Do Lead Halide Hybrid Perovskites Have Hydrogen Bonds?

Hydrogen bonds impact many material properties due to the bond directionality and collective strength. For them to exist, the atoms connected to hydrogen should possess high electronegativity, the distance between the interacting atoms should be less than their van der Waals radii, and there should be a charge transfer between them. In hybrid perovskites, the impact of hydrogen bonds is ascribed to several phenomena. Among them are the effect of passivation at the surface, structural stability of the mixed cation compounds, structural instability of FAPbI3, and ferroelectric behavior. In this paper, we applied a deterministic method to prove the existence of hydrogen bonds using Raman spectroscopy. This approach was successfully applied to water and is based on the emergence of a Raman mode as a result of the intermolecular charge transfer, which is not measured for the isolated molecule. With the help of DFT calculations, we attributed the Raman modes detected at room temperature (RT) for MAI and FAI and compared them with the Raman spectra of MAPbX3 and FAPbX3. This comparison has shown that there are no hydrogen bonds in hybrid lead halide perovskites at RT.