Contrasting Behaviors of FA and MA Cations in APbBr₃

It is known that the organic units in hybrid halide perovskites are free to rotate, but it is not clear if this freedom is of any relevance to the structure-property relationship of these compounds. We have employed quasi-elastic neutron scattering using two different spectrometers, thus providing a wide dynamic range to investigate the cation dynamics in methylammonium lead bromide (MAPbBr₃) and formamidinium lead bromide (FAPbBr₃) over a large temperature range covering all known crystallographic phases of these two compounds. Our results establish a plastic crystal-like phase forming above 30 K within the orthorhombic phase of MAPbBr₃ related to 3-fold rotations of MA units around the C-N axis with an activation energy, E_a, of ∼27 meV, which has no counterpart in the FA compound. MA exhibits an additional 4-fold orientational motion of the whole molecule via rotation of the C-N axis itself with an E_a of ∼68 meV common for the high-temperature tetragonal and cubic phases. In contrast, the FA compound exhibits only an isotropic orientational motion of the whole FA unit with E_a ≈ 106 meV within the orthorhombic phase and a substantially reduced common E_a of ∼62 meV for the high-temperature tetragonal and cubic phases. Our results suggest that the rotational dynamics of the organic units, crystallographic phases, and physical properties of these compounds are intimately connected.