Origin of the anomalous Pb-Br bond dynamics in formamidinium lead bromide perovskites

Extended x-ray absorption fine structure spectroscopy of the light-harvesting formamidinium lead bromide $({\mathrm{FAPbBr}}_{3})$ perovskite, a system with attractive optoelectronic performance, shows anomalously large variance in Pb-Br bond length, some 50% larger than in its inorganic ${\mathrm{CsPbBr}}_{3}$ counterpart. Using first-principles molecular dynamics simulations, we find a significant contribution to this variance coming from the FA cation, and show that the FA does not just tumble in its cuboctahedral ${\mathrm{Br}}_{12}$ cage, but instead stochastically sticks to, and detaches from one of the 12 nearest Br atoms after another, leading to the large variance in Pb-Br bond length. Our results demonstrate dynamic coupling between the FA-Br moiety and perovskite cage vibrations, and that tunability in dynamics can be achieved by changing the cation type and perovskite lattice parameter. Thus, our results provide information that needs to be considered in any of the intensely debated models of electron-phonon coupling in lead halide perovskites.