ICT2014 Abstracts

Originally studied as a super-ionic conductor in the 1970s, Ag₈GeTe₆ has more recently attracted interest for its promise in thermoelectric applications mainly due to its low thermal conductivity. Central to both the fast ionic conduction and low thermal conductivity observed in Ag₈GeTe₆ are its complicated crystal structure and unusual lattice dynamics. Previous studies have shown that (I) Ag₈GeTe₆ adopts a F 4 -3 m cubic structure at room temperature with 60 atoms in a large unit cell; (ii) it undergoes four consecutive phase transitions between 156 K and 245 K, indicating a unstable system; and (iii) the heat capacity is ~ 20 % higher than the Dulong-Petit limit near room temperature, suggesting strong anharmonicity. Notably, there are no published thermoelectric studies of Ag₈GeTe₆ below room temperature. In this work we have melt-grown Ag₈GeTe₆ ingots, and conducted heat capacity, thermal conductivity, electrical conductivity, Seebeck coefficient, and Hall coefficient measurements between a few Kelvin and room temperature. Specifically we elucidate the unusual lattice dynamics and exceptionally low thermal conductivity in the context of a combined Debye-Einstein lattice. These results help in the understanding of how Ag₈GeTe₆ evolves and sets the stage for the coexistence of super-ionic conduction and good thermoelectric performance at higher temperatures.