The Order-Disorder Transition in CH₃NH₃PbBr₃

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[1,2]. 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, suggest that this broadening is due to local variations in the crystal field environment around the hydrogen atoms.

[1] L. M. Herz, Annual Review of Physical Chemistry 67, 65 (2016).
[2] B. Saparov and D. B. Mitzi, Chem. Rev. 116, 4558 (2016).