The Role of Thermal Fluctuations and Vibrational Entropy: A Theoretical Insight into the δ-to-α Transition of FAPbI 3

• Published in: The journal of physical chemistry letters Vol. 13; no. 13; pp. 3089 - 3095
• Main Authors: Yin, Junwen, Teobaldi, Gilberto, Liu, Li-Min
• Format: Journal Article
• Language: English
• Published: United States 07.04.2022
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.2c00454

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