The lattice thermal conductivity (${\kappa }_L$) of materials is fundamentally influenced by atomic mass and chemical bonding. Conventional wisdom suggests that heavier elements typically lead to lower ${\kappa }_L$ due to reduced phonon group velocities. Here, we challenge this paradigm by revealing an inverted trend in the chalcohalides ${\mathrm{Ag}\mathrm{Bi}\mathrm{SCl}}_2$ and ${\mathrm{Cu}\mathrm{Bi}\mathrm{SCl}}_2$ through ab initio calculations incorporating four-phonon scattering. Despite the larger atomic mass of silver, ${\mathrm{Ag}\mathrm{Bi}\mathrm{SCl}}_2$ exhibits a higher ${\kappa }_L$ than its copper-based counterpart. We attribute this anomaly to the intensified $s$-$d$ orbital coupling between $\mathrm{Cu}$ $3d$ and $4s$ states. This coupling, permitted by symmetry and facilitated by their small energy separation, flattens the interatomic potential and enhances high-order scattering. In contrast, the larger energy difference between $\mathrm{Ag}$ $4d$ and $5s$ orbitals weakens this effect in ${\mathrm{Ag}\mathrm{Bi}\mathrm{SCl}}_2$, resulting in longer phonon lifetimes and higher ${\kappa }_L$. Our work underscores the critical role of cation-specific $s$-$d$ coupling in dictating lattice anharmonicity, reducing phonon group velocity and thermal transport, providing a design principle for discovering low-${\kappa }_L$ thermoelectric materials beyond conventional mass-dependent considerations.

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• Electronic structure
• Lattice thermal conductivity
• Phonons
• Energy materials
• Thermoelectrics
• Ab initio molecular dynamics
• Boltzmann theory
• Density functional calculations