Fabrication and Thermoelectric Properties of Single-Crystal Argyrodite Ag8SnSe6

Recently, polycrystalline argyrodite Ag8SnSe6 has shown promising thermoelectric performance, which largely relies on the ultralow lattice thermal conductivity. However, the thermoelectric properties of single-crystalline Ag8SnSe6 have been unavailable so far. In this work, Ag8SnSe6 single crystals, with a highly preferred orientation of (110), are grown by a vertical Bridgman technique. Thermoelectric properties parallel and perpendicular to this direction are investigated, revealing a similar to 40% higher hall mobility but a very similar lattice thermal conductivity in single crystals as compared to that in polycrystals. Both transport and elastic properties are found to be nearly isotropic, even for the low-temperature phase with an anisotropic structure. This work, for the first time, not only successfully synthesizes high-quality single-crystal Ag8SnSe6 but also enables a fundamental understanding on the materials physics.