National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan.
Dalton Trans. 2018 Nov 22;47(45):16205-16210. doi: 10.1039/c8dt02955e.
As-Based Zintl compounds Ba1-xKxCd2As2 crystallized in the CaAl2Si2-type structure (space group P3[combining macron]m1) were prepared using solid-state reactions followed by hot-pressing. We have successfully substituted K for Ba up to x = 0.08, producing hole-carrier doping with concentrations up to 1.60 × 1020 cm-3. We have determined the band-gap value of non-doped BaCd2As2 to be 0.40 eV from the temperature dependence of the electrical resistivity. Both the electrical resistivity and the Seebeck coefficient decrease with hole doping, leading to a power factor value of 1.28 mW m-1 K-2 at 762 K for x = 0.04. A first-principles band calculation shows that the relatively large power factor mainly originates from the two-fold degeneracy of the bands comprising As px,y orbitals and from the anisotropic band structure at the valence-band maximum. The lattice thermal conductivity is suppressed by the K doping to 0.46 W m-1 K-1 at 773 K for x = 0.08, presumably due to randomness. The effect of randomness is compensated by an increase in the electronic thermal conductivity, which keeps the total thermal conductivity approximately constant. In consequence, the dimensionless figure-of-merit ZT reaches a maximum value of 0.81 at 762 K for x = 0.04.
基于 Zintl 的化合物 Ba1-xKxCd2As2 采用固态反应和热压法合成，具有 CaAl2Si2 型结构（空间群 P3[combining macron]m1）。我们成功地用 K 替代了 Ba，最大取代度可达 x = 0.08，从而实现了浓度高达 1.60 × 1020 cm-3 的空穴掺杂。我们通过电阻率的温度依赖性确定了未掺杂 BaCd2As2 的带隙值为 0.40 eV。随着空穴掺杂，电阻率和 Seebeck 系数都降低，导致 x = 0.04 时在 762 K 下的功率因子值为 1.28 mW m-1 K-2。第一性原理能带计算表明，较大的功率因子主要源于包含 As px,y 轨道的能带的二重简并性以及价带最大值处的各向异性能带结构。晶格热导率通过 K 掺杂抑制到 0.46 W m-1 K-1，在 x = 0.08 时在 773 K，这可能是由于无序造成的。无序的影响通过电子热导率的增加得到补偿，这使得总热导率基本保持不变。因此，在 x = 0.04 时，无量纲优值 ZT 在 762 K 达到最大值 0.81。
