Short-range ion dynamics in methylammonium lead iodide by multinuclear solid state NMR and ¹²⁷I NQR

We explore the short-range ion dynamics in methylammonium lead iodide (MAPbI3, the archetypal halide perovskite) by means of solid-state NMR (1H, 13C, 14N, 15N and 207Pb) and Nuclear Quadrupolar Resonance (127I NQR), in combination with molecular dynamics simulations. We focus on the rotational motion of the methylammonium (MA) cation, and on the interaction between MA and the inorganic lattice, since these processes are linked to electronic carrier lifetimes, optical and electronic properties and even structural stability of this promising solar cell material. We show that the motion of the MA cation can be described by a bi-axial rotation, with similar interactions of CH3 and NH3+ groups with the inorganic framework. This motion becomes nearly isotropic above the cubic phase transition, dominating the spin-lattice relaxation of 1H, 13C and 15N through spin-rotational interactions. In addition, we observe strong cross-relaxation between 207Pb and 127I, which fully controls spin-spin and spin-lattice relaxation in 207Pb.