Structural phase transitions and molecular dynamics in aziridinium lead iodide perovskites

Hybrid lead halide perovskites are among the most promising materials for optoelectronic applications due to their exceptional dielectric response and charge-transport properties. Among these, aziridinium lead iodide (AzrPbI3) has recently emerged as a novel composition with a cubic perovskite structure at room temperature. Previous studies report order disorder phase transitions at low temperatures, but the symmetries of the non-cubic phases remain uncertain. In contrast to methylammonium lead iodide (MAPbI3), whose structural transitions and hydrogen-bonding interactions have been resolved by neutron diffraction, the positions and dynamics of the aziridinium cation, particularly its hydrogen atoms, have never been experimentally determined. This proposal aims to perform the first neutron diffraction investigation of AzrPbI3 to determine its low-temperature crystal structure, locate hydrogen atoms, and understand the role of molecular dynamics in driving its phase transitions. Measurements will be conducted between 6 K and 300 K on high-purity AzrPbI3 powder. The results will provide a complete structural model including hydrogen positions and bonding geometry, establishing the connection between cation orientation, dielectric screening, and phase stability. The findings will not only clarify the fundamental structure of AzrPbI3 but also serve as a reference for interpreting and improving polycrystalline thin-film materials for optoelectronic devices.