Atomistic Mechanisms of Temperature-Induced Phase Transitions of 2D Lead Halide Perovskite by Molecular Dynamics Simulation

A series of phase transitions has been experimentally observed in 2D lead halide perovskites within the Ruddlesden-Popper category in response to temperature changes. With increasing temperature, their crystal symmetry transitions from a low-symmetry phase to a high-symmetry phase, evident from the transition from a triclinic to an orthorhombic crystal system. This phase transition has been primarily attributed to the melting of organic spacer molecules. Separate experimental observations have also suggested that the transition is prompted by the disruption of hydrogen bonds between the ammonium headgroup of the spacer molecules and the iodides of the octahedra in the inorganic layers. To the best of our knowledge, there is a notable absence of atomistic modeling in the literature to provide a clear understanding of the atomistic mechanisms underpinning this transition.<br/><br/>In this study, we utilize molecular dynamics (MD) simulations to gain an in-depth understanding of the phase transition mechanisms in BA₂MAPb₂I₇, a compound containing n-butylammonium (BA) as the organic spacer molecule. In agreement with prior experiments, our results indicate that both the melting of the BA ligands and the hydrogen bond breakage play pivotal roles in facilitating the phase transition. At the onset of phase transition as temperature rises, the BA ligands begin localized rattling motion and undergo partial melting at the tail C-C bond. As the temperature continues to increase, the melting extends through the entire ligand backbone, intensifying the rotational motion of the ligands and causing the top and bottom ligands to swipe against each other. In addition, our study offers solid evidence supporting earlier experimental hypothesis regarding the in-plane shear deformation between adjacent organic layers at the onset of phase transition during the contraction of BA ligands. Intriguingly, our analysis unveiled another synchronized in-plane shear deformation within the inorganic layer. Such dual shearing effect results in notable octahedral distortions, which are characterized by the bond length distortion and bond angle variation of the octahedron.<br/><br/>In summary, our study bridges the gap between experimental observations and atomistic modeling and offers some unique understanding of the complex mechanisms underlying phase transition of 2D lead halide perovskites. Our findings pave the way for future research aiming to optimize the thermal stability and performance of these materials in various applications.