Molecular dynamics of hybrid halide perovskite (CH3NH3)2CuX4 (X = Br and Cl) determined by nuclear magnetic resonance relaxation processes

Abstract

The thermal stability of hybrid halide perovskite (CH3NH3)2CuBr4 crystals are investigated using differential scanning calorimetry and thermogravimetric analysis. To elucidate the local environment of the cations, temperature-dependent chemical shifts and spin-lattice relaxation times are measured using 1H magic angle spinning nuclear magnetic resonance (MAS NMR) and 13C cross-polarization/MAS NMR techniques. The chemical shifts of 1H and 13C decrease with increasing temperature, which indicates that the environment around 1H and 13C changes with temperature. Moreover, the 1H and 13C spin-lattice relaxation times T1ρ increase significantly at high temperatures. These T1ρ results reveal that the cations are subject to high mobility at high temperatures. A comparison of the molecular dynamics of (CH3NH3)2CuX4 (X = Br, Cl) crystals is made, together with a discussion of the effect of substituting Cl ions for the Br ions in the structure.