Anomalously mass-dependent thermal conductivity in TM₂SiCO₂ (TM = Ti, Zr, and Hf)

Atomic mass is traditionally considered an important factor governing lattice thermal conductivity, with heavier atoms generally leading to reduced phonon group velocities and lower thermal conductivity. In this work, we report an anomalous mass dependence of thermal transport in two-dimensional TM₂SiCO₂ (TM = Ti, Zr, Hf) using first-principles calculations combined with solutions of the phonon Boltzmann transport equation including both three-phonon and four-phonon scattering processes. Contrary to conventional expectations, the thermal conductivity increases anomalously with increasing transition metal atomic mass, with Zr₂SiCO₂ exhibiting the lowest thermal conductivity and Hf₂SiCO₂ the highest. Detailed analysis reveals that although acoustic phonon group velocities decrease monotonically with increasing atomic mass, Hf₂SiCO₂ exhibits significantly reduced scattering phase space and suppressed scattering rates between acoustic phonons and low-frequency optical phonons. This reduction leads to prolonged acoustic phonon lifetimes, which outweigh the adverse effect of reduced group velocities. Our results demonstrate that phonon scattering phase space and lattice anharmonicity can dominate over atomic mass effects in determining thermal conductivity, providing new insights into the design of materials with tailored thermal transport properties.