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 H-1, C-13, and Pb-207 magic angle spinning NMR and N-14 static NMR. The chemical shifts of the H-1 nuclei in CH3 and NH3 remained constant with increasing temperature, whereas those of the C-13 and Pb-207 nuclei varied near the phase transition temperature (T-C=236 K), indicating that the structural environments of C-13 and Pb-207 change near T-C. The spin-lattice relaxation time T-1 rho values for H-1, C-13, and Pb-207 nuclei increased with increasing temperature and did not exhibit an abrupt change near T-C. In addition, the two lines in the N-14 NMR spectra superposed into one line near T-C, 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 T-C is a change in the surroundings of the Pb-207 nuclei in the PbBr6 octahedra and of the C-N groups in the CH3NH3 cations.