Enhancement of thermoelectric performance of argyrodite Ag8GeSe6 via isoelectronic substitution of Sn for Ge

Argyrodite compound Ag8GeSe6 (AGS) as one of the superionic conductors possesses unique crystal structures: the rigid anionic framework [GeSe4](4-) and mobile cation (Ag). Hence its lattice thermal conductivity is rather low due to the action of the mobile cation (Ag). However, this compound has neither high carrier concentration (0.15 x 10(16) cm(-3) at 300 K and 0.61 x 10(16) cm(-3) at 370 K) nor high thermoelectric performance (the figure of the merit (ZT) of<0.4). So in this work, we enhance the carrier concentration by more than one order of magnitude at 373 K through the band structure engineering via the substitution of Sn for Ge. In the mean time, the lattice thermal conductivity (kappa(L)) reduces from 0.26 W/mK to 0.16 W/mK at 750 K. As a consequence, the ZT value of Ag8Ge0.9Sn0.1Se6 is boosted to similar to 1.05. This value is about 2.8 times that of the pristine AGS (0.38 at 750 K) and stands among the highest in this family so far. The above results confirm that the isoelectronic substitution of Sn for Ge in the anionic framework of AGS is an effective way to realize its high TE performance.