Phase Behavior and Polymorphism of Formamidinium Lead Iodide

A greater understanding of the structure–property relationships of hybrid perovskites for solar cells is crucial for enhancing their performance. The low-temperature phases of formamidinium lead iodide (FAPbI3) have been investigated using rapid neutron powder diffraction. On cooling, the metastable α-polymorph descends in symmetry from the cubic unit cell phase present at room temperature through two successive phase transitions. Between 285 and 140 K a tetragonal phase, adopting the space group P4/mbm, is confirmed and the orientation of the disordered FA cation over this temperature range determined. The cation dynamics have also been investigated, over the same temperature range, at the atomic scale by using ab initio molecular dynamics simulations, which indicate contrasting FA motion in the cubic and tetragonal structures. Below 140 K the neutron powder diffraction data display weak Bragg scattering intensities not immediately indexable to a related unit cell. Data collected at 100 K from N-deuterated FAPbI3 did not reveal any indication of the fundamental Bragg reflections at high d-spacing expected for an expanded supercell model as previously reported. Other hypotheses of a mixture of phases or a simple tetragonal cell below 140 K are also rejected on the basis of the observed data, and our observations are consistent with a locally disordered low-temperature γ-phase.