  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.

Materials Science