High-Temperature Structural and Thermoelectric Study of Argyrodite Ag₈GeSe₆

Argyrodites with a general chemical formula of A₈BX₆ (A = Cu, Ag; B = Si, Ge, Sn; and X = S, Se, and Te) are known for the intimate interplay among mobile ions, electrons, and phonons, which yields rich material physics and materials chemistry phenomena. In particular, the coexistence of fast ionic conduction and promising thermoelectric performance in Ag₈GeTe₆, Ag₈SnSe₆, Ag₈SiTe₆, Ag₈SiSe₆, and Cu₈GeSe₆ at high temperatures ushered us to their chemical neighbor Ag₈GeSe₆, whose high-temperature crystal structure and thermoelectric properties are not yet reported. In this work, we have employed a growth-from-the-melt technique followed by hot pressing to prepare polycrystalline Ag₈GeSe₆ samples, on which the crystal structure, micro-morphology, compositional analysis, UV-vis absorption, specific heat, speed of sound, and thermoelectric properties were characterized as a function of the Se-deficiency ratio and temperature. We found that (i) the crystal structure of Ag₈GeSe₆ evolved from orthorhombic at room temperature to face center cubic above 410 K, with a region of phase separations in between; (ii) like other argyrodite 816 phases, Ag₈GeSe₆ exhibited ultralow thermal conductivities over a wide temperature range as the phonon mean free path was down to the order of interatomic spacing; and (iii) varying Se deficiency effectively optimized the carrier concentration and power factor, a figure of merit zT value ∼0.55 was achieved at 923 K in Ag₈GeSe_5.88. These results not only fill a knowledge gap of Ag₈GeSe₆ but also contribute to a comprehensive understanding of 816 phase argyrodites at large.