From soft harmonic phonons to fast relaxational dynamics in CH${}_3$NH${}_3$PbBr${}_3$

Abstract

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.

(main text - 5 pages, 4 figures; supplementary information - 3 pages, 3 figures )