From soft harmonic phonons to fast relaxational dynamics in CH$_{3}$NH$_{3}$PbBr$_{3}$

Abstract: The lead-halide perovskites, including CH${3}$NH${3}$PbBr${3}$, are components in cost effective, highly efficient photovoltaics, where the interactions of the molecular cations with the inorganic framework are suggested to influence the electronic and ferroelectric properties. CH${3}$NH${3}$PbBr${3}$ undergoes a series of structural transitions associated with orientational order of the CH${3}$NH${3}$ (MA) molecular cation and tilting of the PbBr${3}$ host framework. We apply high-resolution neutron scattering to study the soft harmonic phonons associated with these transitions, and find a strong coupling between the PbBr${3}$ framework and the quasistatic CH${3}$NH${3}$ dynamics at low energy transfers. At higher energy transfers, we observe a PbBr$_{6}$ octahedra soft mode driving a transition at 150 K from bound molecular excitations at low temperatures to relatively fast relaxational excitations that extend up to $\sim$ 50-100 meV. We suggest that these temporally overdamped dynamics enables possible indirect band gap processes in these materials that are related to the enhanced photovoltaic properties.