Argyrodite-Type Cu8GeSe6-xTex (0 ≤ x ≤ 2): Temperature-Dependent Crystal Structure and Thermoelectric Properties

Substitution of Se by Te in argyrodite-like Cu8GeSe6 leads to compounds Cu8GeSe6-xTex (0 <= x <= 2) and involves a structure change from the Cu8GeSe6 type to the Ag8GeTe6 type. The crystal structure of Cu8GeSe4.25Te1.75 between 50 degrees C and 500 degrees C reveals pronounced disorder of Cu atoms that is best described by anharmonic atomic displacement parameters using the Gram-Charlier expansion. Diffusion pathways are visualized by isosurfaces of joint probability density functions. The lattice parameter a increases with temperature in two linear regimes from 10.4682(18) angstrom to 10.609(3) angstrom with a superionic transition at 400 degrees C. The potential barrier for Cu is about 90 meV at 50 degrees C, which is in the range of typical ion conductors. Cu8GeSe4Te2 exhibits a higher thermoelectric figure of merit (zT = 0.65 at 350 degrees C) than Cu8GeSe6 (zT = 0.2 at 350 degrees C). The application of an effective mass model reveals that the charge carrier concentration of 6 x 10(20) cm(-3) is close to optimum. In addition, argyrodite-type precipitates form heterostructures with a Cu- and Se-doped matrix of germanium antimony tellurides in the quinary system Cu/Ge/Sb/Se/Te, whose transport properties were characterized.