Phase Transitions in 2D Halide Perovskites Using Machine Learned Potentials

Hybrid halide perovskites are a promising class of materials for various applications, including high-efficiency solar cells, lasers, and light-emitting diodes. One of the drawbacks of these bulk 3D materials is that they often exhibit relatively low stability. So-called two-dimensional (2D) halide perovskites, composed of a small number of perovskite layers stacked on top of each other and separated by organic cations that act as spacers, have been shown to exhibit much improved stability compared to their 3D counterparts.<br/><br/>Here, we demonstrate that the dimensionality of 2D halide perovskites and the choice of organic linker molecules can have a strong impact on phase transitions in these materials. This is investigated through large-scale molecular dynamics simulations using machine-learned potentials. We focus on the prototypical combination of the linker molecule phenethylammonium (PEA) with the perovskite methylammonium lead halide (MAPI), but also consider the organic linker molecules butylammonium (BA) and phenylmethylammonium (PMA). Phase transitions from a high-temperature phase without octahedral tilting to a lower temperature structure with a global out-of-phase octahedral tilting pattern are found. These phase transitions can be directly observed via thermodynamic and structural parameters such as heat capacities and octahedral tilt angles. We analyze the phase transition temperatures and characteristics with varying numbers of perovskite layers to understand how the transition properties change as a function of the system's dimensionality. For a larger number of perovskite layers, the 3D bulk MAPI phase transition temperature is recovered, whereas, for only a few perovskite layers, the phase transition temperature shifts up by about 50 K. Additionally, we observe surface effects, such as the surface layers (closest to the organic linker) exhibiting stronger octahedral tilting and undergoing phase transitions at higher temperatures (about 100 K) compared to the interior bulk layers.