Syntheses, Structures, and Magnetic and Thermoelectric Properties of Double-Tunnel Tellurides: A_xRE₂Cu_6–xTe₆ (A = K–Cs; RE = La–Nd)

Novel double-tunnel compounds A_xRE₂Cu_6–xTe₆ (A = K–Cs; RE = La–Nd) have been synthesized from the elemental mixtures in the ACl flux at 1123 K and crystallized in the hexagonal P6₃/m, with a = 9.955(3)–10.0841(6) Å, c = 4.1487(6)–4.2366(6) Å, and Z = 1. A nice reconstruction of the CuTe₄-based anionic network has been driven by a cationic substitution of 3 Ba²⁺ with a combination of x A⁺ and 2 RE³⁺ on going from Ba₃Cu₆Te₆ to the title compounds. The undoped sintered polycrystalline La-, Ce-, and Pr-sample (containing 2–4% CsCl impurity) have realized the figure of merit (ZT) of 0.26 at 614 K, 0.17 and 0.23 at 660 K. Typical behaviors of a narrow gap semiconductor or a semi metal are revealed by the electrical conductivity, Seebeck coefficient measurements and the VASP calculations. The presence of the multiple minima in the conduction band and the monotonic increase of ZT with temperature indicate that further enhancement of TE property is possible.