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In this work, we investigate the microscopic mechanisms of anharmonic latticedynamics and thermal transport in lead-free halide double perovskite Cs2AgBiBr6from first principles. We combine self-consistent phonon calculations withbubble diagram correction and a unified theory of lattice thermal transportthat considers both the particle-like phonon propagation and wave-liketunnelling of phonons. An ultra-low thermal conductivity at room temperature(~0.21 Wm-1K-1) is predicted with weak temperature dependence(~T-0.45), insharp contrast to the conventional ~T-1 dependence. Particularly, thevibrational properties of Cs2AgBiBr6 are featured by strong anharmonicity andwave-like tunnelling of phonons. Anharmonic phonon renormalization from boththe cubic and quartic anharmonicities are found essential in preciselypredicting the phase transition temperature in Cs2AgBiBr6 while the negativephonon energy shifts induced by cubic anharmonicity has a significant influenceon particle-like phonon propagation. Further, the contribution of the wave-liketunnelling to the total thermal conductivity surpasses that of theparticle-like propagation above around 340 K, indicating the breakdown of thephonon gas picture conventionally used in the Peierls-Boltzmann TransportEquation. Importantly, further including four-phonon scatterings is required inachieving the dominance of wave-like tunnelling, as compared to the dominantparticle-like propagation channel when considering only three-phononscatterings. Our work highlights the importance of lattice anharmonicity andwave-like tunnelling of phonons in the thermal transport in lead-free halidedouble perovskites.

Unravelling Ultralow Thermal Conductivity in Double Perovskite  Cs2AgBiBr6: Dominant Wave-like Phonon Tunnelling, Strong Quartic  Anharmonicity and Lattice Instability