  Lead-halide organic-inorganic perovskites consist of an inorganic host
framework with an organic molecule occupying the interstitial space. The
structure and dynamics of these materials have been heavily studied recently
due to interest in their exceptional photovoltaic properties. We combine
inelastic neutron scattering, Raman spectroscopy, and quasielastic neutron
scattering to study the temperature dependent dynamics of the molecular cation
in CH3NH3PbBr3. By applying high resolution quasielastic neutron scattering, we
confirm the [CH3NH3]+ ions are static in the low temperature orthorhombic phase
yet become dynamic above 150 K where a series of structural transitions occur.
This molecular melting is accompanied by a temporal broadening in the
intra-molecular modes probed through high energy inelastic spectroscopy.
Simultaneous Raman measurements, a strictly |Q|=0 probe, are suggestive that
this broadening is due to local variations in the crystal field environment
around the hydrogen atoms. These results confirm the strong role of hydrogen
bonding and also a coupling between molecular and framework dynamics.
