Lead-free halide vacancy-ordered double perovskites ${\mathrm{Cs}}_{2}B{X}_6$ have recently attracted significant attention due to their intrinsically ultralow lattice thermal conductivity (${\kappa }_{\mathrm{L}}$), which is highly desirable for thermal insulation and thermoelectric applications. In this work, we systematically investigate the anharmonic lattice dynamics and thermal transport properties of ${\mathrm{Cs}}_{2}B{X}_6$ ($B$ = Zr, Pd, Sn, Te, Hf, and Pt; $X$ = Cl, Br, and I) using state-of-the-art first-principles calculations, based on a unified theory of thermal transport for crystals and glasses. All studied compounds are found to exhibit ultralow ${\kappa }_{\mathrm{L}}$ below 1.0 ${\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ at room temperature and large derivation from the conventional $T^{-1}$ temperature dependence. Our analysis combined with the machine-learning approach show that sound velocities (1100 – 1600 m ${\mathrm{s}}^{-1}$), which originate from the intrinsically weak chemical bonding, are strongly correlated with ${\kappa }_{\mathrm{L}}$ in these compounds. Furthermore, the influence of $B$- and $X$-site elements on phonon dispersion, anharmonicity, and scattering phase space is clarified. Our results provide microscopic insights into the origin of ultralow ${\kappa }_{\mathrm{L}}$ in ${\mathrm{Cs}}_{2}B{X}_6$ and offer guiding principles for the rational design of halide-based materials with tailored thermal transport properties.

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• Heat transfer
• Lattice dynamics
• Lattice thermal conductivity
• Crystal structures
• Semiconductors
• Boltzmann theory
• Density functional calculations