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

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This study investigates the lattice dynamics and thermal properties of silver halide AgX (X = Cl, Br, I) compounds using machine learning potentials. The accurate extraction of third-order atomic force constants and the consideration of coherent thermal transport are found to be important for the calculation of the low thermal conductivity KL of AgX. However, even with the inclusion of four-phonon scattering, the calculated KL for AgI still overestimates the experimental values significantly. Molecular dynamics simulations using machine learning potential suggest that the higher-than-fourth-order lattice anharmonicity plays a crucial role in the low-frequency phonon linewidths of AgI at room temperature, which relates to the limitations of three- and four-phonon phase spaces. The KL of AgI calculated using MD phonon lifetimes including full-order lattice anharmonicity shows better agreement with experiments.