In this study, we constructed an efficient framework based on phonon self-consistent theory and unified transport theory combined with machine learning potential, and systematically reveal some interesting lattice dynamics in simple transition metal halide crystals ${\mathrm{CaXF}}_6$ (X = Ti, Zr, Hf) with cubic structures. Although ${\mathrm{CaTiF}}_6$ exhibits a lighter average atomic mass, its lattice thermal conductivity is lower than that of ${\mathrm{CaZrF}}_6$ and ${\mathrm{CaHfF}}_6$. Such behavior is in contrast to the conventional Keyes theory that typically associates light atomic systems with higher thermal conductivity. Further analysis indicates that this deviation from the expected mass-dependent behavior stems from the stronger anharmonic scattering induced by the weaker $s\text{−}d$ orbital bonding in the Ti-F bond. Notably, the rotating mode vibrations in the soft phonon dispersion curves of ${\mathrm{CaTiF}}_6$ exhibit a double-potential-well, and lead to more pronounced quartic residuals (quartic anharmonicity). As a result, the strong four-phonon scattering reduces the room-temperature thermal conductivity of ${\mathrm{CaTiF}}_6$ by 59%, a suppression markedly stronger than in the Zr and Hf systems. Moreover, the stronger anharmonic scattering (including three- and four-phonon processes) induces substantial coherent phonons in ${\mathrm{CaTiF}}_6$, resulting in the wave-like coherent thermal conductivity even exceeding the particle-like propagate thermal conductivity at high temperatures $(>700 \mathrm{K})$. Our results not only deepen the fundamental understanding of the lattice dynamics in transition metal halides but also provide new perspectives for designing thermal management devices with desirable thermal transport properties.

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• Anharmonic lattice dynamics
• Chemical bonding
• Lattice thermal conductivity
• Phonons
• Thermal expansion
• First-principles calculations
• Machine learning