Molecular Derailment via Pressurization in Methylammonium Lead Iodide

Hybrid organic–inorganic perovskites combine outstanding optoelectronic properties with low‐cost fabrication, yet their structural fragility under environmental factors limits device stability. In this work, we have employed high-resolution inelastic neutron scattering in the GPa regime alongside first‐principles calculations to probe the pressure‐temperature phase behavior of methylammonium lead iodide ({\MA}). Below 1 GPa, we observe a systematic stiffening of NH$\cdots$I hydrogen bonds concomitant with a contraction of the inorganic framework. Between 1 and 1.25 GPa, the INS data exhibit a pronounced broadening of cation librational features, corresponding to a transition to highly disordered organic-cation environments reminiscent of (the maximally tilted) high-pressure cubic phase. This hitherto unexplored "derailed" state of {\MA} is characterized by a broad distribution of NH$\cdots$I bond lengths, in stark contrast with the well‐defined hydrogen‐bond network of the low‐temperature phase observed at lower pressures. Our experimental and computational results bring to the fore the central (and rather subtle) role played by NH$\cdots$I hydrogen bonds across organic and inorganic sub-lattices in dictating the regions of physical stability and metastability of this important material.