Phonon thermal transport in M Te X 4 ( M = Zr, Hf; X = S, Se) monolayers: Role of antibonding states and higher-order phonon anharmonicity

Exploring materials with low lattice thermal conductivity is critical for advancing thermal barrier coatings and thermoelectrics. In this work, we reveal the significant influence of antibonding states and higher-order anharmonicity on the thermal transport properties of $M\mathrm{Te}{X}_{4}$ ($M=\mathrm{Zr}$, Hf; $X=\mathrm{S}$, Se) monolayers. Our results indicate that the valence bands close to the Fermi level in the Te-$X$ bonds exhibit significant antibonding states, which weaken the bonding strength and induce soft optical phonon modes in $M\mathrm{Te}{X}_{4}$. The softened bonding and phonon modes significantly enhance the phonon anharmonic scattering rates, thereby dramatically reducing the thermal conductivity of $M\mathrm{Te}{X}_{4}$. Meanwhile, the in-plane vibrational modes with quartic potential wells lead to strong four-phonon scattering, further limiting the thermal transport performance in these materials. Moreover, it is found that the abnormal increase in bond energy within the Hf system gives rise to an unusual mass dependence of thermal conductivity, that is, the thermal conductivity of the heavy atomic system $\mathrm{Hf}\mathrm{Te}{X}_{4}$ exceeds that of the light atomic system $\mathrm{Zr}\mathrm{Te}{X}_{4}$. Our results highlight the critical role of antibonding states and higher-order anharmonicity in suppressing thermal transport, and could provide fundamental insights and theoretical guidance for exploring and designing materials with low thermal conductivity.