Thermal transport property of Ge₃₄ and d-Ge investigated by molecular dynamics and the Slack's equation

In this study, we evaluate the values of lattice thermal conductivity \kappa_\rm L of type II Ge clathrate (Ge₃₄) and diamond phase Ge crystal (d-Ge) with the equilibrium molecular dynamics (EMD) method and the Slack's equation. The key parameters of the Slack's equation are derived from the thermodynamic properties obtained from the lattice dynamics (LD) calculations. The empirical Tersoff's potential is used in both EMD and LD simulations. The thermal conductivities of d-Ge calculated by both methods are in accordance with the experimental values. The predictions of the Slack's equation are consistent with the EMD results above 250 K for both Ge₃₄ and d-Ge. In a temperature range of 200—1000 K, the \kappa_\rm L value of d-Ge is about several times larger than that of Ge₃₄.