Understanding the lattice dynamics and heat transport physics in the lead-free halide double perovskites remains an outstanding challenge due to their lattice dynamical instability and strong anharmonicity. In this work, we investigate the microscopic mechanisms of anharmonic lattice dynamics and thermal transport in lead-free halide double perovskite Cs₂AgBiBr₆ from first principles. We combine self-consistent phonon calculations with bubble diagram correction and a unified theory of lattice thermal transport that considers both the particle-like phonon propagation and wave-like tunnelling of phonons. An ultra-low thermal conductivity at room temperature (~0.21 Wm⁻¹K⁻¹) is predicted with weak temperature dependence( ~ T^−0.34), in sharp contrast to the conventional ~T⁻¹ dependence. Particularly, the vibrational properties of Cs₂AgBiBr₆ are featured by strong anharmonicity and wave-like tunnelling of phonons. Anharmonic phonon renormalization from both the cubic and quartic anharmonicities are found essential in precisely predicting the phase transition temperature in Cs₂AgBiBr₆ while the negative phonon energy shifts induced by cubic anharmonicity has a significant influence on particle-like phonon propagation. Further, the contribution of the wave-like tunnelling to the total thermal conductivity surpasses that of the particle-like propagation above around 310 K, indicating the breakdown of the phonon gas picture conventionally used in the Peierls-Boltzmann Transport Equation. Importantly, further including four-phonon scatterings is required in achieving the dominance of wave-like tunnelling, as compared to the dominant particle-like propagation channel when considering only three-phonon scatterings. Our work highlights the importance of lattice anharmonicity and wave-like tunnelling of phonons in the thermal transport in lead-free halide double perovskites.

由于其晶格动力学不稳定性和强非谐性，了解无铅卤化物双钙钛矿的晶格动力学和热传输物理仍然是一个突出的挑战。_{在这项工作中，我们从第一原理研究了无铅卤化物双钙钛矿Cs 2} AgBiBr ₆中非谐晶格动力学和热传输的微观机制。我们将自洽声子计算与气泡图校正和统一的晶格热传输理论结合起来，该理论考虑了类粒子声子传播和声子类波隧道效应。预测室温下的超低导热系数（~0.21 Wm ^-1 K ^-1）具有弱温度依赖性（~  T ^-0.34），与传统的~ T ^-1依赖性形成鲜明对比。_{特别是，Cs 2} AgBiBr ₆的振动特性具有强非谐性和声子波状隧道效应。三次和四次非谐性的非谐声子重正化对于精确预测 Cs ₂ AgBiBr ₆中的相变温度至关重要，而三次非谐性引起的负声子能量变化对类粒子声子传播具有显着影响。此外，在 310 K 左右以上，波状隧道效应对总热导率的贡献超过了粒子状传播，这表明佩尔斯-玻尔兹曼输运方程中传统使用的声子气体图被破坏。重要的是，与仅考虑三声子散射时的主要粒子状传播通道相比，需要进一步包括四声子散射才能实现波状隧道效应的主导地位。我们的工作强调了晶格非谐性和声子的波状隧道效应在无铅卤化物双钙钛矿热传输中的重要性。