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arXiv · 1510.01834

From soft harmonic phonons to fast relaxational dynamics in CH$_{3}$NH$_{3}$PbBr$_{3}$

I. P. Swainson·C. Stock·S. F. Parker·L. Van Eijck·M. Russina·J. W. Taylor
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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.

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I. P. Swainson, C. Stock, S. F. Parker, L. Van Eijck, M. Russina, J. W. Taylor. 2015-10-07. From soft harmonic phonons to fast relaxational dynamics in CH$_{3}$NH$_{3}$PbBr$_{3}$. https://doi.org/10.1103/physrevb.92.100303

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