Thermal transfer in amorphous superionic Li 2 S

Using atomic simulations, we characterize the scattering time, the nature, and the weight of thermal excitations in the example system ${\mathrm{Li}}_{2}\mathrm{S}$ from its amorphous solid state to its superionic and liquid states. For the amorphous solid state at 300 K, the vibration scattering time was found to range from a few femtoseconds to several picoseconds. As a result, both propagons and diffusons are the main heat carriers and contribute largely to the total thermal conductivity. The enhancement of scattering among vibrations and between vibrations and the free ions' flow due to the increase of temperature leads to a large reduction of the scattering times and of the acoustic vibrational thermal conductivity, i.e., 0.8 W/mK at 300 K to 0.56 W/mK in the superionic ${\mathrm{Li}}_{2}\mathrm{S}$ at 700 K. In this latter state, the thermal conductivity contributed by convection increases to half of the total, as a result of the cross correlation between the virial heat flux and the free ions' one in liquid ${\mathrm{Li}}_{2}\mathrm{S}$ at 1100 K. The vibrational scattering time can be large ($\ensuremath{\sim}1.5$ ps) yet, and the vibrational conductivity is reduced to a still significant fraction highlighting the unexpected role of acoustic transverse and longitudinal vibrations. Our study provides a fundamental understanding of the thermal excitations at play in amorphous materials from solid to liquid, including the intermediate superionic phase.