Energy Transport in Superionic Crystals

In this Letter, we propose a rigorous concept based on the Onsager reciprocal theorem to describe the thermal transport behaviors in superionic crystals. Our results show that thermal energy in superionic crystals can be transferred through the conduction of atomic vibrations, the enthalpy diffusion caused by ions' diffusion and the thermodiffusion coupling. The thermal conductivity resulting from the heat conduction pathway decreases with temperature as scatterings among vibrations and between vibrations and ions become stronger. However, the thermal conductivity due to enthalpy diffusion increases with temperature, which is because ions diffuse farther at high temperatures. The thermal conductivity owing to the thermodiffusion coupling is negligible since the chemical potential gradient under steady state is small. The apparent thermal conductivity of superionic crystals is therefore determined by the competition between these two thermal pathways, and can exhibit a diverse behavior of negative, weak, and positive temperature dependence as observed in experiments. This Letter unveils the thermal transport mechanisms in superionic crystals, which explains the long-standing and confusing thermal conductivity temperature dependence of superionic crystals.