Symmetry, Dynamics, and Defects in Methylammonium Lead Halide Perovskites

Abstract

In order to better understand the structure and dynamics of methylammonium lead halide perovskites, we performed NMR, NQR, and DFT studies of CHNHPbI 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 N NMR andI NQR spectra, and the effects of high-temperature annealing on defects can be observed. A strong increase in T relaxation time of the Pb NMR signal on cooling is found, and is an indication of slow motions in the PbI 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.

Short-range ion dynamics in methylammonium lead iodide by multinuclear solid state NMR and127I NQR

“…Interestingly, a rather different w Q ( 14 N) of 0.68 MHz with Z = 0.52 has been obtained by averaging over all nitrogen sites (Table S1, ESI †), indicating a significant influence of the Pb-I framework on the EFG. These calculated EFG parameters are very close to those recently reported by Franssen et al, 25 and have been used in our 14 N spectral simulations. As shown in Fig.…”

“…7b), reaching a peculiar 3-fold enhancement at 183 K, even though the value falls back to 20 ms at 173 K (near the orthorhombic phase transition), in agreement with previous reports. 25 This unusual behavior, which is also found in T 1 measurements (Fig. 7b), cannot be ascribed to standard relaxation mechanisms and common spectral density functions, but can be easily understood by taking into account strong spin-spin interactions between the spin-1/2 207 Pb nucleus and the quadrupolar 127 I nucleus.…”

“…[20][21][22] Even though decreasing the temperature naturally hinders MA reorientation, rotation about the C-N axis in a correlated fashion was observed down to 50 K. The phase transitions (orthorhombic-tetragonal at 162 K and tetragonal-cubic at 327 K) 23 are not visible in the T 1 measurements of 2 H and 14 N, 21 but can be detected by 1 H measurements, 22 albeit only the lower temperature transition was investigated. More recently, NMR spectroscopy has been applied to halide perovskites to investigate the effects of degradation, 24 extended defects and crystal quality 25 and details of Pb-X environment and cation dynamics. 13,[26][27][28][29] In this work, we report on NMR and NQR spectra discussed in light of molecular dynamics simulations and relaxation effects.…”

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

“…Since the first preparation of the hybrid lead halide perovskites, 12 the behavior of the molecular cations has been studied by calorimetry, 13,14 infrared spectroscopy, 13,15,16 dielectric spectroscopy, 14,[17][18][19][20] Raman spectroscopy, 21 quasi-elastic and inelastic neutron scattering, [22][23][24][25][26] nuclear magnetic resonance (NMR) spectroscopy, [27][28][29][30][31][32] isotopic substitution, 33 and ab initio methods, [34][35][36][37][38][39] and these findings have been correlated to the structure evolution from crystallographic techniques. 19,[40][41][42][43][44][45][46] Most of these efforts have focused on MAPbX 3…”

The Nature of Ion Conduction in Methylammonium Lead Iodide: A Multimethod Approach

“…Our experiments also clearly establish electron holes as dominant electronic carriers in MAPbI 3 ,ifthe iodine partial pressure is not extremely small. We focus on long-range motion; ad etailed discussion about short-range dynamics (rotational/vibrational motion), as addressed in earlier studies, [40][41][42][43][44] is outside the scope of this paper.…”