Four-Phonon Scattering and Wave-Like Phonon Tunneling Drive Glassy Ultralow Lattice Thermal Conductivity in Cs2AgInCl6

RR Ma and XX Zhang and SQ Guo and M Ge and JF Zhang and JJ Ma, CHINESE PHYSICS LETTERS, 42, 090709 (2025).

DOI: 10.1088/0256-307X/42/9/090709

Lead-free halide double perovskites (HDPs) provide a promising platform for high-performance thermoelectric due to their intrinsically ultralow lattice thermal conductivity kappa l. In this study, we comprehensively investigate the lattice dynamics of Cs2AgInCl6 using first-principles calculations. By explicitly incorporating four-phonon scattering and wave-like phonon tunneling, we predict a kappa l of 0.52 W & sdot;m-1 & sdot;K-1 with a remarkably weak temperature dependence (kappa l proportional to T-0.31), confirming the intrinsically glass-like ultralow kappa l in Cs2AgInCl6. Further analyses reveal that hierarchical chemical bonds, loosely bonded rattling atoms and a mixed crystalline-liquid state collectively induce strong anharmonicity manifested in flat phonon modes. These factors dominate the glass-like thermal transport component of kappa l. This work uncovers the underlying mechanisms governing the unusual thermal transport properties in lead-free HDPs and offers guiding principles for designing novel energy conversion technologies.

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