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Hydrogen bonds (HBs) play an important role in the rotational dynamics oforganic cations in hybrid organic/inorganic halide perovskites, affecting thestructural and electronic properties of the perovskites. However, theproperties and even the existence of HBs in these perovskites are not wellestablished. We investigate HBs in perovskites MAPbBr$_3$(MA$^+$=CH$_3$NH$_3^+$), FAPbI$_3$ (FA$^+$= CH(NH$_2$)$_2^+$), and their solidsolution (FAPbI$_3$)$_{7/8}$(MAPbBr$_3$)$_{1/8}$, using ab initio moleculardynamics and electronic structure calculations. We consider HBs donated by X-Hfragments (X=N, C) of the organic cations and accepted by the halides (Y=Br,I), and characterize their properties based on pair distribution functions andon a combined distribution function of hydrogen-acceptor distance withdonor-hydrogen-acceptor angle. By analyzing these functions, we establishgeometric criteria for HB existence based on hydrogen-acceptor distance$d(H-Y)$ and donor-hydrogen-acceptor angle $\measuredangle(X-H-Y)$. Thedistance condition is defined as $d(H-Y)<0.3$ nm, for N-H-donated HBs, and$d(H-Y)<0.4$ nm for C-H-donated HBs. The angular condition is$135{^\circ}\le\measuredangle(X-H-Y)\le 180{^\circ}$ for both types of HBs. Atthe simulated temperature (350 K), the HBs dynamically break and form. Wecompute time correlation functions of HB existence and HB lifetimes, whichrange between 0.1 and 0.3 picoseconds at that temperature. The analysis of HBlifetimes indicates that N-H--Br bonds are relatively stronger than N-H--Ibonds, while C-H--Y bonds are weaker. To evaluate the impact of HBs onvibrational spectra, we present the power spectra, showing that peaksassociated with N-H stretching modes in perovskites are redshifted andasymmetrically deformed compared with the peaks of isolated cations.

Hydrogen bonds in lead halide perovskites: insights from ab initio molecular dynamics