Phase Transitions of Formamidinium Lead Iodide Perovskite under Pressure

The pressure-induced structural evolution of formamidinium-based perovskite FAPbI₃ was investigated using in situ synchrotron X-ray diffraction and laser-excited photoluminescence methods. Cubic α-FAPbI₃ (Pm3m) partially and irreversibly transformed to hexagonal δ-FAPbI₃ (P6₃mc) at a pressure less than 0.1 GPa. Structural transitions of α-FAPbI₃ followed the sequence of Pm3m → P4/mbm → Im3 → partial amorphous during compression to 6.59 GPa, whereas the δ-phase converted to an orthorhombic Cmc2₁ structure between 1.26 and 1.73 GPa. During decompression, FAPbI₃ recovered the P6₃mc structure of the δ-phase as a minor component (∼18 wt %) from 2.41-1.40 GPa and the Pm3m structure of the α-phase becomes dominant (∼82 wt %) at 0.10 GPa but with an increased fraction of δ-FAPbI₃. The photoluminescence behaviors from both the α- and δ-forms were likely controlled by radiative recombination at the defect levels rather than band-edge emission during pressure cycling. FAPbI₃ polymorphism is exquisitely sensitive to pressure. While modest pressures can engineer FAPbI₃-based photovoltaic devices, irreversible δ-phase crystallization may be a limiting factor and should be taken into account.