Origin of low lattice thermal conductivity and mobility of lead-free halide double perovskites

Lead-free halide double perovskites of Cs2BiAgBr6 and Cs2BiAgCl6 have attracted extensive attention due to potential application in optoelectronic field. The transport behavior plays an important role in electronic devices based on semiconductors, which includes lattice thermal conductivity and carier mobility. Since impurities and defects may exist in the samples, it is difficult to investigate the intrinsic transport behavior by experimental measurements. The theoretical prediction of their transport behaviors can provide a guidance for the application of materials. In this work, we first develop machine learning interatomic potential (MLIP) to explore their lattice thermal conductivities. They exhibit extremely low lattice thermal conductivity at 300 K with the values of xx = 0.122 & PLUSMN; 0.009/xz = 0.103 & PLUSMN; 0.004 Wm-1K-1 for Cs2BiAgBr6, and xx = 0.352 & PLUSMN; 0.014/xz = 0.337 & PLUSMN; 0.012 Wm-1K-1 for Cs2BiAgCl6, respectively. It is revealed that the thermal conductivity is mainly controlled by low-frequency phonons. Subsequently, we apply semi-empirical models to predict their mobilities. The mobility is mainly determined by the scattering from longitudinal optical phonon with the values of /4e,x= 22.0//4h,x= 18.8cm2V-1s-1 for Cs2BiAgBr6, and /4e,x= 19.2/ /4h,x= 19.5cm2V-1s-1 for Cs2BiAgCl6, respectively. The stretching between Bi and Br/Cl atoms limits the mobility. Finally, the origin of low thermal conductivity and mobility on double perovskites is revealed.