Phase transition model of ferroelectric 2D hybrid perovskites

Abstract

Understanding structural phase transitions and cation dynamics in low-dimensional hybrid lead halide perovskites is essential for optimizing their photovoltaic and ferroelectric properties. Here, we propose a model describing the ordering of MA and BA cations in the two-dimensional ferroelectric hybrid perovskites BA₂MAPb₂X₇ (X = I, Br). Our model is based on the available structural data and involves short-range framework-mediated interactions between molecular cations. The effective model Hamiltonian is solved using Monte Carlo simulations, which allow us to explore the interactions between the molecular cations reproducing the structural phase transitions and long-range molecular cation order in both perovskites. Our results reveal the appearance of spontaneous electric polarization at the phase transition confirming the experimentally observed ferroelectricity of BA₂MAPb₂Br₇. We discuss the broader applicability of our model for investigating other two-dimensional Ruddlesden–Popper perovskites incorporating different molecular cations.