<div>The vibrational dynamics of pure and methylammonium-doped formamidinium lead iodide perovskites</div><div>(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</div><div>calculations validated by the neutron data. Inelastic Neutron Scattering experiments on FAPbI3</div><div>provide direct evidence of the formation of a low-temperature orientational glass, unveiling the</div><div>physico-chemical origin of phase metastability in the tetragonal structure. Further analysis of these</div><div>data provides a suitable starting point to understand and explore the stabilization of the perovskite</div><div>framework via doping with small amounts of organic cations. In particular, we ?find that hydrogen</div><div>bonds in FAPbI3 are strengthened in a synergistic manner as a result of cage deformation induced</div><div>by the dynamics of the neighbouring methylammonium cations.</div>

<div>The vibrational dynamics of pure and methylammonium-doped formamidinium lead iodide perovskites</div><div>(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</div><div>calculations validated by the neutron data. Inelastic Neutron Scattering experiments on FAPbI3</div><div>provide direct evidence of the formation of a low-temperature orientational glass, unveiling the</div><div>physico-chemical origin of phase metastability in the tetragonal structure. Further analysis of these</div><div>data provides a suitable starting point to understand and explore the stabilization of the perovskite</div><div>framework via doping with small amounts of organic cations. In particular, we ?find that hydrogen</div><div>bonds in FAPbI3 are strengthened in a synergistic manner as a result of cage deformation induced</div><div>by the dynamics of the neighbouring methylammonium cations.</div>