The lead-halide perovskites, including CH3βNH3βPbBr3β, 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.
CH3βNH3βPbBr3β undergoes a series of structural transitions
associated with orientational order of the CH3βNH3β (MA) molecular
cation and tilting of the PbBr3β 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 PbBr3β framework and
the quasistatic CH3βNH3β dynamics at low energy transfers. At higher
energy transfers, we observe a PbBr6β 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 βΌ 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.Comment: (main text - 5 pages, 4 figures; supplementary information - 3 pages,
3 figures