Octahedral Dynamics and Local Symmetry in Hybrid Perovskite FAPbI₃ under Thermal Excitation

Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations have been employed to investigate the evolution of local motifs within the tetragonal phase of FAPbI₃ under thermal excitation. Our results reveal a distinct broadening in the distribution of PbI₆ octahedral volumes with increasing temperature, indicating a gradual breakdown of symmetry and emergence of diverse local environments. These octahedral volume distortions are primarily driven by the dynamic behavior of the FA cation, leading to softening of PbI₆ octahedra, evident from the calculated mean octahedral volume and Pb-I-Pb bond angles. The examination of the electronic structure confirmed that this dynamic structural phenomenon is directly responsible for the change in fundamental band gap value, highlighting the role of PbI₆ octahedra in modifying and modulating the electronic properties in FAPbI₃. The results demonstrate the microscopic origin of thermally induced dynamical behavior to the macroscopic electronic properties and underscore the pivotal role of local motifs in hybrid perovskites.