Structural dynamics of CH3NH3+ and PbBr3− in tetragonal and cubic phases of CH3NH3PbBr3 hybrid perovskite by nuclear magnetic resonance

Understanding the structural dynamics of lead-halide perovskites is essential for their advanced use as photovoltaics. Here, the structural dynamics of the CH3NH3 cation and PbBr6 octahedra in the perovskite CH3NH3PbBr3 were studied via nuclear magnetic resonance (NMR) to determine the mechanism of the transition from the tetragonal to cubic phase. The chemical shifts were obtained by 1H, 13C, and 207Pb magic angle spinning NMR and 14N static NMR. The chemical shifts of the 1H nuclei in CH3 and NH3 remained constant with increasing temperature, whereas those of the 13C and 207Pb nuclei varied near the phase transition temperature (TC = 236 K), indicating that the structural environments of 13C and 207Pb change near TC. The spin–lattice relaxation time T1ρ values for 1H, 13C, and 207Pb nuclei increased with increasing temperature and did not exhibit an abrupt change near TC. In addition, the two lines in the 14N NMR spectra superposed into one line near TC, indicating the occurrence of a phase transition to a cubic phase with higher symmetry than tetragonal. Consequently, the main factor causing the phase transition from the tetragonal to cubic phase near TC is a change in the surroundings of the 207Pb nuclei in the PbBr6 octahedra and of the C–N groups in the CH3NH3 cations.