DOI: https://doi.org/10.29363/nanoge.hpatom.2022.011
Publication date: 30th October 2021
The methylammonium lead halides, CH3NH3PbX3 (X = Cl, Br, I), were first described by D. Weber in 1978 [1]. The compounds were reported to have cubic perovskite structures; the chloride was colorless, the bromide was red-orange, while the iodide was black. Also, it was indicated that there was no evidence of conductivity under normal conditions. My interest in these compounds began in the early 1980s when my colleague, Professor Osvald Knop at Dalhousie University, Halifax, Nova Scotia, described some interesting problems that he had encountered in examining single-crystal X-ray diffraction data for CH3NH3PbCl3. I suggested that we prepare deuterium labelled methylammonium lead halides and use 2H and 14N NMR line-shapes and spin-lattice relaxation data to examine the dynamics of the methylammonium cation. Our preliminary results were published in the fall of 1985 [2]; this was followed by a more detailed paper in 1990 [3]. Briefly, in addition to the high-temperature cubic phase, all three methylammonium lead halides exhibit one or more tetragonal phases and a low-temperature orthorhombic phase [2], [3], [4], [5]. Our early 2H and 14N NMR relaxation data indicated that in the high-temperature cubic phase of all three compounds, the methylammonium cations reorientate at a rate approaching the inertial limit (i.e., 10-12 s; i.e., ps time-scale) [2] which I found intriguing! Together, the 2H and 14N NMR relaxation data allowed us to separate the end-to-end motion of the C-N axis from the faster jump-motion about the C-N bond. In the lowest temperature phase, the 2H NMR line shapes allowed us to confirm that motion of the C-N bond was essentially “frozen”!
In this last decade, the methylammonium lead halides, particularly the iodide, have attracted considerable attention because of their performance as light harvesters for solar cells [6], [7], [8]. In view of the enormous activity in this area we extended our earlier NMR studies which I will attempt to highlight in my presentation [9]. See D.J. Hubicki, S.D. Stranks, C.P. Grey and L. Emsley for a recent review of the role that NMR has played in this research area [10].
First, I would like to thank my friend and former colleague, Professor Osvald Knop, for introducing me to the methylammonium lead halides and for many enlightening discussions during my years at Dalhousie. Unfortunately, “Ossie” died in April, 2015 at the age of 93. Also, special thanks to Professor Mary Anne White for her friendship and early adiabatic calorimetry data which has been of tremendous value. The beautiful NMR spectra in our 2018 paper were acquired by Dr. Guy Bernard and Dr. Victor Terskikh and were invaluable in helping us develop a better understanding of the dynamics, particularly in the tetragonal phases of the MAPbX3 Perovskites. Here, I want to especially thank Dr. Christopher Ratcliffe for his insights and contributions. Qichao Wu and Tate Hauger prepared most of the samples. Guy’s assistance in helping me put our 2018 manuscript together are much appreciated. Finally, Professor Jillian Buriak is thanked for her encouragement and keeping us on track. This research would have been impossible without the support of the Natural Sciences and Engineering Research Council over forty plus years as well as the Canada Research Chair’s Program.
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