Abstract
The lead-halide perovskites, including , 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. undergoes a series of structural transitions associated with orientational order of the (methylammonium) molecular cation and tilting of the 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 framework and the quasistatic dynamics at low energy transfers. At higher energy transfers, we observe a 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 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.
- Received 10 May 2015
DOI:https://doi.org/10.1103/PhysRevB.92.100303
©2015 American Physical Society