Glassy lattice thermal conductivity of Li6PS5X (X = Cl, Br, I) argyrodites from first principles

We compute lattice thermal conductivities of Li6PS5X (X = Cl, Br, I) argyrodites from first principles within the Wigner transport formulation. We find that these systems are afflicted by unstable vibration of Li atoms off of their symmetric trigonal planar environment. We efficiently calculate effective temperature-dependent phonons using anharmonic renormalization. Good agreement with experimental conductivity measurements is obtained for the three materials. Like other argyrodite-class systems, these materials are found to possess very low, glassy lattice thermal conductivity that is nearly constant with temperature. The coherence effect is found to contribute comparably to the standard Boltzmann transport channel to the total conductivity. The study shows that the Wigner formalism is adapt at characterizing thermal conduction in solid electrolytes and ionic conductors.