Syntheses, Structures, and Magnetic and Thermoelectric Properties of Double-Tunnel Tellurides: A_<i>x</i>RE₂Cu_6<i>–x</i>Te₆ (A = K–Cs; RE = La–Nd)

Novel double-tunnel compounds A_<i>x</i>RE₂Cu_6<i>–x</i>Te₆ (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 <i>P</i>6₃/<i>m</i>, with <i>a</i> = 9.955(3)–10.0841(6) Å, <i>c</i> = 4.1487(6)–4.2366(6) Å, and <i>Z</i> = 1. A nice reconstruction of the CuTe₄-based anionic network has been driven by a cationic substitution of 3 Ba²⁺ with a combination of <i>x</i> 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 (<i>ZT</i>) 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 <i>ZT</i> with temperature indicate that further enhancement of TE property is possible.