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[NH 2 ] 2 + , FA) and methylammonium (CH 3 NH 3 + , MA) cations in FA 1- x MA x PbI 3 with x = 0 and 0.4 and compare it to the dynamics in MAPbI 3 . For FAPbI 3 , 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 0.6 MA 0.4 PbI 3 , the dynamics of the respective organic cations evolve from a relatively similar behavior to FAPbI 3 and MAPbI 3 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 MAPbI 3 . 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- x MA x PbI 3 .