Intrinsic ultralow lattice thermal conductivity in lead-free halide perovskites Cs 3 Bi 2 X 9 (X = Br, I).

Lead-free halide perovskites have recently garnered significant attention due to their rich structural diversity and exceptionally ultralow lattice thermal conductivity ( κ L ). 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 κ L s of two typical vacancy-ordered halide perovskites, denoted with the general formula Cs 3 Bi 2 X 9 (X = Br, I). Ultralow κ L s of 0.401 and 0.262 W mK -1 at 300 K are predicted for Cs 3 Bi 2 Br 9 and Cs 3 Bi 2 I 9 , respectively. Our findings reveal that the ultralow κ L 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 Cs 3 Bi 2 Br 9 , the face-sharing feature and weaker bond strength in Cs 3 Bi 2 I 9 lead to a softer elasticity modulus and stronger anharmonicity. Additionally, we demonstrate the presence of wave-like κ C in Cs 3 Bi 2 X 9 by evaluating the coherent contribution. Our work provides the physical microscopic mechanisms of the wave-like κ C in two typical lead-free halide perovskites, which are beneficial to designing intrinsic materials with the feature of ultralow κ L .