Multinuclear NMR as a tool for studying local order and dynamics in CH₃NH₃PbX₃(X = Cl, Br, I) hybrid perovskites

Abstract

We report on ²⁰⁷Pb, ⁷⁹Br, ¹⁴N, ¹H, ¹³C and ²H NMR experiments for studying the local order and dynamics in hybrid perovskite lattices. ²⁰⁷Pb NMR experiments conducted at room temperature on a series of MAPbX₃ compounds (MA = CH₃NH₃⁺; X = Cl, Br and I) showed that the isotropic ²⁰⁷Pb NMR shift is strongly dependent on the nature of the halogen ions. Therefore ²⁰⁷Pb NMR appears to be a very promising tool for the characterisation of local order in mixed halogen hybrid perovskites. ²⁰⁷Pb NMR on MAPbBr₂I served as a proof of concept. Proton, ¹³C and ¹⁴N NMR experiments confirmed the results previously reported in the literature. Low temperature deuterium NMR measurements, down to 25 K, were carried out to investigate the structural phase transitions of MAPbBr₃. Spectral lineshapes allow following the successive phase transitions of MAPbBr₃. Finally, quadrupolar NMR lineshapes recorded in the orthorhombic phase were compared with simulated spectra, using DFT calculated electric field gradients (EFG). Computed data do not take into account any temperature effect. Thus, the discrepancy between the calculated and experimental EFG evidences the fact that MA cations are still subject to significant dynamics, even at 25 K.