Phase Transitions of Formamidinium Lead Iodide Perovskite under Pressure.

The pressure-induced structural evolution of formamidinium-based perovskite FAPbI was investigated using in situ synchrotron X-ray diffraction and laser-excited photoluminescence methods. Cubic α-FAPbI ( Pm3̅ m) partially and irreversibly transformed to hexagonal δ-FAPbI ( P6 mc) at a pressure less than 0.1 GPa. Structural transitions of α-FAPbI followed the sequence of Pm3̅ m → P4/ mbm → Im3̅ → partial amorphous during compression to 6.59 GPa, whereas the δ-phase converted to an orthorhombic Cmc2 structure between 1.26 and 1.73 GPa. During decompression, FAPbI recovered the P6 mc structure of the δ-phase as a minor component (∼18 wt %) from 2.41-1.40 GPa and the Pm3̅ m structure of the α-phase becomes dominant (∼82 wt %) at 0.10 GPa but with an increased fraction of δ-FAPbI. 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 polymorphism is exquisitely sensitive to pressure. While modest pressures can engineer FAPbI-based photovoltaic devices, irreversible δ-phase crystallization may be a limiting factor and should be taken into account.