Abstract The structure of the hybrid perovskite HC(NH 2 ) 2 PbI 3 (formamidinium lead iodide) reflects competing interactions associated with molecular motion, hydrogen bonding tendencies, thermally activated soft octahedral rotations, and the propensity for the Pb 2+ lone pair to express its stereochemistry. High‐resolution synchrotron X‐ray powder diffraction reveals a continuous transition from the cubic α‐phase (Pm m, #221) to a tetragonal β‐phase (P4/mbm, #127) at around 285 K, followed by a first‐order transition to a tetragonal γ‐phase (retaining P4/mbm, #127) at 140 K. An unusual reentrant pseudosymmetry in the β‐to‐γ phase transition is seen that is also reflected in the photoluminescence. Around room temperature, the coefficient of volumetric thermal expansion is among the largest for any extended crystalline solid.