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 (${\ensuremath{\kappa}}_{\mathrm{L}}$) in silver halide $\mathrm{Ag}X$ ($X=\mathrm{Cl},\mathrm{Br},\mathrm{I}$) compounds are not yet well understood. Herein, we study the lattice dynamics and thermal property of $\mathrm{Ag}X$ 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 ${\ensuremath{\kappa}}_{\mathrm{L}}$ of $\mathrm{Ag}X$, and the coherence thermal transport is also non-negligible. In AgI, however, the calculated ${\ensuremath{\kappa}}_{\mathrm{L}}$ 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 ${\ensuremath{\kappa}}_{\mathrm{L}}$ of AgI calculated using MD phonon lifetimes including full-order lattice anharmonicity shows a better agreement with experiments.