Temperature dependence of overdamped 2D fluctuations in metal halide perovskites

All inorganic perovskites has been attracting great attentions because of their promising optoelectronic properties, ultralow thermal conductivity for both photovoltaic and thermalelectric applications. Zone-boundary soft modes that associated with the octahedral rotations has been found to be highly anharmonic and closely related to the phase transitions driven by structural distortion. It remains unclear whether these overdamped modes behave similarly with the soft modes found in oxide perovksites that will harden on warming above phase transition temperature, and how the competition between M and R distortions evolves away from the phase transitions. The different ground state structure of Sn-based and Pb-based halide perovskites may also result in different temperature dependence of the anharmonic phonon modes in cubic phase. We propose to measure CsPbBr3 and CsSnBr3 in high temperature range to reslove these questions regarding their unusual atomic dynamics, which is crucial for further optimization of their optical and thermal properties.