Phase Transitions of Formamidinium Lead Iodide Perovskite under Pressure

The pressure-induced structural evolution of formamidinium-based perovskite FAPbI 3 was investigated using in situ synchrotron X-ray diffraction and laser-excited photoluminescence methods. Cubic α-FAPbI 3 ( Pm 3̅ m ) partially and irreversibly transformed to hexagonal δ-FAPbI 3 ( P 6 3 mc ) at a pressure less than 0.1 GPa. Structural transitions of α-FAPbI 3 followed the sequence of Pm 3̅ m → P 4/ mbm → Im 3̅ → partial amorphous during compression to 6.59 GPa, whereas the δ-phase converted to an orthorhombic Cmc 2 1 structure between 1.26 and 1.73 GPa. During decompression, FAPbI 3 recovered the P 6 3 mc structure of the δ-phase as a minor component (∼18 wt %) from 2.41–1.40 GPa and the Pm 3̅ m structure of the α-phase becomes dominant (∼82 wt %) at 0.10 GPa but with an increased fraction of δ-FAPbI 3 . 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. FAPbI 3 polymorphism is exquisitely sensitive to pressure. While modest pressures can engineer FAPbI 3 -based photovoltaic devices, irreversible δ-phase crystallization may be a limiting factor and should be taken into account.