From soft harmonic phonons to fast relaxational dynamics in CH3NH3PbBr3

The lead-halide perovskites, including ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbBr}}_{3}$, 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. ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbBr}}_{3}$ undergoes a series of structural transitions associated with orientational order of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$ (methylammonium) molecular cation and tilting of the ${\mathrm{PbBr}}_{3}$ 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 ${\mathrm{PbBr}}_{3}$ framework and the quasistatic ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$ dynamics at low energy transfers. At higher energy transfers, we observe a ${\mathrm{PbBr}}_{6}$ 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 $\ensuremath{\sim}50\text{--}100$ 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.