Argyrodite-Type Cu₈GeSe_6–xTe_x (0 ≤ x ≤ 2): Temperature-Dependent Crystal Structure and Thermoelectric Properties

Substitution of Se by Te in argyrodite-like Cu₈GeSe₆ leads to compounds Cu₈GeSe_6–xTe_x (0 ≤ x ≤ 2) and involves a structure change from the Cu₈GeSe₆ type to the Ag₈GeTe₆ type. The crystal structure of Cu₈GeSe_4.25Te_1.75 between 50 °C and 500 °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) Å to 10.609(3) Å with a superionic transition at 400 °C. The potential barrier for Cu is about 90 meV at 50 °C, which is in the range of typical ion conductors. Cu₈GeSe₄Te₂ exhibits a higher thermoelectric figure of merit (zT = 0.65 at 350 °C) than Cu₈GeSe₆ (zT = 0.2 at 350 °C). The application of an effective mass model reveals that the charge carrier concentration of 6 × 10²⁰ 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.