Materials with heavier constituents typically exhibit lower lattice thermal conductivity (${\kappa }_L$). Herein, we report an anomalous trend in orthorhombic $\mathrm{Ba}M{\mathrm{S}}_3$ ($M=\mathrm{Zr}$, Hf) chalcogenide perovskites, where heavier ${\mathrm{BaHfS}}_3$ shows a higher ${\kappa }_L$ than ${\mathrm{BaZrS}}_3$. Our first-principles approach, which incorporates temperature-dependent force constants, enables us to trace this anomaly to the competition between the mass effect and the lanthanide contraction effect. While the former reduces phonon group velocities in ${\mathrm{BaHfS}}_3$, the latter strengthens chemical bonding, suppresses lattice anharmonicity, and thus elongates phonon lifetimes. The resulting reduction in phonon-phonon scattering overcompensates the velocity reduction, leading to the enhanced ${\kappa }_L$ in ${\mathrm{BaHfS}}_3$. Our finding provides insights into the interplay of mass effect and lanthanide contraction effect of compounds with postlanthanide elements, which also guides the exploration of materials for thermoelectric and thermal barrier coating applications.

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• Lattice thermal conductivity
• Phonons
• Perovskites
• Density functional calculations