Syntheses, Structures, and Magnetic and Thermoelectric Properties of Double-Tunnel Tellurides: AxRE2Cu6–xTe6 (A = K–Cs; RE = La–Nd)

Novel double-tunnel compounds AxRE2Cu6–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.本研究通过以ACl（A为K~Cs）为助熔剂，在1123 K下由单质原料混合物合成了新型双隧道化合物AxRE2Cu6–xTe6（其中RE=La~Nd）。该化合物结晶于六方晶系P63/m空间群，晶胞参数为a=9.955(3)~10.0841(6) Å，c=4.1487(6)~4.2366(6) Å，晶胞内化学式单元数Z=1。相较于Ba3Cu6Te6，从该母体化合物向目标化合物转变时，通过x个A+与2个RE3+共同取代3个Ba2+的阳离子取代策略，实现了基于CuTe4的阴离子骨架的规整重构。对未掺杂的烧结多晶La、Ce、Pr样品（含有2~4%的CsCl杂质）进行性能测试，其在614 K时的热电优值（ZT）可达0.26，在660 K时分别为0.17与0.23。通过电导率、塞贝克系数测试以及VASP计算，证实该化合物具备窄带隙半导体或半金属的典型电学行为。导带中存在多个能量极小值，且ZT值随温度呈单调升高趋势，表明该材料的热电性能具备进一步优化提升的潜力。