Symmetry, Dynamics, and Defects in Methylammonium Lead Halide Perovskites

In order to better understand the structure and dynamics of methylammonium lead halide perovskites, we performed NMR, NQR, and DFT studies of CH₃NH₃PbI₃ in the tetragonal and cubic phase. Our results indicate that the space group of the tetragonal phase is the nonpolar I4/mcm. The highly dynamic methylammonium moiety shows no indication of the occurrence of additional orientations of the C-N bond close to the c-axis at temperatures approaching the cubic phase. Crystal quality effects are shown to influence the ¹⁴N NMR and ¹²⁷I NQR spectra, and the effects of high-temperature annealing on defects can be observed. A strong increase in T₂ relaxation time of the ²⁰⁷Pb NMR signal on cooling is found, and is an indication of slow motions in the PbI₆ octahedra at room temperature. These results aid in the understanding of the structure of methylammonium lead halides and enable further studies of defects in these materials.