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-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).