Positive temperature-dependent thermal conductivity induced by wavelike phonons in complex Ag-based argyrodites

The phonon transport mechanisms and the anomalous temperature-dependent lattice thermal conductivities (kL) in Ag-based argyrodites have not been fully understood. Herein, we systematically study the phonon thermal transport of five Ag-based crystalline argyrodites Ag7PS6, Ag7AsS6, Ag8SnS6, Ag8GeS6 and Ag9GaS6 utilizing perturbation theory and the unified theory thermal transport model. Our results show that, as the complexity of the unit cell increases, the proportion of the population terms falls while the coherence contributions become more significant, leading to the relatively weak temperature-dependent kL of Ag7PS6 and Ag7AsS6, while the more complex crystalline argyrodites, Ag8SnS6, Ag8GeS6 and Ag9GaS6, exhibiting a glass-like behavior in their temperature dependence of kL. We attribute the positive temperature-dependent and ultralow kL of Ag8SnS6, Ag8GeS6 and Ag9GaS6 to the dominance of wavelike phonons and the strong phonon broadening. Furthermore, using laser flash measurements and the homogeneous non-equilibrium molecular dynamics simulations based on accurate machine learning neuroevolution potentials, we provide further evidence for the glass-like temperature-dependent kL of Ag8SnS6 and Ag8GeS6.