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Influence of the Halide Ion on the A Site Dynamics in FAPbX3 (X = Br and Cl)

The optoelectronic properties and ultimately photovoltaic performance of hybrid lead halide perovskites, is inherently related to the dynamics of the organic cations. Here we report on the dynamics of the formamidinium (FA) cation in FAPbX3 perovskites for chloride and bromide varieties, as studied by neutron spectroscopy. Elastic fixed window scan measurements showed the onset of reorientational motion of FA cations in FAPbCl3 to occur at a considerably higher temperature compared to that in FAPbBr3. In addition, we observed two distinct dynamical transitions only in the chloride system, suggesting a significant variation in the reorientational motions of the FA cation with temperature. Quasielastic neutron scattering data analysis of FAPbCl3 showed that in the low temperature orthorhombic phase, FA cations undergo 2-fold jump reorientations about the C-H axis which evolve into an isotropic rotation in the intermediate tetragonal and high temperature cubic phases. Comparing the results with those from FAPbBr3, reveal that the time scale, barrier to reorientation and the geometry of reorientational motion of the FA cation are significantly different for the two halides. We note that this dependence of the dynamic properties of the A-site cation on the halide, is unique to the FA series; the geometry of methylammonium (MA) cation dynamics in MAPbX3 is known to be insensitive to different halides.