The Role of Thermal Fluctuations and Vibrational Entropy: ATheoretical Insight into theδ-to-αTransition of FAPbI3

Formamidinium lead iodide as a typical organometal perovskite has attractedconsiderable interest due to its suitable electronic structure. However, the intrinsic mechanismsof its unwanted delta-to-alpha phase transition remain elusive. By combinedfirst-principlescalculations, lattice dynamics analysis, and molecular dynamics simulations, we assign the alpha phase to the highly dynamic tetragonal phase, with the high-symmetry cubic structure emergingas a dynamically unstable maximum in the system's potential energy landscape. Finite-temperature Gibbs free energy calculations confirm that the delta-to-alpha transition should beconsidered as a hexagonal-to-tetragonal transition in contrast to the previous hexagonal-to-cubic assignment. More importantly, phonon thermal property calculations indicate that thedriving force of the process is the vibrational entropy difference. These results point out thedynamical nature of the alpha phase and the key role of the vibrational entropy in perovskite-related phase transitions, the harnessing of which is critical for the successful uptake oforganometal perovskites in commercial applications.