Temperature-Induced Phase Transition in 2D Alkylammonium Lead Halide Perovskites: A Molecular Dynamics Study

Molecular dynamics simulations are utilized to unravel the temperature-driven phase transition in double-layered butylammonium (BA) methylammonium (MA) lead halide perovskite (BA)$_{2}$(MA)Pb$_{2}$I$_{7}$, which holds great promise in a wide range of optoelectronics and sensor applications. The simulations successfully capture the structural transition from low to high symmetry phases with rising temperatures, consistent with experimental observations. The phase transition initiates at two critical interfaces: the first is between the inorganic and organic layers, where the melting of N-H bonds in BA leads to significant reduction in hydrogen bonding between BA and iodides, and the second is at the interface between top and bottom organic layers, where the melting of the tail bonds in BA triggers the phase transition. Following this, BA cations exhibit a patterned and synchronized motion reminiscent of a conical pendulum, displaying a mix of ordered and disordered behaviors, prior to evolving into a completely molten and disordered state. While the melting of BA cations is the primary driver of the phase transition, the rotational dynamics of MA cations also plays a critical role in determining the phase transition temperature, influenced by the BA-MA interaction. Such interaction alters the polarization patterns of MA cations across the phase transition. In particular, an anti-parallel polarization pattern is observed in low temperature phase. Additionally, displacive elements of the phase transition are identified in the simulations, characterized by the shear and distortion of the inorganic octahedra. Notably, at lower temperatures, the octahedral distortion follows a bimodal distribution, reflecting significant variations in distortion among octahedra. This variation is attributed to an anisotropic hydrogen bonding network between iodides and BA cations. Our study reveals new phenomena and mechanisms extending beyond the order-disorder transition mechanism, shedding new light on potential phase engineering through strategic tuning of organic and inorganic components.