Argyrodite‐Type Cu₈GeSe_6–<i>x</i>Te<i>_x</i> (0 ≤ <i>x</i> ≤ 2): Temperature‐Dependent Crystal Structure and Thermoelectric Properties

Abstract

Substitution of Se by Te in argyrodite‐like Cu 8 GeSe 6 leads to compounds Cu 8 GeSe 6– x Te x (0 ≤ x ≤ 2) and involves a structure change from the Cu 8 GeSe 6 type to the Ag 8 GeTe 6 type. The crystal structure of Cu 8 GeSe 4.25 Te 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. Cu 8 GeSe 4 Te 2 exhibits a higher thermoelectric figure of merit ( zT = 0.65 at 350 °C) than Cu 8 GeSe 6 ( zT = 0.2 at 350 °C). The application of an effective mass model reveals that the charge carrier concentration of 6 × 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.