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.