Rotational Dynamics of Organic Cations in Formamidinium Lead Iodide Perovskites

We report results from quasi-elastic neutron scattering studies on the rotational dynamics of formamidinium (HC[NH₂]₂⁺, FA) and methylammonium (CH₃NH₃⁺, MA) cations in FA_1-xMA_xPbI₃ with x = 0 and 0.4 and compare it to the dynamics in MAPbI₃. For FAPbI₃, the FA cation dynamics evolve from nearly isotropic rotations in the high-temperature (T > 285 K) cubic phase through reorientations between preferred orientations in the intermediate-temperature tetragonal phase (140 K < T ⩽ 285 K) to an even more complex dynamics, due to a disordered arrangement of the FA cations, in the low-temperature tetragonal phase (T ⩽ 140 K). For FA_0.6MA_0.4PbI₃, the dynamics of the respective organic cations evolve from a relatively similar behavior to FAPbI₃ and MAPbI₃ at room temperature to a different behavior in the lower-temperature phases where the MA cation dynamics are a factor of 50 faster as compared to those of MAPbI₃. This insight suggests that tuning the MA/FA cation ratio may be a promising approach to tailoring the dynamics and, in effect, optical properties of FA_1-xMA_xPbI₃.