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 context, the use of pressure offers the enticing possibility of unveiling the microscopic mechanisms behind structural changes and the eventual collapse or decomposition of the material. In this work, we have employed high-resolution inelastic neutron scattering in the gigapascal regime alongside first-principles calculations to probe the pressure-temperature phase behavior of methylammonium lead iodide (MAPbI₃). Below 1 GPa and 150 K, pressurization leads to a stiffening of spectral features sensitive to NH···I hydrogen-bonding motifs, concomitant with a contraction of the inorganic framework. Between 1 and 1.25 GPa at these low temperatures, the INS data undergo a pronounced broadening, corresponding to the formation of an orientational glass of organic cations whose immediate environment is reminiscent of the high-pressure cubic phase. This hitherto unexplored <i>derailed</i> state of MAPbI₃ is characterized by a broad distribution of NH···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 rather subtle role played by the NH···I bonding topology across organic and inorganic sublattices in dictating the regions of physical stability and metastability of this important material.