Role of high-order lattice anharmonicity in the phonon thermal transport of silver halide AgX (X=Cl,Br,I)

The phonon transport mechanisms and ultralow lattice thermal conductivities (𝜅L) in silver halide Ag⁢𝑋 (𝑋=Cl,Br,I) compounds are not yet well understood. Herein, we study the lattice dynamics and thermal property of Ag⁢𝑋 under the framework of perturbation theory and the two-channel Wigner thermal transport model based on accurate machine learning potentials. We find that an accurate extraction of the third-order atomic force constants from largely displaced configurations is significant for the calculation of the 𝜅L of Ag⁢𝑋, and the coherence thermal transport is also non-negligible. In AgI, however, the calculated 𝜅L still considerably overestimates the experimental values even including four-phonon scatterings. Molecular dynamics (MD) simulations using machine learning potential suggest an important role of the higher-than-fourth-order lattice anharmonicity in the low-frequency phonon linewidths of AgI at room temperature, which can be related to the simultaneous restrictions of the three- and four-phonon phase spaces. The 𝜅L of AgI calculated using MD phonon lifetimes including full-order lattice anharmonicity shows a better agreement with experiments.