Intrinsic ultralow lattice thermal conductivity in lead-free halide perovskites CsBiX (X = Br, I).

Lead-free halide perovskites have recently garnered significant attention due to their rich structural diversity and exceptionally ultralow lattice thermal conductivity (). Here, we employ first-principles calculations in conjunction with self-consistent phonon theory and Boltzmann transport equations to investigate the crystal structure, electronic structure, mechanical properties, and s of two typical vacancy-ordered halide perovskites, denoted with the general formula CsBiX (X = Br, I). Ultralow s of 0.401 and 0.262 W mK at 300 K are predicted for CsBiBr and CsBiI, respectively. Our findings reveal that the ultralow s are mainly associated with the Cs rattling-like motion, vibrations of halide polyhedral frameworks, and strong scattering in the acoustic and low-frequency optical phonon branches. The structural analysis indicates that these phonon dynamic properties are closely relevant to the bonding hierarchy. The presence of the extended Bi-X antibonding states at the valence band maximum contributes to the soft elastic lattice and low phonon group velocities. Compared to CsBiBr, the face-sharing feature and weaker bond strength in CsBiI lead to a softer elasticity modulus and stronger anharmonicity. Additionally, we demonstrate the presence of wave-like in CsBiX by evaluating the coherent contribution. Our work provides the physical microscopic mechanisms of the wave-like in two typical lead-free halide perovskites, which are beneficial to designing intrinsic materials with the feature of ultralow .