Experimental Phonon Dispersion and Lifetimes of Tetragonal CH3NH3PbI3 Perovskite Crystals.

Hybrid organic-inorganic perovskites were reported to have ultralow thermal conductivity in recent studies. In this Letter, we report the first experimental phonon dispersion and lifetimes of tetragonal CH3NH3PbI3 single crystals at both 200 and 300 K by high-energy resolution inelastic X-ray scattering, which enables a thorough understanding of the underlying mechanisms for the ultralow thermal conductivity. Notably, we observed unusual and significant phonon dips along the [100] and [110] directions at both temperatures. The ultralow thermal conductivity can be attributed to small group velocities due to ultrasoft acoustic modes and short phonon lifetimes originating from the strong acoustic-optical coupling. We further provided the structural origins for these peculiar phonon features. Moreover, our results and interpretation are consistent with the reported temperature-dependent trend for thermal conductivity of CH3NH3PbI3. Our work offers critical guidelines for accelerating the design and discovery of novel hybrid materials for energy applications including photovoltaics and thermoelectrics.