Revealing the Low-Temperature Phase of FAPbI₃ Using a Machine-Learned Potential

Formamidinium lead iodide (FAPbI₃) is a material of interest for its potential in solar cell applications, driven by its remarkable optoelectronic properties. However, the low-temperature phase of FAPbI₃ remains poorly understood, with open questions surrounding its crystal structure, octahedral tilting, and arrangement of formamidinium (FA) cations. Using our trained machine-learned potential in combination with large-scale molecular dynamics (MD) simulations, we provide a detailed investigation of this phase, uncovering its structural characteristics and dynamical behavior. Our analysis reveals the octahedral tilt pattern and sheds light on the rotational dynamics of FA cations in the low-temperature phase. Strikingly, we find that the FA cations become frozen in a metastable configuration, unable to reach the thermodynamic ground state. By comparing our simulated results with experimental nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) spectra, we demonstrate good agreement, further validating our findings. This phenomenon mirrors experimental observations and offers a compelling explanation for the experimental challenges in accessing the true ground state. These findings provide critical insights into the fundamental physics of FAPbI₃ and its low-temperature behavior, advancing our understanding of this important material.