Multinuclear Magnetic Resonance Tracking of Hydro, Thermal, and Hydrothermal Decomposition of CH3NH3PbI3

An NMR investigation of methylammonium lead iodide, the leading member of the hybrid organic- inorganic perovskite class of materials, and of its putative decorriposition products. as :a result of exposure to heat and humidity, has been undeitaken. We show that the Pb-207 NMR spectra of the compound of interest and of the proposed lead containing decomposition products, CH3NH3PbI3.H2O, (CH3NH3)4PbI(6).2H(2)O, and PbI2, have distinctive chemical shifts: spanning over 1400 ppm, making Pb-207 NMR an ideal tool for investigating this material; further infiirmation may be gained from C-13 and H-1 NMR spectra. As reported in many investigations of CH3NH3PbI3 on films, the bulk material hydrates in the presence of high relative humidity (approximately 80%), yielding the monohydrated perovskite CH3NH3PbI3.H2O. This reaction is reversible by heating the sample to 341 K. We show that neither (CH3NH3)(4)PbI6.2H(2)O nor PbI2 is observed as a decomposition product and that, in contrast to many studies: on CH3NH3PbI3 films, the bulk material does not decompose or degrade beyond CH3NH3PbI3.H2O upon prolonged exposure to humidity at ambient temperature. However, exposing CH3NH3PbI3 concurrently to heat and humidity, or directly exposing it to liquid water, leads to the irreversible formation of PbI2. In spite of its absence among the decomposition products, the" response of (CH3NH3)(4)PbI6.2H(2)O to heat was also investigated. It is stable at temperatures below 336 K but then rapidly dehydrates, first to CH3NH3PbI(3).H2O and then to CH3NH3PbI3. The higher stability of the bulk material as reported here is a promising advance, since stability is a major concern in the development of commercial applications fot this material.