Methylammonium\nLead Iodide across Physical Space:\nPhase Boundaries and Structural Collapse

Hybrid perovskites exhibit complex structures and phase\nbehavior\nunder different thermodynamic conditions and chemical environments,\nthe understanding of which continues to be pivotally important for\ntailoring their properties toward improved operational stability.\nTo this end, we present for the first time a comprehensive neutron\nand synchrotron diffraction investigation over the pressure–temperature\nphase diagram of the paradigmatic hybrid organic–inorganic\nperovskite methylammonium lead iodide (MAPbI<sub>3</sub>). This ambitious\nexperimental campaign down to cryogenic temperatures and tens of kilobars\nwas supported by extensive <i>ab initio</i> molecular dynamics\nsimulations validated by the experimental data, to track the structural\nevolution of MAPbI<sub>3</sub> under external physical stimuli at\nthe atomic and molecular levels. These combined efforts enable us\nto identify the mechanisms underpinning structural phase transitions,\nincluding those exhibiting negative thermal expansion across the boundary\nbetween the cation-ordered low-temperature phase and the dynamically\ndisordered high-pressure cubic phase. Our results bring to the fore\nhow pronounced octahedral distortions at high pressures ultimately\ndrive the structural collapse and amorphization of this material.