Two-Channel Thermal Transport in Ordered-Disordered Superionic Ag₂Te and Its Traditionally Contradictory Enhancement by Nanotwin Boundary

A recent experiment [J. Mater. Chem. A 2015, 3, 10303] has proved that superionic Ag₂Te can achieve a figure of merit as high as 1.39 due to its extremely low thermal conductivity. However, the traditional lattice vibration concept, i.e., phonons regarded as heat carriers, fails to explain the governing mechanism in such structures where anions vibrate around their equilibrium positions while cations flow like a liquid. As a result, the underlying physics for thermal transport properties in superionic Ag₂Te is still a mystery. In this study, two-channel heat transport in such ordered-disordered systems (i.e., lattice vibrations and liquid-like mobile ions coexist) is quantitatively characterized on the basis of the heat flux linear response theory. Our results show that the convective thermal conductivity is dominant in the system, which results from the free movement of Ag ions. As a consequence, the total thermal conductivity increases abnormally with temperature due to the strengthened cations' mobilities at elevated temperatures. Meanwhile, the effect of experimentally observed nanotwin boundaries [ Acta Mater. 2017, 128, 43 ], which facilitate electrical transport in crystals, on heat carriers are also investigated. In contradiction to the classical heat transport theory, in which heat carriers are hindered by boundaries, the nanotwin boundary in superionic Ag₂Te unexpectedly improves thermal transport due to the enhanced movement of Ag ions around the grain boundary. This exhaustive explanation of thermal transport properties in superionic Ag₂Te will support the future design of superionic conductors based on thermoelectrics and more broad energy systems composed of ordered-disordered materials.