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

Abstract

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 °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.

Entropy-Induced Multivalley Band Structures Improve Thermoelectric Performance in p-Cu7P(SxSe1–x)6 Argyrodites

“…30−32 When the configuration entropy is high enough, the phase transition could be shifted to even below room temperature, which results in maintaining the required crystal structure symmetry in all temperature ranges of the operation and at the same keeping improved electrical transport properties. 29,33 A series of argyrodite compounds, such as Cu 8 GeSe 6 , 34,35 Cu 7 PSe 6 , 29,36,37 Ag 9 AlSe 6 , 38 Ag 8 SnSe 6 , 39−41 Ag 9 GaSe 6 , 42−44 Ag 9 GaS 6 , 45 Ag 8 SiSe 6 , 46 Ag 8 GeSe 6 , 47 Ag 8 GeTe 6 , 48,49 and Ag 9 TlTe 5 , 50 have been demonstrated as the promising TE materials with a high ZT of 1.0 and above at high temperatures. The best reported ZT parameter for the n-type argyrodites reaches a value of ∼1.62 for Ag 8.28 Cu 0.72 GaSe 6 .…”

“…A series of argyrodite compounds, such as Cu 8 GeSe 6 , , Cu 7 PSe 6 , ,, Ag 9 AlSe 6 , Ag 8 SnSe 6 , − Ag 9 GaSe 6 , − Ag 9 GaS 6 , Ag 8 SiSe 6 , Ag 8 GeSe 6 , Ag 8 GeTe 6 , , and Ag 9 TlTe 5 , have been demonstrated as the promising TE materials with a high ZT of 1.0 and above at high temperatures. The best reported ZT parameter for the n -type argyrodites reaches a value of ∼1.62 for Ag 8.28 Cu 0.72 GaSe 6 .…”

Improved Thermoelectric Performance of p-Type Argyrodite Cu8GeSe6 via the Simultaneous Engineering of the Electronic and Phonon Transports

“…Besides, its band gap ( E g ) can be reduced by alloying Te . This is important as the narrowing of the band gap will enhance the carrier concentration and improve the TE performance. , Furthermore, some work has revealed that the pristine CGS has a high carrier concentration of ∼10 21 cm –3 and exhibits a metal-like conducting behavior, while some work indicates that it has a carrier concentration of only 10 17–18 cm –3 in magnitude and is a typical semiconductor . These inconsistent results suggest that it is strongly necessary to determine the carrier concentration accurately.…”

PHASE EQUILIBRIA IN THE Ag8SiTe6–Ag8GeTe6 SYSTEM

“…Binary and more complex chalcogenides of copper and silver are valuable functional materials [1][2][3]. In particular, copper and silver chalcogenides with heavy p-elements are of interest as thermoelectric materials [4][5][6][7][8]. Several representatives of this class, due to their unique crystal structure, have ionic conductivity for Cu + or Ag + cations, which makes them very promising for use in the development of photoelectrode materials, electrochemical solar energy converters, ion-selective sensors, etc.…”