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Abstract
The vibrational dynamics of pure and methylammonium-doped formamidinium lead iodide perovskites
(FAPbI3) has been investigated by high-resolution neutron spectroscopy. For the ?first time, we provide an exhaustive and accurate analysis of the cation vibrations and underlying local structure around the organic moiety in these materials using ?first-principles electronic-structure
calculations validated by the neutron data. Inelastic Neutron Scattering experiments on FAPbI3
provide direct evidence of the formation of a low-temperature orientational glass, unveiling the
physico-chemical origin of phase metastability in the tetragonal structure. Further analysis of these
data provides a suitable starting point to understand and explore the stabilization of the perovskite
framework via doping with small amounts of organic cations. In particular, we ?find that hydrogen
bonds in FAPbI3 are strengthened in a synergistic manner as a result of cage deformation induced
by the dynamics of the neighbouring methylammonium cations.
