Investigation of the Solution Chemistry of Hybrid Organic-Inorganic Indium Halides for New Material Discovery.

Recently, metal halide perovskites (MHPs) have emerged as a new class of materials for optical and electronic applications such as solar cells and ionizing radiation detectors. Although the solution-processability of MHPs is among their greatest advantages, the solution chemistries of most metal halide systems and their relationship with the observed structural and chemical diversity are poorly understood. In this work, we study the solution chemistry of a model indium halide system, methylammonium (MA)-In-Br, using a combination of the UV-vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS) measurements, small-angle X-ray scattering (SAXS), and density functional theory (DFT) calculations. Our results show that indium could form either octahedral [InBr 6 3- ] or tetrahedral [InBr 4 - ] anions in solution or a combination of both, depending on the loading ratios of MABr and InBr 3 reactants. Understanding the solution chemistry of this system and recognizing the optical fingerprints of these polyanions allow for targeted crystallization of two novel compounds: MAInBr 4 featuring tetrahedral [InBr 4 - ] anions and MA 2 InBr 5 containing both octahedral [InBr 6 3- ] and tetrahedral [InBr 4 - ] anions. Further increase of the MABr content leads to the formation of previously reported MA 4 InBr 7 , containing only octahedral [InBr 6 3- ] anions separated by Br - anions. Our results suggest that understanding the solution chemistry of multinary metal halide systems could be a valuable tool for discovering functional materials for practical applications.