Enhancement of Power Factor for Inherently Poor Thermal Conductor Ag₈GeSe₆ by Replacing Ge with Sn

Superionic argyrodites have been identified as thermoelectric materials with inherent ultralow lattice thermal conductivity because of liquid-like behavior of cations in a relatively larger unit cell. The major drawback for these as potential thermoelectric materials is low carrier concentration and low electrical conductivity, which however can be improved by controlling the compositions while keeping the advantageous poor thermal conductivity intact. Here, we report ∼8 times enhancement of thermoelectric power factor of Ag8GeSe6 by replacing Ge with metallic Sn, at high temperatures. Thus, Ag8SnSe6 has been demonstrated as an efficient thermoelectric material with ZT ∼ 0.62 at 623 K, even at a very low carrier concentration (∼1016 cm–3). The extremely low thermal conductivity has been explained with weakly bonded Ag ions to rigid anion sublattice and presence of low-frequency Einstein optic modes, which provide the possibility of decoupling of charge and heat transport