Conventional Peierls theory cannot fully capture the thermal transport in strongly anharmonic compounds (e.g., lead-free halide double perovskites) because it does not account for the significant wavelike behavior of phonons—a consequence of their inherent capability to interfere and tunnel. Here, we elucidate the mechanism behind the ultralow lattice thermal conductivity in model systems ${\mathrm{Cs}}_2{\mathrm{AgInCl}}_6$ and ${\mathrm{Cs}}_2{\mathrm{AgInBr}}_6$ through a combined evaluation of the particlelike and wavelike thermal transport channels. The rattling vibrations of Cs atoms suppress the particlelike channel through enhanced anharmonicity and promote the wavelike channel by flattening phonon branches to enhance coherence. Furthermore, the $T_{1g}$ rotational mode of the ${\mathrm{InX}}_6$ octahedron induces stronger quartic anharmonicity in ${\mathrm{Cs}}_2{\mathrm{AgInBr}}_6$ than in ${\mathrm{Cs}}_2{\mathrm{AgInCl}}_6$, which primarily enhances four-phonon scattering processes to further suppress the particlelike channel. In contrast, the wavelike contributions in both compounds remain comparable due to a compensating effect between the stronger coherent coupling in ${\mathrm{Cs}}_2{\mathrm{AgInBr}}_6$ and the larger group velocities in ${\mathrm{Cs}}_2{\mathrm{AgInCl}}_6$. Overall, the dominance of the wavelike channel is more pronounced in ${\mathrm{Cs}}_2{\mathrm{AgInBr}}_6$. Finally, the dominant wavelike contribution leads to a markedly weaker temperature dependence of the total thermal conductivity in both compounds, which is more notable in ${\mathrm{Cs}}_2{\mathrm{AgInBr}}_6$ with a scaling exponent of $T^{-0.19}$ compared to $T^{-0.52}$ in ${\mathrm{Cs}}_2{\mathrm{AgInCl}}_6$. Our work establishes the wavelike channel as a vital mechanism for thermal transport in lead-free halide double perovskites, providing novel insights into their fundamental thermal properties.

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• Anharmonic lattice dynamics
• Lattice thermal conductivity
• Thermal transport
• Perovskites
• Density functional calculations