From soft harmonic phonons to fast relaxational dynamics in CH\u2083NH\u2083PbBr\u2083

The lead-halide perovskites, including CH\u2083NH\u2083PbBr\u2083, 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\u2083NH\u2083PbBr\u2083 undergoes a series of structural transitions associated with orientational order of the CH\u2083NH\u2083 (methylammonium) molecular cation and tilting of the PbBr\u2083 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\u2083 framework and the quasistatic CH\u2083NH\u2083 dynamics at low energy transfers. At higher energy transfers, we observe a PbBr\u2086 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 3c50\u2013100 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.Peer reviewed: YesNRC publication: Ye