Lattice dynamics of CH(NH)PbI and CH(NH)SnI investigated by inelastic X-ray scattering and comparison of their elastic properties

Tamatsukuri, Hiromu   ; Murakami, Yoichi*; Saito, Noriko*; Ohashi, Naoki*; Tsutsui, Satoshi*

Using the inelastic X-ray scattering (IXS) technique, we investigate the lattice dynamics of inorganic-organic hybrid iodide perovskites FAPbI and FASnI (FA CH(NH), formamidinium) in their cubic phase and tetragonal phase. We find that the IXS spectra of FASnI are highly broad at all momentum points, as has been widely observed in the Pb-based halide perovskites. The speeds of sound (phonon group velocities) and absolute values of elastic constants of FASnI are smaller than those of FAPbI in both phases. In addition, no significant difference in the phonon lifetimes of FAPbI and FASnI is observed. These results correlate better with the reported tendency that the lattice thermal conductivity of ASnX is lower than that of APbX. We discuss that our results also favor the up-conversion mechanism by acoustic phonons in the phonon bottleneck effect for the long lifetime of hot carriers.