The role of thermal fluctuations and vibrational entropy for the delta-to-alpha transition in hybrid organic-inorganic perovskites: the FAPbI3 case

FAPbI3, as a typical hybrid organic-inorganic perovskite, has attracted considerable interest due to its band gap suitable for visible light absorption and good thermal stability. A barrier to the use of FAPbI3 in commercial, stable devices is its unwanted black-to-yellow (non-perovskite to perovskite, commonly known as delta-to-alpha) phase transition at around 300 K. The intrinsic mechanisms of such phase transition are far from clear, being the detailed structural description for the alpha-phase still missing. By combined Density Functional Theory (DFT) calculations, lattice dynamics analysis and DFT molecular dynamics simulations, we assign the alpha-phase to the highly dynamic tetragonal phase, with the high-symmetry cubic structure emerging as a dynamically unstable maximum in the system potential energy landscape. We demonstrate computationally that the diffraction-observed cubic structure is the result of the averaging of different tetragonal distortions sampled in the experimental detection time scale as a result of the enhanced FA dynamics, instead of a static system of cubic symmetry. Further finite-temperature Gibbs free energy calculations confirm that the delta-to-alpha transition should be considered as a hexagonal-to-tetragonal transition in contrast to the previous hexagonal-to-cubic assignment. More importantly, the simulations indicate that the driving force of the process is the vibrational entropy difference rather than the rotational entropy as previously proposed. These results point out the dynamical nature of the alpha-phase, the importance of the overlooked tetragonal structure, and the key role of the vibrational entropy in perovskite-related phase transitions, the harnessing of which is critical for successful uptake of ABX3 hybrid organic-inorganic perovskites in commercial applications.