Intrinsic ultralow lattice thermal conductivity in lead-free halide perovskites Cs3Bi2X9 (X = Br, I)

Abstract

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_Ls of two typical vacancy-ordered halide perovskites, denoted with the general formula Cs₃Bi₂X₉ (X = Br, I). Ultralow κ_Ls of 0.401 and 0.262 W mK⁻¹ at 300 K are predicted for Cs₃Bi₂Br₉ and Cs₃Bi₂I₉, respectively. Our findings reveal that the ultralow κ_Ls 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 Cs₃Bi₂Br₉, the face-sharing feature and weaker bond strength in Cs₃Bi₂I₉ lead to a softer elasticity modulus and stronger anharmonicity. Additionally, we demonstrate the presence of wave-like κ_C in Cs₃Bi₂X₉ 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.